self build waterproof basement formwork membrane concrete architect

Basement Construction. Our page for architects' FAQs.

My name is Phillip Sacre. I have been building and helped build basements for years. I sell my expertise and some products I sourced. They are better than the famous brands. I sell them mainly to self-builders.

You might be here because your client has asked you to read this page so you understand where he is coming from. If he has read this web site he will be trying to influence your design.

If you could read the banner at the top of the page, you will get the gist; though there is more assistance here than just waterproofing.
  1. Basement waterproofing. How to avoid litigation and what the client wants. Unfortunately what you want and what the client wants are different.

  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.
  self build basement

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

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's 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 your client does not need you wasting his money on a BBA certified concrete additive, such as Caltite or Sika. Your client just needs to do his work well. And this web site is about teaching him to do his work well.


Basement waterproofing.

How to avoid court and what the client wants are unfortunately different.

The client wants no leaks while the specifier does not want to end up in court.

The difference is to do with workmanship. The specifier needs to be certain his choice will still work even if installed badly. The client wants to pay for one thing that works; not six things that don't.

The industry seems fixated with a court case that set a precedent. Architects are told that the precedent means the client must have internal drainage. But is that the precedent the judge intended to set?

You can find one report about the High Court ruling following the Outwing Construction v Thomas Weatherald (1999) case here.

Outwing Construction. 1999.

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.

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

  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
    AND 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.

The industry seems to have adopted the second way and interpreted it to mean that internal drainage membrane is essential and that there should be a backup pump and a backup power supply, sometimes costing tens of thousands of pounds.

Whereas 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.
The case report is on the web site of a membrane supplier, who seems to choose the lack of internal drainage as the negligent mistake by the designer.

And it seems to suit Warranty Providers, Building Control Officers and Specifiers to assume the same and not look at what would work but cost less.

According to this report, amongst the evidence heard, the judge favoured this:

1. Clause 3.3 of BS 8102, Code of Practice for the Protection of Structures Against Water from the Ground, states that the designer should
i) Consider the consequence of less than adequate workmanship,
ii) Consider the consequence of leaks and
iii) Consider the form and feasibility of remedial work.

These 3 issues to consider do not leave internal drainage as the only option. Once a basement is plasterboarded and lived in no one could get to the membrane any easier than they could concrete walls.

Supervising the concrete, then not covering over the concrete till you have had a chance to search for any leaks after heavy rain, then repairing any leaks with a tub of stuff costing under £100, satisfies the judge's summing up just as well.

This first section explains why specifiers have to produce 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.

There is no doubt, workmanship in the UK can be terrible and corners do get cut. I am training your client to do the work well instead.

These two photos are of a commercial basement near Hammersmith. The basement is in river gravel close to the Thames, so the water table is high and tidal. Note the pile in the corner in the wrong place so that the 'waterproof concrete' specified doesn't cover it.

If there is no other information available, the internal drainage designer will provide a system that can cope with this:
About 50 litres a minute for ever and ever and ever switching the pump on and off every 40 seconds. The problem with this property (see the video here) is that every few months a pump or pipe connection would fail and the basement flooded in about 8 minutes. So it flooded many times over the 7 years and in the end the owner couldn't rent it out any more.

I was asked to visit to quote repairs. What it demonstrates is that if you specify expensive internal drainage but the pump or the pipework fail because they switch on and off too often, the 10 year guarantee you made your client pay a huge amount of money for is void and useless.

The trouble is, no one insures or guarantees the pump or the damage caused if it fails.

But very often I see that architects have specified the most comprehensive and expensive systems available (E.g. Delta or Newton) long before they know how much water will be leaking into the basement.

I believe passionately that the reinforced concrete is the only water-resisting product that cannot get damaged later so it is the one product the client wants to make sure is done properly.

  1. The first extreme.

    The developer with a site close to the Thames, in gravel with a tidal water table just a metre or so down.

    He gets the basement built by the cheapest gang he can (or he employs a main contractor who gets the basement built by the cheapest gang he can - and probably has no intention of paying them in full so they do the job as fast as possible with no regard to quality).

    After all. No one supervises or inspects these days.

basement architect leak

This next photo is a link to a very short video

basement sump pumping
  1. The other extreme.

    The developer whose basement is being built into the side of a hill, or on chalk, or with a very effective land drain.

    This developer, no matter how badly the basement structure is built, is not going to have any leak anyway.

    So why have 3 pumps, backup battery power and a water level alarm?

    In fact, why have internal drainage?

    It is up to the architect to recognise a situation where a basement could not leak, usually from the soil investigation report, and make a sensible judgement.


    Visit the property after the roof is on, the windows are in and the basement has been cleared and dried and assess then just how comprehensive an internal drainage system does his client need?

    Unrestrained internal drainage designers will specify £45,000 of internal drainage and other items if they feel the architect has given them free rein.

    The Grenfell Tower disaster is going to change everything, including clients' belief that they cannot sue their architect for wasting money.
  1. The private client and self builders.

    People who are going to live in the new property have an entirely different requirement.

    They want a basement that will not leak.

    This is what I have been doing for many years. I build basements and guarantee that if there is any visible ingress of water I will fix it. I hardly ever have any leaks to fix.

    How do I manage this amazing feat?

    1. Good old fashioned training.

    2. Good old fashioned supervision.

    3. Some lessons I learned along the way, such as how to pour concrete properly.

The architect needs to decide, before work starts, which sump needs creating in the basement floor slab?

We would recommend the middle sump because the first could be fitted inside if needed or the third could be formed above it later, ignoring it.

wykomol sump pump   basement sump   completed waterproof basement sump

My advice to your client throughout this web site is firstly to understand that at the start you have to put 'internal drainage' on the drawings. Then to build his basement as well as he can; continue till the roof is on, the windows are in and he is weathertight; clean and dry the basement; after some heavy rain call you back in and between you make a sensible decision about what he actually needs. If he has done his work properly he won't need internal drainage. But I have told him, he has to have done his work properly before you can change your specification.


This second section explains the truth about waterproof concrete.

Also, why paying for any big-name brand, such as Caltite or Sika, is a waste of clients' money. After Grenfell clients will look to specifiers to get wasted money back if they learn that the BBA certificate the specifier relied upon
(but did not read) provides the evidence that these products don't do anything of value.

At the moment specifiers and inspectors don't fully understand these certificates because they don't read them in full. But I think they soon will because a Grenfell enquiry will condemn this practice as negligent. At the moment, most specifiers and inspectors just tick a box if a product has BBA. No further questions or research.

It is concrete that can be made waterproof. There is not any admixture that waterproofs un-waterproof concrete used below ground for basement construction.

BBA certification is completely inappropriate, being little more than a licence for the supplier to print money if he invests in some meaningless testing by BBA.

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).

My evidence is from The UK's Concrete Society report 2013* as well as BBA certificates.

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 cladding and insulation for the Grenfell Tower both have BBA certificates. The insulation certificate got taken down. The cladding certificate states: "For resistance to fire, the performance of a wall incorporating the product, can only be determined by tests from a suitably accredited laboratory, and is not covered by this Certificate."

How could the cladding have been allowed? Unless everyone in authority saw it had BBA and ticked their box?

BBA certificates for water-resisting admixtures for concrete are just as bad.
What BS 8102 says about waterproofing a domestic basement.
  1. Two defences, different types from
    1. External
    2. Integral
    3. Internal
  2. The first should substantially reduce the amount of water that gets through that the second defence will have to deal with.
I am telling my customers that they can have a completely waterproof structure from the concrete alone. But the specifier does not need to worry that a completely waterproof structure will be achieved.

The specifier needs only to know that if completely waterproof concrete is the target, then substantially reduced water ingress will be achieved. The specifier does not need to worry about any brand of anything with a BBA certificate because concrete is inherently water resistant anyway.

At the design stage, the specifier simply notes on the drawing
  1. First defence, water-resistant concrete, C35A or better. Ideally all workmanship supervised.
  2. Second defence Internal Drainage System by Approved Supplier.
If the customer thinks that he built the basement completely waterproof from the concrete alone, he can invite the specifier, building control and his insurer back
and if you are all happy that there will never be a drop of water to pump you can agree on just a vapour barrier of waterproof paint or polythene instead.

If, after inspection, you still insist on internal drainage, then if you did not see any water to pump it needn't be the £40,000 from Delta or Newton. I'm sure you could agree something cheaper with your client.

Caltite's does not prove it makes concrete any more water resistant than a structural concrete without it.

Sika's improvement is barely measurable.

Most of the rest might at first appear to have made a small improvement but, actually, the small improvements can be explained by a reduction in water cement ratio* rather than any clever chemical.

All this from analysing the various BBA certificates*.

For years I always used the concrete mix that is proven to always be completely impermeable. Without any proprietary admixture not proven to do anything but cost a lot of money.

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.

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.
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 better 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, so I tend not to pay for this test (£600 plus VAT) any more. But you can specify tests if you wish.

Every BBA certificate for a water-resisting admixture for concrete tells the user to buy an already water-resistant mix of concrete. This is 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 had 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 OPC and water a maximum 50% by weight.

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.

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

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

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.

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.

Basements are usually waterproofed to BS 8102.

Two defences against ingress of water.

One must substantially reduce the amount of water the second has to deal with.

For years now, our concrete and our work was always absolutely waterproof.

  1. We use waterproof concrete.

  2. We cast fibreglass threaded rods in walls instead of leaving holes through plastic tubes.

  3. We pour walls to only half height or so. This means we get concrete down to the bottom in better (perfect) condition and we can poker the concrete right at the bottom properly.

  4. We don't use kickers. Kickers are impossible to form to be waterproof.

  5. All joints are waterproof to BS 8007 (instead of using swellable strips or carpets that swell before the concrete is poured and don't overcome poor workmanship anyway).

  6. I train my own workers. I don't use experienced workers experienced at doing the work badly.

  7. I supervise all our concreting.
I am telling your client that he can do the same, or at least do a lot better than usual if he tries to do his work well rather than just doing all the work the usual, slapdash, corner-cutting way.

If you have external drainage, our concrete won't face any pressure.

If you have internal drainage, the pump should never switch on - which gives great confidence that it won't break down at a bad time.


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

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

  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.

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 the internal drainage and only if that is lapped up and over the ceiling.

Therefore, if you specify a beam and block ground floor, either the client will have to pay for a complex way to waterproof around a beam and block floor or it will leak.

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

Here you see 3 options.
  1. Beam and block against an upstand.

    But this has the effect of making the basement smaller and it costs more to make the corbel.

  2. Engineered timber joists.

    Really cheap and simple.

  3. Cast insitu reinforced concrete podium deck.

    This might be chosen for an outside patio, beneath underfloor heating or if you are really trying to maximise thermal mass.
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 block and beam independently of the retaining wall.

In our 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 pre cast slabs.

waterproof basement construction upstand
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


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.

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