self build waterproof basement formwork membrane concrete
Structural warranties, from NHBC, Premier, LABC, BuildZone etc., do not include below ground waterproofing in their cover. Some inspectors seem willing to allow you to build a basement that water floods in. Though they usually want to see that any water can be pumped away.   self build basement

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.

These 3 requirements are completely distinct and different from each other.

  1.   The Occupier wants good workmanship*.

  2.   The Developer just wants the Latent Defects Warranty his buyer's solicitor will look for.

  3.   The Latent Defect Warranty provider does not include below ground waterproofing in his cover.
    To keep this secret some will probably insist on a guaranteed insurance-backed internal drainage system.
    Therefore, if the basement leaks, there won't be a claim, the water will be pumped away
    (until the pump breaks down years later, but that's the occupier's problem).

Due to past court cases your design team, including your architect, is compelled by law to assume that your waterproofing work will be carried out badly and will leak. So they feel they have to specify a remedy that is usually very, very expensive - internal drainage, a sump, a pump, a backup pump and a backup power supply.

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

  2. Use our concrete additive and get proof all the concrete you used in your basement is completely impermeable.

  3. Do all the concrete work properly including using our 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.

Complete peace of mind for the occupier needs only the first - good workmanship* 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. So he may have no choice but to include internal drainage. £40,000 sometimes, depending on who he chose for the warranty.

No one needs all 3.

But neither do 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 client persuaded NHBC to drop their insistence for internal drainage as well, 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
This basement was built so badly and 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 many times.

  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.

  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 has any visible leak.

None of the basements we built in the last 7 years has any 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.

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.
  1. NHBC inspectors are 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.

    The 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 and you have no 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. Trapped with no space to spare between a wall and the backfill, it swells to form a continuous layer of impermeable clay.
    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. Especially after the concrete admixture supplier with a BBA certificate demanded he buy strip for 19 joints because of the pour sequence.

    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.

    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.

    I sell the best plasticiser for the job on its own and I took many test cubes all proving that the concrete used on site was impermeable.

    On other pages I explain that you get the concrete delivered with only 60mm slump, so you know it isn't wet. You mix in the plasiciser at site increasing the slump to what is easily pumped.

    If you wish, you get one set of cubes taken and 3 of them sent to be tested to prove the concrete you used in your basement is impermeable.

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."

If I was building my own basement and I needed a warranty, I would probably choose BuildZone or LABC. 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

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. 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.

  3. or

  4. 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.

  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.
This 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.

This may be why designers seem to go over the top at the drawing board stage,

it also explains how our clients avoided internal drainage once our workmanship was proven to be sound.

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.

We were 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 our competitors give you 2 and 3. We believe we are unique giving you 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 concrete already waterproof.

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

  6. You can take samples of concrete you 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 in 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.)

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. Our founder 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 should easily get insurance for the next five years.