Building a waterproof basement.

Introduction page for architects who want to do a proper job. For clients who want a proper job done. Designing reinforced concrete to be waterproof when built. The first and only rule, designing and building a waterproof basement, and therefore fully complying with Building Regulation C2 and BS8102:2022, is: supervision. Supervision from the start of the design until all leaks are known to be fixed. My supervision of your design really only means my being as useful as I can. But, on site, my supervision needs to have teeth. The next most important rule is, stop inferring into literature or a certificate something the sales rep told you the product does - but isn't in writing. For instance 'we waterproof concrete'. It isn't true, not used beneath ground where it won't bake dry. 'The insulation is fire resistant', 'the sticky-back membrane always sticks', 'the joint strip works', etc. etc. etc. One more important rule is set in law now. "pass on necessary information to contractors and explain to them how to demonstrate that elements are built properly." Building Safety Act 2022. Applies to all projects from October 2023. In most cases the architect is responsible, as the Principal Designer, for ensuring "that elements are built properly." Perhaps you need to insist the client pays for frequent Clerk of Works inspections or a full-time site engineer. If your client is a self-builder with an education, he can read this web site and I might only supervise concrete pours. Otherwise, particularly if there is experienced labour, I need to be there almost all the time so they cannot cheat. Or do what they think will do. Contractors have a lot of bad habits. Habits that were OK before internal drainage was banned in new habitable accommodation. Bad habits that need ironing out with training on site. Are you aware that the laws surrounding building recently changed because of the Grenfell Tower fire? Some of the changes affect every project subject to at least one building regulation. The press only told us about tall buildings, but basement construction is materially affected as well. From the Grenfell Tower Inquiry, "Decades of failure   by government" ; "a construction industry in a race to the bottom" ; a "merry-go-round of buck-passing" ; "regulation was viewed as guidance to be bent" . Here's why. Government cut red tape allowing 'interpretation'; the architect didn't have the competence; the suppliers held back vital safety information; the client ignored safety when cutting costs; and the main contractor ignored poor workmanship. For the past 15 years or so, this is evidenced in new basements under new-builds by 'pumping the water out is a no-brainer'. But not fixing leaks let in mud. Mud blocked pumps. Basements flooded. There was mould. There was illness and therefore harm. C2 not complied with. The law broken. PII costs through the roof. A series of new laws, regulations and procedures mean that if that happens again, the developer faces jail while the architect and main contractor face sanctions from the new Building Safety Regulator. I have been helping people build their basement compliant with the new rules for almost 20 years. Since 2013 no self-builder had a leak to fix. Only the very few basements employing experienced labour had some minor leaks - easily fixed. I save tens of thousands of pounds. The architect's responsibility is the shape. To prevent water ingress around openings, penetrations, and over the top. I will guide you and I will guide the structural engineer on preventing cracking. I will supervise the contractor to prevent leaks. Waterproof concrete is absolutely essential for a new domestic basement about to be built. The Standard says no water ingress or damp area is acceptable. Building regulation C2 says it is the floors, walls, and roof that must protect. (Note. Waterproof concrete can only be made. No one can waterproof non-waterproof concrete). BS8102:2022 states, at 6.2.5, "Continuity of waterproofing protection    The need for continuity in the waterproofing protection should also be determined when selecting a type of protection. In most circumstances, the protection should be continuous (typically from DPC level or 150 mm above ground throughout the below ground structure). " Only continuous waterproof reinforced concrete can achieve this. See the sketch at the top of every page. Added together, Building Regulation C2, BS8110, BS8102:2022, the new Building Safety Act 2022, and the New Building Safety Regulator, make it imperative that the design team both ENABLE and DIRECT the contractor AND CLIENT to build a new domestic basement not to leak. It needs to be specified, clearly where contractors preparing a tender will notice it, •  from Table 2 of BS8102:2022 "Grade 3. Any water ingress or damp area is unacceptable". To keep YOU and the design team out of trouble, the structure needs to be inspected for leaks. This is now the responsibility of the Principle Designer, which is probably you. (Keep reading to 'Principle Designer Duties', lower down) Inspected for leaks After it is cleaned, dried and ventilated, before the inside of the basement structure is covered with anything, soon after a period of heavy rain. It might be wise to also make it clear on the tender documents, that there will be a clerk of works type of inspection, by someone who knows and understands what to look for, before each and every stage payment is authorised. If the contractor is not paying attention to preventing leaks, the payment will be held up until they pass a repeat inspection. Beware of BBA certificates. Roundly discredited during the Grenfell Tower Inquiry. BS8102:2022 states that "relevant manufacturers product information should be checked to confirm that the system selected is suitable for the structure to which it is to be applied". A judge summed up 'BBA certificates do not amount to a form of guarantee or a passport to compliance with Building Regulations'.Mulalley and Co v Martlet Homes Ltd (2022). Open a legal page here. The design, specification, methodology, and workmanship must all prevent failure. The contractor needs to understand they keep repairing leaks until the structure passes inspection. "Complete replacement if repairs not possible". The architect does not want any comeback because they make an error that, previously (when water was pumped out), did not matter. Success does not require any products. It requires good design, satisfactory materials, and good workmanship, nothing else. Therefore supervision and inspections need to be the new norm. Products actually cause failures, because they rarely meet their claims, and the team thinks that products will do the magic and the team can cheat. Products incentivise leaks. These "extras" do not cost the client anything, because there will be no Tape or strip in the joints No stop ends in the walls No threaded tie bar holes to seal No concrete kickers to try to repair later No honeycombing in wall concrete to repair later NO INTERNAL DRAINAGE SYSTEM No breach of building regulation C2.   This page is all about why you must design differently. The details you want are are on other pages. Links to other pages useful to the architect. These open in new tabs. Soil investigation. Windows and openings (and fire escapes). Ducts and pipes. Basement foul waste. Joints in concrete. The test to prove concrete is waterproof. 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. The floor over a basement may only be engineered timber joists or RC cast insitu. Here's why. External waterproofing. The New Waterproofing Specialist on the Design Team who also supervises and inspects. My review of BS8102:2022. This page includes evidence why BBA certificates cannot and should not be trusted. Basement warranties and insurance backed guarantees. Typical problems of bad design and poor workmanship. basement insulation and thermal mass. Green ideas that failed my clients. Swimming pools. Quick links to sections of this page. Second waterproofing defence options. Important guidance from DLUHC. What the new regime will prevent. Products you are used to, but should not use. The principle Designer is responsible for inspections. Maclennan waterproofing case study. How you were mislead with the Outwing court case. I had an IBG for basement waterproofing before sumps and pumps. This page is all about why you must design differently. Use the menu listings above. These two images below take you to the main menus for self-builders and sub-contractors.

Your domestic basement design must comply with Building Regulation C2. From Part C "Site preparation and resistance to contaminants and moisture".

For new application approvals since October 1st 2023, building control must file away evidence of who was responsible for what. Backed up by the new Building Safety Regulator that has the power to prosecute. The Building Safety Act and the Procedural Changes to Building Control (October 1st 2023) make the architect the new Principal Designer, in most domestic design cases. But I am finding, nearly a year later, that architects have no idea that carrying on doing the same old is now breaking the law. This is a link to an article in RIBA Journal: What you need to know. It says in the article "This legislation affects all projects" Here are the headline changes: New duty-holder roles: client, principal designer, principal contractor. Plus duties on designers and contractors. Competence: duty holders need to ensure they are 'competent' for the work they undertake and may be called to document this. Duty holders must ensure that arrangements and systems for planning, managing and monitoring work comply with the regulations The new regulations forbid an architect accepting instruction for what they are not competent to design. Unless they find and accept the advice of a suitable specialist. With regard only to new domestic basements, this is usually the structural design and waterproofing. Architects commonly find a structural engineer. But all too often waterproofing is what someone selling something tells the architect to use. The Grenfell Tower fire deaths were due to someone selling something telling the main contractor and client, who did not involve the architect in their decision to change materials, to use it. This government web page outlines the new duties to prevent a Grenfell Tower disaster happening again. The new duties apply not just to tall towers but to all work that has to meet at least one building regulation. Go straight to Principal Designer sections: Principal Designers' duties. Principle designers: competence requirements. "Principal designers should be able to manage other designers and reach a consensus that design work is compliant with building regulations. They should be able to monitor identified compliance risks and assess gaps in other designers' competencies." And, INSPECTIONS. The Principle Designer's completely new AND legal duty work with the principal contractor and share information about planning, managing, monitoring and co-ordinating the design and building work PRINCIPLE DESIGNER     MONITORING     BUILDING WORK. These few words add a whole new, very practical, site, responsibility to the Principal Designer role. The Principal Designer is responsible for Clerk of Works inspections. The Principle Designer might be wise to appoint a Clerk of Works, me, for the basement structure. If basement waterproofing fails, Building Regulation C2 is not complied with. The Building Safety Regulator will be after the Principal Designer.   "pass on necessary information to contractors and explain to them how to demonstrate that elements are built properly." My warning sign is because anything you tell the main contractor to pass on to the sub-contractor's sub-contractor's tradesman won't reach him. He will tell someone he will read it, but I promise you he won't have the capacity to understand or take it in, if he reads it. He has to be shown. That is how he learnt everything he knows. He was shown. My page about training, explains the tremendous and particular problems for the Principle Designer complying with this new duty, in the case of a new domestic basement. "Principal designers should understand the law as it relates to their role and competency requirements as set out in building regulations." If the client did not appoint a principal designer, then "the designer in control of the design phase of the project is the principal designer" And, a CSSW, who has no valid waterproofing measure to sell you, is NOT a waterproofing specialist, despite the plainly wrong advice from NHBC, Premier Guarantee, LABC Warranty and other warranty providers. If a design detail lets in leaks; the architect risks sanctions from the new Building Safety Regulator; and their client faces a criminal trial in court for choosing an incompetent architect. You can get more detail on the home page. Opens in a new tab here.

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. At this time, the DLUHC would have known about the new regime on its way just a few days later. I didn't.

Every "professional" contractor will leave holes through plastic tubes after they withdraw the steel threaded rod to keep re-using it. The number of holes they will fill ranges from zero to about 90%. What they will not do is make sure they fill every hole to be waterproof - because architects have always put a sump and a pump on the drawing. The sump and the pump are the root cause of bad workmanship.

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. And, when mud blocks the pumps, and rain persists, the basement will flood. Mould. Harm. The client or developer will be prosecuted for instructing you beyond your competence. The architect will face the wrath of the new Building Safety Regulator. Building control will face the wrath of the Building Safety Regulator as well. Then there will be a claim.

Leaks cannot all be identified and fixed until after the structure is completely weathertight, it has been cleaned and dried, and inspected before anything covers the basement inside. Because it fills up with rain and muck as soon as the wall formwork is put up. Internal drainage suppliers do not include cleaning and fixing leaks in their scope of works. They might say they will paint the inside with a waterproof paint, or render. But neither would stick where a leak of water is pushing it off. Products from internal drainage suppliers, including tapes and strips, renders, copolymer paints and so on, do not waterproof leaking structures. Unless you fix all the leaks first, they don't work. If you fix all the leaks first you don't need any of them. That's why none of them are waterproofing specialists.

1. External drainage if it is maintainable and always drains away by gravity. Actually, a no-brainer if it will always work.


2. Waterproof paint all over the inside of the basement - but this only adheres where there is no leak, no matter how slight. And only if each coat is thin enough to dry against the substrate before a skin dries on the surface. Great care is required.


3. A very tiny internal drainage system that might be there primarily for plumbing spills, but would double up to remove extremely minor leaks that were missed during inspection.

This is a shallow tray I cast in a structural floor slab during 2012, 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 and maintenance (rodding eyes or inspection chambers), 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. Neither do they say they will fill holes if the contractor left the tie bar plastic tubes through the walls without filling them. Their two, valid, approved, 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. And not if external drainage cannot drain away. Internal drainage is allowed only in non-habitable accommodation (e.g. workshop, car parking) if leaks are reduced to seepage or less. It may only be designed to cope with seepage or less. It is not a waterproofer. Maclennan 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 the sentences I reproduced in blue above. 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. The quotation PDF document is too large for this web server. If you would like to see the original, please just email me and ask me to send it to you. 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 recognised the overdue need for change. The Building Safety Act 2022 takes change even further. 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.

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