Building a waterproof basement.

Introduction page for architects. Your design must comply with Building Regulation C2. From Part C "Site preparation and resistance to contaminants and moisture".   NHBC guidance is wrong. Premier Guarantee guidance is wrong. LABC Warranty guidance is wrong. They all make you let the water in and pump the water out. For years, from 2009 probably, their advice, that they insist you adopt, has caused architects to contravene Building Regulation C2. Do you have problems with PII for basements ? Now you know why. Sadly, you will find that most building control bodies and most structural warranty providers follow NHBC as the gold standard. The only Approved Document for basement waterproofing is BS8102:2022. It says fix leaks. Don't let water in. You don't have to follow an Approved Document. But if you don't you have to justify and explain why. Letting water in breaches the Building Regulation. Fixing leaks is a necessity. As this image shows, if you let an internal drainage system company in, and their CSSW who spent 4 days in a classroom to become your expert, your waterproofing specialist, they won't wait. They will cover the leaks over long before the structure is weathertight. They won't let you check for leaks. They won't let you fix leaks. They won't let you comply with Building Regulation C2. To the right of the picture above are some quick links for architects. No.8 is my review of BS8102:2022. Above those are two menu buttons. The first answering questions and the second is my manual showing how to build a new basement without a leak. Without ever needing a repair.

This web site is written in harmless plain script. No forms, no cookies. Please note. I do not go underneath anything already there. My guidance is not about going underneath anything already there. The two images below take you to extensive menus. Quick links to sections of this page. new alternative to a big sump in the floor of a basement. Why an architect will soon lose their licence if they rely on advice from a CSSW. Products you are used to but should not use. How you were mislead with the Outwing court case. Quick links to other pages useful to the architect. These open in new tabs. The all-new BS8102:2022 basement waterproofing. Structural issues are dealt with on the page for Engineers. NOTE: It is important that architects understand a little about why the floor over a basement MUST be different to the ground floor of a standard house. How basements should be built waterproof from the reinforced concrete alone. The New Waterproofing Specialist on the Design Team. Basement warranties and insurance backed guarantees. This page includes evidence why BBA certificates cannot and should not be trusted. Typical problems of design and workmanship. basement insulation and thermal mass

Last year, trying to get to the bottom of what is obviously quite a bit of a mess, I wrote to Dame Judith Hackitt hoping that DLUHC would forward my letter to her. I copied in the Secretary of State, Housing Minister and Parliamentary Committee. A few months later I got a very useful reply. Click on the thumbnails to see the original.

1. External drainage that is maintainable and drains by gravity to a soakaway or ditch.


2. The structure itself, which would have to be
   
   
   • continuous from its base to 150mm above outside ground level or dpc,
   
   
   • and of reinforced water resistant concrete in order to be repairable.


3. Waterproof paint applied all over the inside of the basement structure after
   
   
   • the entire structure is weathertight, meaning roof on and windows in
   
   
   • the entire basement pumped empty of rain that collected
   
   
   • the basement cleaned and dried
   
   
   • all before the basement was fitted out with anything, no cables, no pipes, nothing
   
   
   • the basement inspected after a period of heavy rain, leaks fixed, reinspected until signed off. Perhaps by the architect.


4. I have another defence you might like to include as a belt and braces idea. Look for the shallow tray lower down.

1. internal drainage systems,
2. sticky-back membrane,
3. single-coat render outside or any render inside,
4. precast concrete,
5. ICF.

Our internal drainage system.

This is a shallow tray I cast in a structural floor slab, positioned in the plant room to help deal with plumbing spills. This client decided to put drainage membrane over the floor before his basement floor insulation. He did this himself. The 25mm deep tray is in the waterproof, reinforced, structural concrete.

I was given a copy of a quotation by Maclennan Waterproofing dated 31st May 2023. A standard sentence in the introduction is:   and this is from the same Maclennan quotation. "As BS8102:2022 (Protection of below ground structures against water ingress - code of practice) recommends two forms of waterproofing, with the current design we would use an externally applied membrane and cavity drain to comply. But if shuttered RC concrete wall adopted for the walls then we could change the design to Crystalcoat in leiu of the external membrane system. A cavity drain system would be fitted inside the walls and floor." They don't specifically claim that internal drainage is a waterproofing measure, which BS8102:2022 states it isn't. But is that what you understood the first time you read it? External drainage is, subject to suitable conditions, a good waterproofing measure. Sticky-back membrane is a poor waterproofer. The Standard says sticky-back membrane should only be applied to impermeable surfaces. It might happen to work where concrete has a crack. The Crystalcoat will be basically some cement powder that gets mixed with water and painted or rubbed on to the concrete. The idea is that it will slightly penetrate the surface of porous concrete where it will set and improve the density of the surface, slightly increasing its resistance to water penetration. But it isn't going to magically fix cracks or voids the result of poor workmanship. Nor seal joints. It might reverse much of the bad effect of too much water in the concrete after the workers added more water to the concrete when it arrived. But it cannot cure poor concrete placement or poor compaction. Their two, valid, defences are the external drainage and the structural concrete they improved but did not repair, but not if there are still leaking tie bar holes, honeycombing, leaking kickers and so on. Internal drainage is allowed only if leaks are reduced to seepage or less. It may only be designed to cope with seepage or less. It is not a waterproofer. They don't mention reducing leaks to seepage or less but they have specified a massive internal drainage system with inspection ports to check whether the internal drainage channel is blocking up with debris washed in through obvious leaks. These are all serious breaches of BS8102. Ian Maclennan knows full well he is being dishonest. I know about this stuff. Yet I found it very difficult to properly understand what they were saying just in these 3 sentences. Did you? The problem for Maclennan Waterproofing is that they are a major part of the problem, and if BS8102 is properly followed they will lose a very large chunk of their business. They are lying to preserve it. But I think they want an inexperienced eye to conclude that their internal drainage system is both essential and compliant - despite their not saying it is. I think that to an inexperienced eye they want internal drainage to be taken as one of two defences. And they want the reader to believe that because Ian Maclennan wrote the new BS8102 that this is exactly what the new basement specification needs to include. But I think it is a whole lot of lies and I think architects will get themselves into trouble accepting these proposals. It would be a mistake if you allowed your mind to imply wording into the Maclennan quotation that isn't actually there. You can download the quotation from Maclennan Waterproofing here. Be in no doubt. CSSW surveyors and the internal drainage system suppliers that employ them, and who largely wrote the 2009 Standard, are the root cause. As well as NHBC that still gives you little alternative but to use a CSSW. Everyone who follows NHBC instead of the Building Regulations is at grave risk of losing a claim - and their PII. BS8102 recognises the overdue need for change. CSSW surveyors are not the expert you need to listen to. They get no mention in BS8102:2022. The British Standard has changed significantly. Internal drainage for a new build basement is all but forbidden. Bad choices of product: beam and block and sticky-back membrane should be avoided. Other products, such as ICF and twin wall precast concrete (such as Glatthaar Keller) "should be deemed inherently high risk." See 5.1.3 of the Standard. Instead, the building of the structure should be supervised and repaired until it does not leak - before any fitting out of the basement. The architect should not choose a CSSW surveyor from an internal drainage company to be the Waterproofing Specialist if the project is not a Victorian cellar. They should choose someone suitably qualified and experienced at the civil engineering involved in constructing an underground structure that is waterproof, and, in the majority of cases, internal drainage should not be installed in a new basement. I can be the Waterproofing Specialist on the Design Team.

• External render or external sticky-back membrane.
  
  If either external render or membrane are specified, they are not to be relied upon as fully waterproof. Render cracks. Membranes fail to stick. Either may luckily prevent a leak. But they might not prevent all leaks.
  
  They can be damaged too easily by following-on trades and processes. They could not be continuous from the base of the floor slab to well above outside ground level. Neither of these product types work reliably across structural joints, for instance, over the ends of beam and block flooring.
  
  The inside of the basement structure will still need repairing if there is water ingress despite an external render or external membrane.


• Waterproof paint all over the inside of the basement structure.
  
  This must only be applied after the sign off by all interested parties that there is zero water coming through the structure. Or, conceivably, before the basement is backfilled and before there is any water outside to leak through.
  
  In the hands of a labourer, waterproof paint will not work.
  
  If there is any ingress of water, no matter how slight, coming through the surface to be painted, waterproof paint will not dry or adhere.
  
  Manufacturers specify 3 coats. I have found that when I apply waterproof paint I apply about 9 coats before I am satisfied, and every coat is thin so that the base of each coat dries before the surface of each coat dries which would prevent the surface of the wet paint adhering to what is beneath.
  
  This will also need inspecting and signing off, but perhaps only by the architect or a clerk of works.


• Internal drainage. But only if total ingress of water is no more than 'seepage' and all interested parties agree. Otherwise leak repairs must continue.
  
  IT IS ESSENTIAL THAT THERE IS NO MENTION OF INTERNAL DRAINAGE ON ANY DRAWING FOR TENDER THROUGH TO CONSTRUCTION.
  
  NO EXCUSE FOR SITE TEAMS NOT TO TAKE UTMOST CARE.
  
  Note. 'seepage' should not amount to more than a few litres of water a year. A sump need be no bigger than a few litres. A shallow tray cast in the structural floor slab. Described on the drawing as only an aid to clearing up plumbing spills.

This is a 20 second video of a sump and pumps that fail from time to time because they have too much water to pump out.

Internal drainage has allowed earlier trades to do their work badly. Internal drainage specified on drawings has caused more and more water leaking through basement structures. This pump switched on every 40 seconds. Over 8 years that is more than 5 million pumping cycles. Many pipework joints and pumps failed, each failure leading to a flood.

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. 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 a basement expert: 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. 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. 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: This ruling means that a sub contractor cannot have money withheld if he does work badly. or 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. or 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.

Back to the Basement Building Questions Answered menu.

Forward to the Basement Building Construction Manual menu.

For a fixed fee of £199 I will answer all your questions by email. More details here.

Previous Page

Next Page

The Page After That