self build waterproof basement formwork membrane concrete architect

Basement Construction. My page for architects' FAQs.

My name is Phillip Sacre. I have built and helped clients build basements for years. I am the guy who promised waterproof and had to overcome the design details (and BBA certified products) that didn't work.

Nowadays, I sell my expertise, supervision and some products I sourced. They are better than the famous brands. I sell them mainly to self-builders because the only people who seem to care whether the basement will always be dry are those who will build the basement they will live in themselves. I know how to avoid every cause of leaks through a new basement and I personally guarantee your client that I will fix all visible ingress of water through his reinforced concrete structure. I haven't had a single drip through my work built since 2013.

I have so much to tell you that is useful. I worked with private clients. My ways went well. They became friends. This means I later listened to all the other things that went wrong. Top of that list, the things that I can help you avoid going wrong with are:

  1. Ground Source Heat Pumps failed after two years because the ground doesn't warm up again at depth.

  2. Air Source Heat Pumps aren't any better. On frosty days it costs more to power an ASHP than the energy you get.

  3. Treating thermal mass concrete as a cold bridge is a big mistake. Insulation slows the passage of heat energy. Thermal mass slows it even more. Sufficient thermal mass is effective insulation as well.

  4. The underside of a floor over a basement is usually below outside ground level.
    Traditional floor materials that work above outside ground level or allow water under, such as beam and block, cause incurable problems and misery for years when they are specified over basement accommodation.

  5. It is wrong to specify any brand of internal drainage at the design stage.

    BS 8102 says any internal drainage should be designed to cope with the amount of water coming in.

    The proper approach is to mention you will have internal drainage but to wait to see how much water needs to be pumped before choosing the size of the system you need.

    Reinforced concrete is very easy to repair, to reduce the amount of water ingress, if it is repaired during the construction phase. My basements have no ingress of water and therefore no internal drainage.

    However, it is important to understand that all my basements would have had internal drainage had there been any incurable ingress of water. The decision was made later, not at the design stage.
  self build basement

Quick links to sections of this page.
  1. Basement waterproofing. How to succeed without spending a fortune.

  2. The truth about waterproof concrete.

  3. How a floor over a basement is different to the ground floor in a normal house.

  4. Door and window openings in a waterproof concrete wall.

  5. Pumping foul waste from a basement bathroom, toilet or laundry.

  6. Structural issues are dealt with on the page for Engineers. (New page in a new tab).

My name is Phillip Sacre. I have been building and helping to build basements for years. I sell my expertise and some products I sourced because they are better than the famous brands. My clients are self-builders, seemingly the only people who care that a basement is built dry.

When I used to have leaks, none since 2013, I investigated their cause and changed my method to avoid them. I know how to avoid every cause of leaks through a new basement structure. I have been sharing my "secrets" on this web site for years but very few tradesmen have copied them, only self builders, and I am very busy helping many of them.

During the year or so the client lives with a ventilated, bare concrete wall basement, if he sees any ingress of water I will fix it. (None fixed since 2013).

Even if I fail and there is some ingress of water, it won't be a torrent.

A damp patch or a few drips of water don't want a powerful pump or an outlet hose that takes 2 litres to fill before any water is actually pumped away.

Excavating extra deep for a sump is very expensive, especially if the extra depth finds the water table. And extra depth for a deep sump can be dangerous if there is not enough room to safely batter the sides.

Many basements have plumbing and a plumbing spill will be far worse than a leak through my work. If the plumbing spill is catered for, any drips leaking through are catered for as well. One client chose a Baked Bean tin cast in his floor slab and others something the size of a dinner tray 50mm deep. These could be emptied with a WetVac or aquarium pump. They make far more sense and of course the cost is tiny in comparison.

Building Control expect to see "internal drainage, a sump and a pump" on the drawings before work starts; and there is no need to enter into a battle, you can specify exactly what they expect to see.

Your client hopes to see "internal drainage, a sump and a pump" and "shallow depression for plumbing spills in the structural slab within the plant room".

In this way, provision has been made for a pump to be added if there is enough water to pump out and the outlet hose might be 3mm diameter and all but an eggcup of water can be pumped right out.

I appreciate that you have to put 'internal drainage' on your drawings at the design stage and that BS8102 directs designers to withstand even the worst circumstances and allow for remedial measures.

Withstanding the worst circumstances means providing a good design. These days, you and your design team (including the structural engineer) can specify a solution a whole lot better than how they built Victorian cellars with bricks and lime mortar.

It does not mean increasing your client's costs £50;000 with expensive Delta or Newton-type internal drainage, creating an expensive difficulty burying the sump far deeper than the basement often with limited working room, digging the basement deeper and providing taller retaining walls for an extra 150mm of concrete on the floor to create a perimeter drain.

I analyse the Outwing court case nearly half way down this page. I am sure you have been told an incorrect interpretation by the people who would sell you a huge internal drainage system.

BS8102 does not say you must specify exactly what the Type C defence will be before the extent of water ingress is known. You have a choice.

However, you can also specify that genuinely water-resistant concrete is used, for instance C35A, and all waterproofing work to be supervised and approved by someone other than the sub-contractor doing the installation.
I rely on the householder living with a bare concrete basement for at least a year after he moves in, so that a wholly appropriate Type C internal defence against ingress of water can be chosen after the worst weather has been experienced.

My methods are no good to a developer who wants to sell a new dwelling with a basement off-the-drawing before it is complete to a family who expect the basement to be plastered and decorated.

Neither will my methods suit a main contractor who will use a string of sub contractors who don't care about anyone but themselves.
What worked best are, and some of these might still be best choices if your client is a developer or will use a main contractor:
  1. Heat Recovery Ventilation (but only if the filters are very accessible and easily changed).

  2. Waterproof concrete with good workmanship throughout including design, placing, compacting and curing - which means supervision throughout.

  3. Extending the retaining walls to above outside ground level. Engineered timber and cast insitu reinforced concrete are the best floor choices over a basement because both allow the retaining wall to be continuous to above outside ground level.

  4. Leaving the basement properly ventilated but with bare concrete walls until everyone is certain there is no leak anywhere. Bare concrete is very easy to repair.

  5. A basement has to be built with a lot of concrete. That is a lot of thermal mass. Unheated, a basement will always be cave-temperature. Never freezing. Never too hot.

    There are different ways to build basements with zero carbon in mind. An investment in concrete creates thermal mass: a storage heater that costs nothing to install or run that (a) releases heat for free only when required and (b) soaks up excess heat, again only when required and for free.
    Thermal mass explained, opens in a new tab.

These are few design choices that put basements over budget:
  1. Digging considerably deeper than the basement structure to install a sump. Especially if the extra dig finds the water table or if there isn't the space to safely batter the sides as far down as the base of the sump.

  2. Foul waste gravity draining to a deep sump and then pumped out. These sumps block from wet wipes and so on and are horrible to unblock and repair. Macerator pumps are a tiny fraction of the cost and if a wet wipe is used they will block immediately and need clearing. But at least macerators block when the first wet wipe is used. A big, buried sump might have 20 wet wipes in it before it blocks the first time and could block 19 more times before they are all removed.

  3. An internal drainage design might require a perimeter drain channel. This adds digging deeper down, taller retaining walls and a second screed to bury the channel beneath the floor.

  4. A choice of floor over the basement that will always leak, such as beam and block, precast concrete, composite steel or steel beams.

A few ideas how to use waterproof concrete to above outside ground level and create a a lot of thermal mass :

seasonal heating cooling envelope architect architect basement thermal mass architect basement timber floor architect basement traditional

Clients have also been very badly let down by steel and precast concrete over basements. Very hard to insulate and very difficult to waterproof. I must say this again as well. Steel beams, beam and block, precast concrete planks and composite steel floors over a basement have all caused incurable problems of leaks, cold spots or mould. Incurable. A lot of blame aimed at the architect for years.

I particularly DON'T want you to specify any brand of internal drainage, such as Delta or Newton, at an early stage because specifying a brand to design the drainage gives them free rein to go mad with your client's money before the extent of water ingress is known. These are real figures:
  • Delta V3 dual surface water pumps complete with high level alarm and exit pipework £2000.
  • Sub-floor drainage system to perimeter of floor with perimeter rebate £790.
  • Delta MS20 water and vapour proof cavity drainage membrane to the floor area of the internal basement £1500.
  • Delta MS500 membrane to the walls of the basement £2200.
The V3 sump has to go in before the basement floor slab. That means the CSSW designs for the very worst before construction starts and some suppliers want a 40% deposit up front. Yet, if it was a Victorian cellar conversion a far cheaper system would be chosen if very little water came in.

Specifying a brand early on means a system to pump hundreds of litres of water every hour. Crazy if it turns out there isn't any water.

BS8102 states that if, when the dwelling is watertight, there is no visible ingress of water even during and after a heavy storm, it allows you to change your mind away from the all-singing, all-dancing very expensive internal drainage costing thousands of pounds in favour of something more sensible.

This is the joint aim of me and your client. If nothing comes through the Type B, the concrete; then the Type C internal defence can be changed to polythene or waterproof paint.

Successful waterproofing is all about building well with good choices and supervision and proving the basement does not leak before it is fitted out.
  • Your client needs to make sure good solutions are chosen and all workmanship is satisfactory. No exceptions.

  • Your client should complete his house above ground before moving in but leaving the basement with bare concrete for as long as is necessary to know no water leaked in even during the worst weather.

  • If any water does come in, concrete is very easy to repair from the inside if the walls haven't been covered over yet. Even easier, probably, than repairing internal drainage membrane.
What does everyone else specify and build?

Seasoned professionals will all use steel threaded rods in a plastic sleeve. They leave the plastic sleeve as a hole through otherwise waterproof concrete; or else they fill the holes but some leak.

I was sent these images by a prospective client looking for a different basement builder, so I don't know where he found them. He is planning a second basement and he says his first basement leaks - so perhaps this is it.

typical basement leaks

And here we see the hole destined to get the Delta-type sump for a pump, backup pump and backup power supply. All very expensive and not guaranteed to work either.

the sump in a basement

PLUS. HORROR !!!!    The pipes and ducts connecting the outside to the inside are underground but not set in any waterproof concrete.

The shingle you can see in the photo above would have damaged the joints between sections of duct when it was tipped in. When the shingle fills up during a severe storm, water could flood in through these various duct joints far faster than the pump could pump it out. Pushing the drainage membrane off the wall, flooding the basement temporarily and leaving plasterboard, decoration and furnishings ruined.

No fancy warranty from a respected waterproofing company will cover any of this and all the 10 year latent defects insurers exclude waterproofing below ground.

These built-in errors are at the risk of the householder. No one else.

Sometimes you can't trust anyone. Whose fault is this? The architect, the engineer, the internal drainage supplier or the contractor? None of them seem to have realised the terrible mistake being made.

The client won't be happy.


Basement waterproofing.

Many specifiers kill their client's basement dream on the drawing board by specifying another excavation within the basement excavation for a buried pumping chamber to pump out thousands of litres of water from the internal drainage system.

Digging even deeper than the basement gets very expensive and this simple error by an architect can put the basement well beyond the budget.

Most soils aren't as stable 5m down as they were 3.3m down. A big excavation 3.3m deep can be safe but a narrow hole for a pumping chamber inside that is more likely to find the water table and be quite dangerous.
  basement sump chamber

There are some Victorian cellars that leak Thames water continually in Hammersmith, Chiswick and Battersea. But if your client wants a new build basement the important question is 'does the soil investigation report warn that this basement could flood continually or will it sit in mainly dry ground except during an occasional severe storm? '

A big Delta chamber is justified 3 storeys down in Knightsbridge, but not higher up in the Chilterns. Going over the top, wasting money, just kills off the project.

Victorian cellars were built with bricks. A new basement should be built with reinforced concrete. Potentially, the reinforced concrete will keep out far more water than brickwork.

Whether reinforced concrete leaks Sometimes or Never: is down to the workmanship, not your design.

If your client has a Clerk of Works or a Supervisor a simpler design will work better than a complex design thrown in badly.

  1. A basement that does not leak except for a few trickles during bad weather does not require a massive pumping chamber.

    It requires a small chamber the size of a dinner tray and only 50mm deep. With some electronics and a 12V fish aquarium pump a small amount of water like one litre an hour can easily be lifted 3m to the mains drainage. It can be hidden in a plant room where it will also pump away plumbing spills.
    shallow sump

  2. If the workmanship is shocking no amount of hydrophylic strip, sticky back membrane or clay filled carpet will save it because they will be installed just as badly.

    When the reinforced concrete basement structure is complete, the answer is to repair leaks before work continues.

    Probably by a different team and at the expense of the main contractor. If someone does their work properly a basement won't leak.

    The specification should demand that leaks are reduced to one litre an hour or less even during the worst weather.

  3. But,


    With some training, supervision and better methods, 100% dry basement construction is very easy indeed.

    It can be very difficult to persuade seasoned concreting specialists to do the work differently,

    But your client can join the farmers, policemen, university lecturers, IT consultants, business men, accountants, doctors, surgeons, consultants, dentists, marketing and insurance consultants and big game safari guide who have all built a completely dry basement with my help.
This first section explains why every project needs to start with a design specifying extremely expensive internal drainage, a sump, a pump, a backup pump and a backup power supply.

And how I advise the client to try to get you to agree later to change that to a relatively cheap waterproof paint or polythene.

The Outwing Appeal judgment stated that designers should allow for some errors in workmanship and design to overcome them.

Therefore, very wisely, BS8102 requires 2 defences against ingress of water of two quite different types
  • External
  • Integral
  • Internal
and sales people will tell you that to allow for some errors in earlier workmanship you MUST specify the last possible defence which is internal drainage.

During my career I have learned how to make concrete leak-free every time.

Sadly, I seem to be almost unique in making sure all concrete I supervise is completely leak-free.

The concrete only requires more cement and less water. The workmanship needs to be good as well. It only takes good old-fashioned care and supervision (like in the good old days) and a basement can be dry from the concrete alone.

Most people agree that a waterproof concrete structure shouldn't leak. Yet they also agree most do. And some very badly.

Concrete is usually sound, so if it visibly leaks it is through voids, cracks or debris where it wasn't cleaned.

The Concrete Society report (referenced elsewhere on this page) quotes as follows (at 7.1.1.):

" In its introduction to the selection of materials the publication Concrete Basements Guidance on the design and construction of in-situ concrete basement structures to the Eurocodes (The Concrete Centre, 2012) states: "Good concrete is inherently water resistant".

It then goes on to say;

"Before considering selection of materials, it is worth emphasising the fundamental requirements for achieving such performance. These are:
  • correct structural design
  • appropriate concrete mix
  • good workmanship in construction
  • appropriate supervision.
Many water-resistant basements have been successfully constructed on the basis of the above alone. There are a number of admixtures in the market to modify the properties of the fresh and hardened concrete (e.g. porosity, permeability). Such measures should not be necessary if the basic principles noted above are observed."

That is the official authority that if your client's work turns out to have been done well, you no longer need to specify the last possible and most expensive defence against ingress of water: internal drainage.

A major part of doing the work well is to avoid repairs that might not be carried out properly.

Your client will be buying these threaded fibreglass rods for his formwork.
Instead of having hundreds of holes through his basement walls he will cut these off. No holes. No repairs. No leaks.
frp threaded rods for formwork

Your client will not be using a kicker at the bottom of his walls. Kickers are usually made out of old concrete that cannot be compacted properly. kickers leak

Your client will clean the slab concrete thoroughly before he carefully delivers high quality concrete to the bottom of his form, compacts it properly and cures it properly. leaking basement wall concrete
You will have had presentations telling you that you must specify internal drainage because of a particular court case.

The Outwing Construction v Thomas Weatherald (1999) case here.

But I think you have been misled by unscrupulous sales people.

The architects for the Grenfell Tower were misled by unscrupulous sales people. Unscrupulous organisations that cheated the testing, withheld important information, knowingly lied.

Outwing Construction. 1999.

When the basement leaked the main contractor withheld money that the sub contractor, Outwing, successfully sued to be paid.
self build basement house

The situation: The design required 2 skins of blockwork wall filled with concrete to be the retaining wall, covered on the outside with a sticky-back waterproof membrane and in front of that a land drain some way up the wall, not at the bottom. The ground outside was chalk.

The judge agreed that the sticky back membrane outside was not suitable if it were to sit in water and the land drain should have been much lower beneath the concrete slab/wall joint.

If external drainage wasn't going to work then he said that a better solution needed to be chosen.

Nowhere does the judgement say internal drainage, a sump, pump, backup pump or emergency power source needed to be specified.

In my experience, the experience that meant I became an expert:
  1. Two skins of blockwork would both leak and the concrete in between could never be waterproof because the void could not be cleaned of mortar that dropped inside when the blocks were laid, or the joint cleaned or a joint strip protected.

  2. Sticky-back membrane rarely sticks successfully to a basement wall because the atmosphere on the north side down an excavation is usually too moist, which is enough to stop these products sticking even to primer.

  3. The designer may have thought that chalk would always drain anyway, and the drainage superfluous.
It would seem to me this case could be read 3 different ways, not just the one way that suits the suppliers that sell the most expensive solutions:
  1. This ruling means that a sub contractor cannot have money withheld if he does work badly.

  2. or

  3. A basement waterproofing design can only be valid if the waterproofing can be repaired during the life of the basement
    the design will be robust even with water outside 1m deep.

  4. or

  5. This design was particularly poor and so likely to fail that the sub contractor could not be blamed.

Without any doubt, this third way of reading the case is the correct way to read it. Not that basements need up to £40,000 of internal drainage to cope with leaks through this obviously very poor design.

I think that in this case a good soakaway should have been possible in the chalk, the land drain should have been much lower beside the slab and the retaining walls solid reinforced, waterproof concrete checked and repaired before the basement was fitted out.

And it would be very wrong to expect internal drainage to be foolproof.

Where no effort is made to stop water leaking in because workmen think that the internal drainage can cope with it, like everything else internal drainage breaks down more often the more often it is used.

Even if the sump, pump, backup pump and alternative power supply work fine, after a few years the plumbing to remove the water can fail. Every time the pump fires up it kicks the pipework. After a few thousand jolts plastic, rubber or canvas pipes will break, tear or tear themselves out of jubilee clips.


The only way for workmen to probably substantially reduce the amount of water that gets through concrete is to aim for no water to get through. In that way, failure to be perfect is still success in terms of BS8102.


This section of this web page is to prepare you for the phone call from your client that he now lives in his new house, that he hasn't fitted out the basement yet (though he might have a washing machine, table tennis table, drum kit and gym down there) and after that heavy rain last weekend it is still bone dry.

He hasn't put the internal drainage in yet and he doesn't want to.

He wants you to specify waterproof paint as a vapour barrier and Type C defence instead please.


This section explains the truth about waterproof concrete.  There is a certified CPD I wrote, about 2014, somewhere on the Internet.

This is the quote from the Concrete Society report, 2012:
"Many water-resistant basements have been successfully constructed on the basis of the above alone. There are a number of admixtures in the market to modify the properties of the fresh and hardened concrete (e.g. porosity, permeability). Such measures should not be necessary if the basic principles noted above are observed."

The Concrete Society has published reports more than once expressing doubt that brands such as Caltite, Xypex, Pudlo and Sika do anything more than good concrete can achieve anyway.

BBA certification is completely inappropriate because their testing is completely inappropriate. Suppliers just added a bit more cement or reduced the water slightly, got a tiny improvement and got a BBA certificate in exchange for a lot of money. But none of the BBA certificates say that they tested the concrete that would be delivered to site or that site concrete will be waterproof.

Yet concrete can be made waterproof. There is not any admixture that waterproofs un-waterproof concrete used below ground for basement construction. But there is a test that measures the porosity of a cube of hardened concrete delivered to a project and it can prove that the concrete used on site is waterproof.

My evidence is also from another UK Concrete Society report, 2013* and the BBA certificates for Caltite, Pudlo, Sika etc..

The proper certification for waterproof concrete is concrete made to BS EN 206-1, and concrete tested to BS EN 12390: testing of hardened concrete.

Thousands of test cubes are tested to BS EN 12390 part 3 every week. This is the test for compressive strength.

BS EN 12390 part 8 is the test for depth of penetration of water under pressure.

The different parts of BS EN 12390 are listed here on the BSI web site.

Here is a photographed extract from my copy of "Advanced Concrete Technology", the volume called "Processes", the Chapter called "Concrete construction for liquid-retaining structures" by Tony Threlfall, 2003, Butterworth-Heinemann. Page 16.2.   Click on this image to be able to read the text from the whole page.   from Processes, water retaining BS EN 12390 8
It says that permeability less than 20mm is satisfactory. You will see on my certificate of permeability that my results in this case were 1mm, 1mm and 3mm. I have dozens of similar results going back years.

You can specify that cubes should be taken and tested for strength and permeability. The permeability test costs £600 plus VAT.

Every BBA certificate for a water-resisting admixture for concrete tells the user to buy an already water-resistant mix of concrete. This is a pump mix with 325 to 350kg of cement per cubic metre with reduced water.

C35A in BS 8007 is a concrete mix with 325kg of OPC and 55% water by weight of cement and this is a watertight mix.

In the text you will see that the appropriate test for waterproof concrete is to BS EN 12390 part 8. There is no mention that a BBA certificate will prove concrete won't leak.

Many, many large sites routinely test concrete for strength to BS EN 12390 part 3. I have concrete tested to BS EN 12390 part 8 as well. Depth of penetration of water under pressure on concrete. The pressure is equivalent to a depth under water of 30m and the pressure is maintained for 96 hours.

Click on either image to the left to see two original test certificates.
  I always buy concrete over-sanded, with 350kgs of cement and a target slump of 60mm. From experience, concrete suppliers will send concrete too stiff to pump or compact but wet enough not to induce a flash set.

The concrete chemistry books state that by adding a little more cement to C35A and reducing the water a little further that pores of water between remnants of cement grains will be closed off from neighbours by crystals of hydrated cement.

However, such a mix would be too stiff to pump or compact without a more powerful plasticiser than those usually available. We have to make sure that a particularly powerful plasticiser is used.

I add my powerful, PCE based water reducing plasticiser at site and I count the revolutions to make sure it is thoroughly mixed through. The concrete becomes runny, pump-able and easy to compact.

concrete compressive strength report to BS EN 12390 3 concrete permeability report to BS EN 12390 8

A working group of the UK's Concrete Society published a report in 2013* produced by consultant engineers, academics and other experts. They strongly suspect that it is always the concrete that will be waterproof, not any of the admixtures.

* The influence of integral water-resisting admixtures on the durability of concrete. P36. Concrete Society. 2013.

But to make matters worse, most of the BBA tests were on concrete samples that would have been too stiff to pump. On concrete that would have to be rejected on site or, more likely, have more water added which defeats the purpose of trying to waterproof concrete.

Most BBA certificates contain the evidence that the concrete that will be delivered to site was not tested.

Do all products have to have a BBA certificate or else they cannot be used?
(Not according to a statement here from the BBA web site 22 Oct 17 stating that a specifier needs to make a judgement, not just trust a product because it has a certificate).

On November 9th 2020 the Grenfell Tower enquiry was told "This reveals an industry in which Arconic, Celotex and Kingspan were content to push hazardous products into the marketplace and sought to market them dishonestly. These products should have been safe, they should have been tested and certified rigorously, and they should have been marketed in an honest and transparent fashion. None of that happened. The testing and certifying bodies, such as the BRE and the BBA, were quite happy to go along with this process."
(Source: Click on the linked text above, opens in a new tab).

ABI exposes the 'utter inadequacy' of the laboratory tests currently used to check and certify the fire safety of building materials. 25th April 2018.

On 26 February 2021, this article reported on a technician involved with fire testing the products used on the Grenfell Tower.
  1. He failed to notice in 2014 that a test rig for an insulation product from Celotex had been secretly altered to increase its chances of passing.

  2. He claimed that manufacturers could sneak extra components onto test rigs without inspectors knowing

  3. The reliance was very much on the honesty of the client.

  4. If you have got somebody who is going out of their way to deceive, then there was a possibility they could do that, if that was their intention.

During the week to 17 March 2021, the Grenfell Tower enquiry heard published in this and in other articles important reports concluding that a product was unsafe were withheld from BBA by the manufacturer, and BBA granted and renewed certificates even if the manufacturer failed to provide specific information requested.

* In 2013, the UK's Concrete Society published a report by a working group of 18 members, experts in their fields, that states: " water/cement ratio .... primary measure of water penetration and hence the durability of the concrete."

The authors of this report had the bright idea of plotting all the BBA data on to one chart. Here is the same data I plotted on to a chart. It includes my educated guesses where data is missing, see the legend to see which.

chart   Concrete needs to be the right consistency to be pumped into formwork and compacted properly without bleeding. 120mm to 140mm slump would be fairly usual.

So any BBA testing on much drier, stiffer concrete has not tested concrete that would be used on site. Site concrete would have to have more water, which would massively affect test results.

Seemingly, a very similar situation to Grenfell Tower cladding. The cladding resisted a flame in a lab but they (BBA) stated that they didn't know if it would work on a tower, on the certificate.
ABI exposes the 'utter inadequacy' of the laboratory tests currently used to check and certify the fire safety of building materials.

That is what the cladding BBA certificate states and every admixture certificate also warns that BBA didn't really find out if any of the admixtures would work on site.

Slumps of tested concrete
Pudlo 45mm
Sika 40mm
Xypex 35mm
Kryton KIM 45mm
Caltite no information
BASF 135mm. Hooray. Site concrete. But BASF performed very badly.
Only BASF has a certificate for concrete with the consistency used on site.

All these admixtures have to include a water-reducing plasticiser.

Apart from plasticiser what seems to be in them?

With a little more cement or cement replacement:
Krystol KIM

With liquid plastic absolutely useless below ground where it won't dry
Others with polymer.

There is no evidence anywhere that anything other than extra cement and less water can waterproof concrete used beneath ground where it won't dry. The concrete will need a particularly powerful plasticiser as well to make it workable.

If you specify any of the famous brands, you risk litigation after Grenfell enquiries rule that not reading and fully understanding a BBA certificate is negligent.

None of the BBA certificates provide any evidence that any of these products make enough difference to site concrete to be worth any money at all.

The proper procedure is to specify concrete made to BS EN 206-1, and concrete tested to BS EN 12390:8 permeability of hardened concrete.


The floor over a basement is completely different to the ground floor of a normal house

because water getting in under the ground floor of a normal house settles harmlessly on the earth beneath.

But in a basement that same water is a leak.

  1. The ground floor over a basement must prevent a horizontal ingress of water beneath it.

  2. The retaining wall must prevent a horizontal ingress of water over it.
Please do not picture in your mind sticky-back membrane as the solution. Sticky-back membranes are hopelessly unreliable and won't last the lifetime of the building.

Sticky-back membranes stick fabulously well on the south side with the sun shining.

They will not stick to a primed surface or to a neighbouring sheet on the north side where the atmosphere will be moist and these products do not work. Neither will the membrane stick to the neighbouring sheet up the sides of the slab where the tails have been sitting weeks in water.

After they are fitted the bricklayers or the scaffolders will damage a sticky-back membrane and just push it back into place without a repair.

Basements with a flat top to the retaining wall stop just beneath outside ground level, water gets in horizontally beneath the ground floor and over the top of the flat-top retaining wall running down the inside of the basement wall. It cannot be stopped except by internal drainage and only if that is lapped up and over the ceiling. It would be a shame if your client spent thousands of pounds on internal drainage and the only water it removed got in only because of your uninformed choice of floor.

Therefore, if you specify a beam and block ground floor, precast planks or corrugated steel sheet with concrete on top, either the client will have to pay for a complex way to support his floor, losing real estate or it will leak.

I was the first specialist to promote an upstand which I now see has become common practice.

Here you see 4 options.
  1. A continuous, air-tight structure of reinforced concrete. Continuous insulation envelope outside the structure. Maximum thermal mass. No cold bridging. Exceeds Passivhaus Standard. (See my page about bare concrete walls inside here).

    The basement has to be a little smaller and an extra footing for face brickwork.

  2. Beam and block, precast planks or corrugated sheets against an upstand.

    Makes the basement a little smaller and the corbel is expensive.

  3. Engineered timber joists.

    Really cheap and simple.

  4. Cast insitu reinforced concrete podium deck.

    This might be chosen for an outside patio, a garage over a basement, beneath underfloor heating or if you are really trying to maximise thermal mass.
seasonal heating cooling envelope

beam and block sketch        insitu podium deck

Another way, that also reduces the size of a basement, is to build blockwork inside the basement to support beam and block independently of the retaining wall.

In this photo, the right is outside and the left will be inside. I helped with all the waterproof concrete, the formwork, the pouring and the external land drainage. The overall wall width is about 600mm. The insulation had to be put in before the floor over could be installed, it already looks sodden.

waterproof basement blockwork beam and block

In my experience a waterproof concrete upstand is a most essential waterproofing measure.

It is very much easier to accommodate wall construction above a basement with a waterproof concrete upstand if engineered timber joists are used instead of beam and block or precast slabs.

outside brick ledge
waterproof basement construction upstand
  inside wall plate
waterproof basement construction floor ledge
  top chord supported
waterproof basement construction floor ledge
  floor joists in hangers
floor joists on hangers

An engineered timber floor joist can be any width and any height and stronger than concrete (for the same overall floor construction depth including the service void beneath concrete). It can be a trus joist (OSB web), an easi joist (lattice steel sided) or plywood sided. Services can go through it.

a top chord supported floor joist      Nu Heat underfloor heating for timber floors. underfloor heating over basement

Clearly, engineered timber joists offer more choice than concrete and they are flat on top. With services through and not under, and without the need for a screed to overcome curvature, timber floor construction depth is usually less than concrete.

Waterproof concrete gets hotter while curing than ordinary structural concrete so the engineer will be asked to have enough extra steel to control the extra issue of thermal cracking. Therefore there will be enough steel in the walls for their not to benefit from propping by a concrete floor over the top.

Please choose an engineered timber joist floor, supported off an upstand created out of waterproof concrete. The basement will cost far less this way. Indeed this way is often the only way that suits a tight budget.

Unless the ceiling over the basement has to be concrete, for instance it is a garage, outside or, sometimes, a kitchen. In which case concrete cast insitu is flat and absolutely waterproof all round.

Our rods and nuts make forming an insitu concrete podium deck very easy indeed. The top row of rods for the wall are left to use again for an edge shutter. The upstand will need to be formed on the podium deck afterwards.

frp threaded rods reused

Note added March 2018.

I have recently been investigating Near Zero Energy Buildings and am persuaded that a 'Fabric First' approach is best.

On this page I discuss how you might build with maximum thermal mass including a basement.


Door and Window Openings.

In these photos you see that the window opening is formed in a piece of wall the thickness of a concrete block and side steps are 300mm away from the opening to allow the brickwork to come down and look a seamless part of the brickwork for the whole house.

These are before, during and after pictures of the same window.

form window opening in icf  form window opening in icf

form window opening in icf

form window opening in icf  form window opening in icf

form window opening in icf

The ledge beneath the opening is 3 brickwork courses down so the upstand in waterproof concrete creates a cavity tray and some height in case the window well should flood temporarily.

The window frame could be fixed to the concrete or the brickwork in front, as detailed by the architect.


Pumping foul waste from a basement bathroom, toilet or laundry.

The simple choice is between Saniflo-type macerators with built-in pumps or a sump beneath the basement floor containing a pump and perhaps a back-up pump as well.

  1. Either will block and jam if a wetwipe or sanitary towel is put down the loo. But clearing the Saniflo is a lot less bad than clearing a big pump inside a sump.

  2. The macerator sits beside the shower, loo or washing machine. The big pump needs a massive construction far deeper than the basement excavation.
The image below is a client's washing machine in the basement I built for him a few years ago. Beside it is a macerator that pumps his waste water up to his mains drainage.

The photos in the right column are of a sump beneath the basement floor.

basement macerator

If this macerator was dealing with toilet waste it would chew up the poo and paper so it all got pumped up with the water.

But if a guest visits with their own pack of wet-wipes, the plastic in the wet-wipe would block the pump and someone has the unpleasant job of taking the pump apart to get the plastic out. They would then 'educate' the guest to put wet-wipes in a bag and put the bag in a bin.

In comparison, some clients opt for a substantial sump under the basement floor and a much heftier pump to get the waste up to their drainage above.

The diagram is from an American web site.

The second photo is of a sump a client damaged when they tried to put it down the hole in the basement floor, because at that extra depth they found the water table and they couldn't pump the water out fast enough and the fragile fibreglass tank got broken before they could get concrete round it. So it came out and the concrete got sent away.

All of the drainage needs to be in the ground before the basement slab can be cast over it.

Be in no doubt, drainage beneath your basement floor is very expensive and it might be impossible to get right if you find water.

And the wet-wipe?

If the same guest uses wet-wipes they may visit the loo several times before the first wet-wipe blocks the pump.

The pump has to be hauled up out of the sump into the basement and taken apart to remove the wet-wipe.

Whoever does this will fish around for any more wet-wipes in the sump. But they may not get them all, and a short while later the pump could block again and have to be hauled up, taken apart and the wet-wipe cleared. Again.

Unfortunately the guest will have left. Too late for 'education'.

And you will never know if there is still one more wet-wipe waiting to block the pump and put your basement loo out of service again until it is cleared.
basement foul waste

basement foul waste pump sump

basement foul waste pipes

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