self build waterproof basement formwork membrane concrete

This page is about basement warranties and guarantees in general, and how they disappoint and cause immense problems sometimes.

The best UK basement guarantee is from me, and it is free if you buy my products and services. Read about it on this other page.

  self build basement

Structural warranties, from NHBC, Premier, LABC, BuildZone etc., do not include below ground waterproofing in their cover. Some inspectors seem willing to allow water flooding into a basement, content that it will always be pumped away. But the more work a pump and its plumbing have to do the more frequently they will break down, and no one insures against that.

A guarantee means different things to different people.

  1.   Occupier: No leaks.

  2.   Developer: Property easily sold on. (Any leaks hidden).

  3.   Latent Defect Warranty provider. Below ground waterproofing excluded from cover. NHBC, in particular, will insist you have someone else's insurance.

These 3 stakeholders to a new basement are completely distinct and different from each other.

  1.   The Occupier wants good workmanship.

  2.   The Developer probably does not want to pay for the best workmanship; he probably just wants the Latent Defects Warranty his buyer's solicitor will look for.

  3.   The Latent Defect Warranty provider wants the basement waterproofing insured but not by him. Most want the waterproofing to be insured by someone else. The trouble is then: who is this someone else? What is the extent of their cover? Just their products and not the installation?

    The internal drainage supplier loves it if the basement leaks because he is the saviour who will pump the water away
    (until the pump breaks down years later, but that's the occupier's problem).

Construction suffers from a culture that tries to pass responsibility down the supply chain from your designer to a supplier then on and on till someone is trying to do the actual work as cheaply as they can.

Dame Judith Hackitt reported to Government following the Grenfell Tower fire. Hackitt's interim report found that under current building regulations it is too difficult to identify an accountable person as responsibility is often cascaded down the supply chain. She called for a cultural change within the industry.

It is usually believed (I think completely wrongly) that nothing can be done to fix the leaks in a newly built basement. Specifiers seem to do nothing to enforce better construction. Instead they hand all responsibility to an internal drainage supplier to issue a guarantee that all water ingress can be pumped away.

Internal Drainage suppliers are the employers of the people trained to design internal drainage. These CSSW surveyors rely, again I believe quite wrongly, on a court case that they think gives them free rein to specify everything their employer has for sale.

These same internal drainage suppliers are also largely responsible for writing the British Standard for basement waterproofing: BS 8102.

Due to a misinterpretation of the Outwing court case the CSSW starts with the assumption that the basement yet to be built will leak very badly indeed.

Firstly, the CSSW assumes no one will inspect the building work or insist that any leaks will be fixed.

Second and worse the CSSW's employer will sub contract out the installation, moving responsibility further down the line nearer to what Dame Judith Hackitt called 'a race to the bottom'.

Your specifier may believe very strongly that he has chosen a supplier with a good reputation and a good guarantee. But your specifier has no contract or further dealings with the supplier. He is already out of the loop before work starts.

Dame Judith Hackitt said that any future regulatory system should be 'outcome-based'; and that there should be more proper inspections.

What she meant is that, if there had been someone with authority to insist shoddy work was done again properly, the fire may never have got inside Grenfell Tower. If your specifier jumps straight to internal drainage at the design stage he is making exactly the same mistake allowing those who build your basement to do a shocking bad job.

CSSWs specify a remedy that is usually very, very expensive - internal drainage, a sump, a pump, a backup pump and a backup power supply - as if, by some magic, their internal drainage alone will be installed faultlessly.
I am asserting that specifiers tell you to buy completely unsuitable and unnecessary products without waiting to see if they are the correct choice when the time comes to install them. Dame Judith Hackitt has called for a change in culture whereby someone influences the activity on site instead. Currently, as well as over-engineering, specifying solutions to problems that have not yet occurred also results in those solutions being over-whelmingly unsuitable in the eyes of those fitting them, but they fit them anyway because they are specified. People told her things like "we always knew something like this would happen".

Part of her solution is for important decisions about unknown situations to be made at the appropriate time, not years earlier on the drawing board.

You will read here that the way to save money is
  1. Go along with internal drainage but don't allow them to tie you down to a brand or a final design.

  2. Use my concrete additive and site supervision plus get proof all the concrete you used in your basement is completely impermeable.

  3. Do all the concrete work properly, I will show you how, including using my fibreglass threaded rods so you leave no holes.

  4. Later, when the roof is on and the windows are in show them that your workmanship was very good.

    This allows your design team to stop assuming the work will be done badly and they can re-assess what you need as a second waterproofing defence.

    Usually just waterproof paint or polythene sheet.
But you have to prove your completed basement is dry due to your good workmanship before they will be able to drop their demand for an internal drainage system.

How did the industry get where it is today?

One major piece of the jigsaw is the British Standard. BS 8102:2009 Code of practice for protection of below ground structures against water from the ground.

It was written largely by the suppliers of internal drainage and to buy it costs £224 from here.

But you don't need to buy it because the Property Care Association has made its summary public.

From its website:
"The Property Care Association (PCA®) is the trade association representing specialists across the UK who can be trusted to resolve problems affecting buildings."

The British Standard seems to have been written by companies in the business of resolving problems. Not by companies in the business of building basements properly.

Section 4 of the summary, 2nd paragraph, states that a waterproofing specialist needs to be part of the design team.

Near the end of section 4 it states: It is acknowledged that defects can occur in waterproofing systems as a result of workmanship errors.

Section 5 is important, but jump to section 6.

The code clearly states that:-

Waterproofing measures should be designed on the basis of water to the full height of the retained ground at some time during the structure's life where:
  1. no detailed geological or hydrogeological assessment has been undertaken;

  2. the results of the soil investigations are inconclusive with respect to groundwater;

  3. the ground drainage characteristics are unreliable;

  4. the drainage measures (either internal or external) are unreliable or un-maintainable and infiltration cannot be controlled.
What they are saying is, unless you can prove otherwise
  1. d is brilliant. They have written the Standard to say that if you do not have what we sell then you must assume that water outside your basement will at some point surround your basement to the top.

    Surely, there cannot be any correlation?

  2. You cannot have external drainage unless you know it will always work, and I agree. If you have external waterproofing it has to be ignored. (I entirely agree that any external waterproofing will fail). You must have 2 different defences so therefore, they are trying to say, you must have our internal drainage.

  3. Waterproofing specialists are called CSSW surveyors. They work for the suppliers. You are supposed to have this designer, paid and loyal to his employer, not you, in your design team, and he will insist on the £40,000 type internal drainage his company sells and installs.
However we have a lifeline: "and infiltration cannot be controlled".
To avoid the £40,000 type internal drainage you have to prove that infiltration IS CONTROLLED TO THE EXTENT THAT THERE IS NOT ANY.

What I try to help you with on this web site is building your structure so it does not leak.

Yes, you still need an internal defence to satisfy the Standard (unless you have an external defence as well as the concrete), but it need not be internal drainage and it could cost you less than £100 if you can prove there is no water to deal with.

Lower down this page I explain another piece of this jigsaw, the Outwing court case. I am sure that sales people use their interpretation of it to scare architects. But, again, if you do your work well and don't have any leaks, the judge's summing up can be interpreted in an entirely different way that will not cost you money.

  How did I arrive at a figure of £40,000 for what a CSSW surveyor would want you to spend?

I got sent this quotation by someone whose questions I was answering.

Initially, I was a bit put out since I speak so strongly against such things. But it turns out his architect asked for the quote, not the client. Therefore I don't mind sharing it here. It is fairly typical in that they seem to have included everything they can.

It is 29 pages long. The devil, they say, is in the detail.

No account has been taken of what concrete the basement will be built of. They have assumed the worst, which I would say is blocks.

The proposed basement is 11m x 10m. Average. It will be 3m deep.

In the fifth paragraph it is clear the recommendation in the British Standard is for an existing cellar. Yet they are happy to throw everything at a basement that is yet to be built that can obviously be built out of better materials than they built coal cellars with in the 19th century.

They make me angry. I think they are crooks.

Click on the image of the document to open the whole document in .pdf form.

Complete peace of mind for the occupier needs only good workmanship. Something that will work installed correctly so that he has no leaks.

He might be happier with two defences not one and he will want them both installed / fitted / incorporated properly.

The Developer needs the Latent Defects Warranty to be able to sell the new home before he has finished building it. That means he wants the property sold before he can inspect for leaks. So he may have no choice but to include internal drainage. £40,000 sometimes, depending on who he chooses for the warranty.

Neither the developer or the insurer want what the occupier wants.

The occupier wants good workmanship so that there are no leaks. But the court case Outwing v Weatherald seems to make good workmanship unnecessary in law. There is a text box about it lower down.

The warranty provider wants to avoid any future claim.

The NHBC position seems to be, get a CSSW surveyor involved during the design process to specify an insurance-backed internal drainage system that will cope with any amount of water - regardless of the potential for water which in some areas is only minor.

Other warranty providers, building control and architects would like to see internal drainage mentioned on the drawings but would not usually need to see a brand or a design. Just the expectation that leaks will be dealt with.

They can mostly be invited back before the house is finished and persuaded to do away with internal drainage in favour of waterproof paint if they can see with their own eyes a completely dry basement shortly after a period of heavy rain.

Actually, one self-build client persuaded NHBC to drop their insistence for internal drainage, but not until after 6 annual inspections never finding any water ingress.

Could an internal drainage pump fail after a few years?
Go to this page and watch the video on Youtube
Someone built this basement (in Hammersmith) so badly and it leaked so much that the pump switched on and off about a million times over 8 years. Many pumps and pipes failed and the basement flooded regularly.

  1. With my help you can have a basement that does not visibly leak, at all, probably completely waterproof.

  2. If you do get a leak concrete is easy to repair with waterproof fast-setting repair mixture. I will do it for you if you bought my products and services and your team did as they were told.

  3. I will show you elsewhere on this web site how to fit a cheap sump for a pump to enable a fitter who gives you a warranty to install his equipment easily, later. Or you can fit your own. Or you can fill it in.

You probably only have to have a warranty if you borrow to build or might sell within 10 years. See below for some ways round the problem.

But if you never want a pump to fail, either because of a break down or a power cut, you need the basement built never to leak.

No basement built my way since 2013 has any visible leak.

Most were for clients to live in themselves so most don't have internal drainage. They are dry from our workmanship alone.

We built this basement during 2015 and the clients moved in November 2016.

self build basement house

The basement has no insulation, no heating and no internal drainage. So far it has been a constant 10°C. And it is dry.

self build basement house


It is a fact that NHBC do not include beneath ground waterproofing in their warranty. None of the warranty providers do.

That doesn't stop NHBC having some very strong rules. It doesn't stop NHBC suggesting that they pay out millions on basement leak claims as well, but I don't think it is true at all.

I repeatedly hear prospective clients telling me the same usual sad tales about NHBC insisting on huge extra costs for methods I explain elsewhere on my web pages will fail, and sometimes I made notes as they spoke.

Not only would NHBC not give one his warranty if he didn't obey, he was told they wouldn't give him building regulations approval either - which is surely outrageous. If your design meets all building regulations how can you be blackmailed into using unnecessary or unsuitable products on the NHBC approved list? Are NHBC on a commission?

That caller says he tried to check with head office about this approved list but they wouldn't confirm or deny what his inspector had told him.

As his basement structure neared completion, he told me that NHBC sent one of their most experienced basement people who said to him something very like "I don't really know what I'm looking at. You will know more than me."
  1. As far as I know, NHBC inspectors are still insisting that everything you use must have a BBA certificate if a product with one is available.

    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.

    The Grenfell Tower cladding has a BBA certificate and it was used despite BBA stating on the certificate they had not tested for fire in the situation it would be used.

    Many concrete waterproofing admixtures have a BBA certificate and none of them state or claim the product waterproofs - and the figures on the certificates suggest they don't.

    All but one product was tested in concrete too stiff to use on site, not tested in the concrete you would buy.

    The Feb 2021 article suggests that each supplier was able to cheat to get a certificate to allow their product to be used in a basement from completely inappropriate and dishonest testing.

    I could go on, stcky-back membranes, joint strips that swell, clay carpets. All BBA certified. None work reliably.

    Any sensible person would be pleased to be told to use products that will work for him. But sensible people using NHBC seem trapped between an oligarchy, a clique of second-rate people, the NHBC and the BBA, seemingly sitting on each other's boards and getting rich marking each other's homework, working together to make sensible people throw vast amounts of money away on rubbish. Why?

    The other trouble with insisting on a certificate is that the product supplier might think he has you over a barrel if his products are specified. He can sell you extras you don't need. Neither do you have any leverage to get a discount.

  2. NHBC inspectors have insisted on sticky backed membranes and bentonite clay carpets as Type A protection (where BBA certified Type B concreting has not been used).

    Sticky back membranes have probably never been installed 100% successfully. The main issues are:
    1. Even if your slab blinding is as smooth and dust free as a snooker table and the membrane won't be punctured by debris underneath, your steel fixers are likely to puncture it with steel reinforcing bars.
    2. Where membrane is left loose round the slab to fold up the sides later, it gets damaged when the slab formwork is removed. Even if tears are repaired, strips cannot be cleaned and dried before they are joined right beside the bottom of the slab. So they will always leak round the bottom of a basement slab.
    A video I made last time I built an NHBC warranted basement.
    3. The strips will not stick, either, against the walls on the North side that never gets the sun if the excavation is damp. Even with primer, sticky back membrane does not stick if the atmosphere is damp.
    4. Any external membrane is easily damaged during backfilling.

    Bentonite clay carpets are designed to swell when any water tries to get from the earth to the basement walls. They swell to form a continuous layer of impermeable clay in the restricted space between the wall and the backfill.
    1. But it expands very readily on contact with water. In the UK this is at the first opportunity after the packaging is removed and it will be too heavy, as well as already useless, to fit properly if it has already soaked up water and expanded.
    2. It will soak up any water while in the formwork waiting for concrete and the wet clay powder will tear the stitching and sink as one lump to the bottom, preventing any concrete against the steel reinforcement. As well as failing to waterproof, these carpets have the capacity to make the structure fail.
    3. The carpet will soak up any water and the clay sink as one lump in the bottom after the formwork is removed and while the wall concrete gains enough strength before it can be backfilled.

  3. NHBC always insist the builder follows the advice of a CSSW surveyor.

    As far as I know, that means he will have to have internal drainage installed capable of dealing with a substantial flow of water, whether or not he has any leaks.

    For many, this means having to have £40,000 or whatever of internal drainage to satisfy NHBC.
    1. Including a channel around the internal edge of the floor slab. To avoid weakening the structure by reducing the cover to steel reinforcement this means excavating deeper to cast a deeper slab.
    2. A flood test. This means NHBC want to come and watch you throw buckets of water over the floor slab and they expect to see all that water flow to the edge channel and on to the sump - even if the basement does not leak.

    It is quite easy to get concrete smooth and level if you let the concrete gang add loads of water to the wet concrete. But that will weaken the concrete and make even waterproof concrete porous. It is very difficult to get a waterproof concrete mix smooth and level because it is stiff to move and it sets quickly.

    Therefore, it would be ridiculously difficult to use waterproof concrete if you use NHBC.

    But if you don't use waterproof concrete, NHBC will make you use an external protection that is very unlikely to work either.

    That, ironically, makes the extremely expensive internal drainage essential, because NHBC make you have two defences (fair enough) but their other rules prevent any defence ahead of the internal drainage working.
If you use a waterproof concrete Type B protection:
  1. NHBC inspectors insist on kickers at the bottom of basement walls.
    kickers leak. Follow this link.

  2. NHBC inspectors insist on a waterproofing strip along all joints.

    One caller was worried about the cost. (Nearly £1,000). Especially after the concrete admixture supplier with a BBA certificate demanded he buy strip for 19 joints because of the pour sequence they wanted.

    Strips and tapes etc. do not work unless they are installed properly and protected until concreted over and only if the concreting is performed properly. He was told (by BASF) the strip would not swell too early - but it did. As soon as it got wet it swelled and became useless.

    If the concreting is performed properly and the joint formed to BS 8007, it won't need a tape or strip.

  3. You will have to use a concrete admixture with a BBA certificate
    despite none of these certificates proving that they either make any beneficial difference or that the concrete you have to use on site has faced any testing or evidence it works.
    Kryton KIM

    My personal experience is that far too often the batching plant is left alone to make concrete how they usually do. They make concrete with the same water as usual and put the sachets that contain a water-reducing agent in as well.

    That means not making any effort to measure what should be a restricted amount of water.

    I have had many calls about concrete with some of these brands in it cracking across the surface the following day. Cracks after only 24 hours sound to me like too much water (which could not be waterproof). 24 hours is too soon for cooling cracks.

Does all this pedantry mean that if you claim against NHBC you can feel certain that they would organise good tradespeople and good materials?

Not according to one person who wrote to me

"I am desperate for some advice , as no one seems to be able to explain the cause and this has seriously impacted our lives over the last five years. As you can imagine we have had many NHBC technical people over the five years and NHBC have been slow to address the issues.

It was found after the first NHBC subcontractor assigned to job whose standard of work was negligent, and a second NHBC contractor was assigned the claim that the original build had cavity trays installed back the front, and the lead work was not rolled to seal the joints just laid flat but with mastic as a seal."

This link is to a newspaper article that ends with an NHBC spokesperson. NHBC do insure basement structures, I just don't think they insure basement waterproofing. Never the less, this article seems to say NHBC pay out against many leaky basements. So their "inspections" aren't very good, are they.

NHBC quoted in Evening Standard.

I explain the What and How of Waterproof Concrete here.

I sell the best plasticiser for the job. All you need to know here. I took many test cubes all proving that the concrete used on site was impermeable.

You can buy my admixture, pay me to put it in and supervise your concreting and I can guarantee all your work and fix any leaks for you free of charge.

If you wish, I will take one set of 5 cubes for you. 3 of them will be be tested to prove the concrete you used in your basement is impermeable. The other 2 to prove the strength of your concrete.

If I was building my own basement and I needed a warranty, I would probably choose BuildZone. They didn't pretend basement waterproofing is included in their cover.

If I needed building control approval I would without any doubt use the local authority. I find their inspectors far more useful. Other inspectors, especially those doing building control and your warranty at the same time either don't visit very often or when they visit they seem to look only for reasons they can wriggle out of a claim later.

Good workmanship and no leaks.

Workmanship starts with suitable and intelligent design, includes choice of materials and of course includes the work itself.

The industry seems to be fixated with a court case that set a precedent. But what precedent, exactly, did it intend 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 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.
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.

If you ask 101 people for advice about waterproofing a basement you will get 101 different answers.

The reason for that is they usually expect all the workmanship to be very poor.

The best advice will be to BS 8102: Code of practice for the protection of below ground structures against water from the ground.

BS 8102 states two defences and:

"the water resistance of the structure should be improved prior to the installation of the Type C protection".

One of your defences needs to work and concrete is the only defence no one else can damage later.

Bare concrete is also very easy to repair if you discover a leak before your wiring and plasterboarding.

That's why we believe it is the one defence you do as well as possible.
Not just to the standard allowed by the court case.

I was once asked to see a basement in Hammersmith that leaked everywhere.

Over 8 years the various pumps failed many times and the basement kept flooding.

Most basement labour, who see on the drawings that internal drainage will hide and cope with all the leaks, don't care.

When we visited, the screed and floor drainage were still in place, still hiding the worst leaks.

This photo was taken 8 years after the basement was complete and the internal drainage taken off the wall. It leaked all that time.

In our experience, many main contractors employ the cheapest to build the basement for them; and they charge the client double in case they have to rebuild it properly.
leaking basement

The approach I recommend is
  1. You do your best so that your basement will never leak, not even a drop, from the concreting alone.

  2. You have internal drainage as well for peace of mind, if you wish, or more often because your lender or your future buyer want it.

  3. or

  4. You just have a vapour barrier inside, polythene or waterproof paint.
If you have internal drainage, you need to liaise with the supplier before you build your basement, even though they cannot install anything until after your roof is on, your windows are in and your basement cleaned of everything that washed down into it while the house above was built. This is because you might need to put their sump in the floor and their membrane under internal walls before they are built.

Most of my competitors give you 2 and 3. I believe I am unique helping you achieve 1, a basement that will never leak.

Hopefully you can see the difference.

Waterproof Concrete.

I have another website at where I go into waterproof concrete in great detail.

This text box is a few highlights from it.

Some truths about waterproof concrete:
  1. There isn't an additive anywhere that will always turn non-waterproof concrete into waterproof concrete where that concrete is used beneath ground in Britain.

  2. Concrete is waterproof when it has sufficient extra cement and the water to cement ratio is sufficiently reduced.

  3. None of the BBA certificates for any proprietary product, Caltite, Pudlo, Xypex, BASF, Sika and so on, provide any evidence they make much difference. Indeed most weren't even tested on the concrete you would buy.

    This makes them very similar to the cladding used on the Grenfell Tower in that, had the people approving the tower cladding read and fully understood the BBA certificate, they would have known the certificate did not state that the cladding was safe to just use.

    Any architect or engineer who fully understands the BBA certificate for what he supposes is a concrete waterproofer will realise none are concrete waterproofers (below ground where they will never fully dry) so he shouldn't insist you use one.

    If he does insist, I would tell him that after the work is complete you want to sue him for the full cost on the basis that he made you waste your money.

  4. Every admixture with a BBA certificate is added to an already potentially waterproof concrete mix.

  5. All you need to make waterproof concrete is the same mix and a sufficiently powerful water-reducing plasticiser, that I supply cheaply, so that your concrete has less water.

  6. I can take samples of concrete we used in your project and get proof that the concrete is impermeable, which should satisfy everyone including your architect and building control.

BS 8102 says that if you use an internal drainage membrane you need a prior defence to reduce the amount of water the drainage will deal with.

How much would different concretes reduce the amount of water that would get through?

  • Concrete blocks would barely reduce the amount of water that got through compared to nothing at all because they are deliberately not dense.

  • A plain concrete would let less water through than concrete blocks because it would be a bit denser. But it would still let a lot of water through because it would be very porous.

  • Structural concrete should be dense enough not to let water pour through but it would be full of capillary pores. As soon as any water dried off from inside the concrete into the basement it would be replaced by water from the soil outside. The basement would always feel damp and smell musty.

  • Waterproof concrete has sufficiently reduced water so that the cement grains start off closer together and no capillary pores remain once the concrete has cured.

    The basement properly built with waterproof concrete will be dry.

The following mix of concrete from your readymix supplier will be completely impermeable, and can be proven to be completely impermeable if tested to BS EN 12390 part 8.

P350 (This means 350kgs of cement per m³.)

CEM 1 (This means pure OPC cement, though OPC blended with slag can be just as good. But there is a theoretical doubt about flyash that might always have a capillary system throughout.)

Target slump 60mm (This is to get the water reduced. Specifying an amount of water sadly doesn't work because readymix producers cannot, or will not, batch concrete with a known amount of water. Through experience I have found asking for 60mm slump creates far fewer problems.)

A very powerful plasticiser to increase the slump of this mix to over 130mm. (Such as my powerful plasticiser.)

No other chemicals. (Mixing plasticisers, and using too much of any poorer plasticiser, can cause unwelcome results.)

You can view one of the many test certificates I have here.

If you need a mortgage or you plan to sell within 10 years you probably need the insurance backed warranty only available with internal drainage.

If you need to borrow and getting a mortgage for a basement is proving difficult,

Clients in the past have
  1. Funded the basement construction from their savings and only drawn down funds after the basement structure is complete, satisfying their lender that his money is tied up only above ground in the part of the new dwelling that has a full warranty.

  2. I once paid QANW almost £8,000 for a policy later assigned to NHBC, so that although NHBC insured the basement without internal drainage, they were fully insured themselves.

  3. When you wish to sell, if it is within 10 years, buy a policy for the remaining time only. Warranties are only for latent defects, the completely unexpected. So if after 5 years you have no problems you might get insurance for the next five years.

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