waterproof guarantee basement construction build manage
  waterproof guarantee basement construction build manage

Basement Construction. Our page for architects' FAQs.

Basements are usually waterproofed to BS 8102.

Two defences against ingress of water.

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

Our concrete and our work is always absolutely waterproof.

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

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

  During 2012 we built this basement and put a shallow sump in the plant room. The client put cheap internal drainage membrane up before studding the walls. His pump has sat patiently for 4 years without seeing a single drip of water.
Local building control signed his system off.
  completed basement waterproof sump

My ear-bending about waterproof concrete. Sorry, but you will find it educational.
  • Not a single BBA certified admixture has made public how it waterproofs concrete*.

  • However every BBA certified admixture for concrete claiming to make concrete less permeable states on the certificate that the admixture must be added to a concrete mix that would already be waterproof without it.

  • Most if not all the admixture suppliers guarantee only their product. But since none of them say what their product does it must be impossible to prove it did not work. And impossible to bring a claim.

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

A working group of the UK's Concrete Society published a report in 2013* produced by consultant engineers, academics and other experts. They strongly suspect that it is always the concrete that will be waterproof, not any of the admixtures.

But to make matters worse, most of the BBA tests were on concrete samples that would have to be rejected on site (or more water added which defeats the purpose) because the samples were too stiff to pump.
Here is a photographed extract from my copy of "Advanced Concrete Technology", the volume called "Processes", the Chapter called "Concrete construction for liquid-retaining structures" by Tony Threlfall, 2003, Butterworth-Heinemann. Page 16.2.   Click on this image to be able to read the text from the whole page.   from Processes, water retaining BS EN 12390 8
It says that permeability better than 20mm is satisfactory. You will see on our certificate of permeability that our results in this case were 1mm, 1mm and 3mm. We have dozens of similar results going back years, so we tend not to pay for this test (£600 plus VAT) any more. But you can specify tests if you wish.

In a concrete mix that will always be waterproof there is no product added suited to a BBA 'Product' certificate.

This is because waterproof concrete only contains sands, cement and water. The plasticiser quickly disappears.

In the text you will see that the appropriate test for waterproof concrete is to BS EN 12390 part 8. There is no mention that a BBA certificate will prove concrete won't leak.

Many, many large sites routinely test concrete for strength to BS EN 12390 part 3. We had concrete tested to BS EN 12390 part 8 as well. Depth of penetration of water under pressure on concrete. The pressure is equivalent to a depth under water of 30m and the pressure is maintained for 96 hours.

Click on either image to the left to see two original test certificates.
  The text above describes concrete with 325kgs of OPC and 55% water by weight.

We always buy concrete over-sanded, with 350kgs of OPC and water 50% by weight.

The concrete chemistry books state that by adding a little more cement to C35A and reducing the water a little further that pores of water between remnants of cement grains will be closed off from neighbours by crystals of hydrated cement.

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

* In 2013, the UK's Concrete Society published a report by a working group of 18 members, experts in their fields, that states: " water/cement ratio .... primary measure of water penetration and hence the durability of the concrete."
From: The influence of integral water-resisting admixtures on the durability of concrete. P36. Concrete Society. 2013.

In our specialist experience, what your client wants is good materials and good workmanship, not lots of money spent on products that won't work when used badly.
  1. BS 8102. A first defence to substantially reduce the amount of water that can get in. A second defence to stop or deal with the rest.

  2. We satisfy the BS, latent defect warranty companies, specifiers, lenders, building control and clients.

  3. Therefore the basement pump may never switch on, or so rarely it could never become worn out or unreliable.

  4. We do our very best to make our construction completely visible leak free and probably completely waterproof.

  5. We personally guarantee no visible leaks for 12 years**

We use
  1. Waterproof concrete. Actually C35A (as specified in BS 8007) though with modern plasticisers this is supplied with slightly more cement and substantially less water than required by the Standard.

  2. Therefore the concrete is completely impermeable and we can take samples on every project to be tested to BS EN 12390-8, which is the current test for permeability of concrete under pressure from water (4 bar pressure for 96 hours).

  3. We avoid kickers and tie bar holes and we always strike the formwork so we can check there are no flaws in the concrete and repair anything we don't like the look of immediately.
  1. I personally supervise all concrete pours.

  2. I make sure all the workmanship is to BS 8007, which means sufficient steel, concrete poured and compacted correctly, cleanliness throughout and proper curing.

  3. I am Phil Sacre, a civil engineer with 25 years of varied basement experience from a wide variety of projects such as office blocks, retail developments and new schools as well as, most recently, 13 years involved with building domestic basements.

  4. This is why I am happy to give a personal guarantee that our work and our concrete will not visibly leak.
You will want to specify either an external or an internal measure as well as waterproof concrete. You can specify any number of defences against water ingress but we have a site observation to share with you.
When most sub contractors and workmen see there is more than one defence against water ingress it immediately means one thing, and only one thing, to them.

They do not have to do their own work well. Theirs can leak.
That is why our workforce is trained, and supervised throughout, to assume ours is the only defence against water ingress. Their work matters.

In this photo you see the bottom half of the wall concrete in perfect condition. The top half will have an upstand and you can just see the bottom of a window opening.

By specifying us to build and waterproof your client's basement you are specifying how the concreting is to be carried out and site supervision as well.

You might specify:

C35A watertight concrete with a maximum of 0.50 water to cement and a minimum of 350kgs pure OPC per cubic metre.

One set of 5 cubes to be taken from the first pour and tested to BS EN 12390-8-2009. Maximum permissible average penetration 5mm.

Concreting workmanship to be to a high standard, at least BS 8007, and all concrete works to be effectively supervised throughout.

waterproof concrete basement
** Obviously this personal guarantee has to be sensible.

It is subject to being allowed to build our way and it means visibly leaking - the leak can be seen. We cannot be responsible for any cost of removal or replacement of anything that covers the structure we built.

Ideally, the basement structure will not be covered over until the roof is on and the structure above watertight, the basement emptied of rain that collected, dried and finally inspected after a period of subsequent heavy rain.

We have to be paid for wasted journeys if we prove any water is just condensation.


Concrete BS 8007 C35A or better.

Workmanship BS 8007 or better, including effective supervision.

Concrete from every project tested for water penetration under pressure to BS EN 12390-8-2009 by a UKAS accredited laboratory, equalling the prestige of BBA yet exceeding by far the usefulness of a single BBA result in one lab-made sample.

A reinforced concrete structure designed by the structural engineer with our advice when to add extra horizontal steel.

The next web page helps him design a waterproof structure.

Type B waterproofing to Grade 3 of BS 8102: No water penetration and a dry environment. We probably usually achieve Grade 4 from the concrete alone, customers report their basement feeling completely dry even before their heating is installed.

Concrete made water resistant the way the UK's Concrete Society said was best. With a powerful water reducing plasticiser, more pure cement and less water by proportion, in a very dense concrete mix.

Getting Building Control Approval.

As a practitioner, I despaired at some of the waterproofing products that have BBA certification but very rarely work. Membranes, carpets, additives, tapes and so on.

Your client's basement excavation will be wet and dirty, almost certainly not the conditions needed to successfully apply anything that needs to be clean or protected from damage. So, frankly, anything with a BBA certificate can fail and be a waste of money.

By using waterproof concrete supplied by a readymix company to all the usual standards and to the engineer's satisfactory design there are no issues with Building Regulations as long as the basement does not leak.

If it leaks the concrete can be repaired until it doesn't, so that can be your instruction on your drawings. "Basement construction team to repair and stop all visible leaks".

No following on trade will be able to damage concrete.

Internal drainage membrane as the second defence to BS 8102.

This section of wall was cast with my resin rod form work method. It requires no kickers and leaves no holes.

waterproof concrete basement resin rod formwork stripped

It is very simple, very adaptable and strong enough for the concrete to be properly compacted inside.

waterproof concrete basement
Joint strips.

We prefer to scabble and clean joints to BS 8007 and have nothing in a joint to prevent us keeping it clean.

If you feel you must specify something in the joint, we favour metal waterstop from Max Frank.

It is fixed to the slab steel and strong enough not to fold when the concrete is poured on top.

A first strip is removed at the last moment to reveal the bituminous coating that is in the slab concrete and a second strip is similarly removed to reveal the coating that will be in the wall concrete.

metal waterstop
Wall widths and how the main house footprint sits upon them.

Basements with a flat top to the retaining wall stop just beneath outside ground level, water gets in horizontally beneath the ground floor and runs down the inside of the basement wall. It cannot be stopped except by the internal drainage and only if that is lapped up and over the ceiling.

Ours was the first business to promote an upstand which we now see has become common practice.

In our experience a waterproof concrete upstand is a most essential waterproofing measure.

It is very much easier to accommodate wall construction above a basement with a waterproof concrete upstand if engineered timber joists are used instead of beam and block or pre cast slabs.

waterproof basement construction upstand

waterproof basement construction floor ledge

top chord supported
waterproof basement construction floor ledge

floor joists in hangers
floor joists on hangers

An engineered timber floor joist can be any width and any height and stronger than concrete (for the same overall floor construction depth including the service void beneath concrete). It can be a trus joist (OSB web), an easi joist (lattice steel sided) or plywood sided. Services can go through it.

a top chord supported floor joist

Clearly, engineered timber joists offer more choice than concrete and they are flat on top. With services through and not under and without the need for a screed to overcome curvature timber floor construction depth is usually less than concrete.

Wet underfloor heating is available (from Nu Heat and possibly others) for timber floors.

underfloor heating over basement

Timber shuttered walls can be any thickness and change thickness easily to create a corbel and the upstand.

An absolute minimum concrete width of 200mm for waterproof concrete walls, 225mm would be better and 250mm better still because of the increased working room to deliver concrete to the bottom of the form in perfect condition and compact it properly.

An apparent advantage of a concrete floor over timber would be the support a concrete floor gives to the top of the retaining walls by propping the walls one side against the walls the other side and the backfill beyond.

But this would mean that the floor and internal walls had to be in place before the basement was backfilled and that causes problems.

Waterproof concrete gets hotter while curing than ordinary structural concrete so the engineer will be asked to have enough extra steel to control the extra issue of thermal cracking. Therefore there will be enough steel in the walls anyway for their not to benefit from propping by a floor over the top.

Please choose an engineered timber joist floor, supported off an upstand created out of waterproof concrete. The basement will cost far less this way. Indeed this way is often the only way that suits a tight budget.

Unless the ceiling over the basement has to be concrete, for instance it is a garage, outside or, sometimes, a kitchen. In which case concrete cast in situ is flat and absolutely waterproof all round.

What makes concrete waterproof?

According to the UK's Concrete Society report in 2013 not proprietary admixtures with BBA certificates.

They proved that they make much less difference than better concrete.

Extra sand, extra cement, correct cement, reduced water.

Fly ash in most concrete is barely reactive. It doesn't use water.

The important ratio is water to Ordinary Portland Cement. CEM 1.

Or water to a blend of Ordinary Portland Cement and Slag or Micro silica. CEM 3.

More Here
The usual mix required is
min P350
60mm slump
over sanded
plus PCE plasticiser

If the concrete delivery ticket says all these things and the concrete looks cohesive and consistent, it is waterproof.
It will be so dense that capillaries will become discontinuous.
What can be used to construct a fully waterproof basement?

Over 20 years our founder worked with traditional formwork, proprietary panel systems and Insulated Concrete Forms (polystyrene); waterproof concrete, sticky back membranes, bituminous coatings, carpet membranes, tapes, strips and bars, external and internal drainage.

These do not work with any certainty:
1. ICFs.
2. Sticky back membranes.
3. Hydrophylic tapes.
4. Clay filled carpets.

Internal drainage works but only if the pump turns on rarely.

If the pump had to turn on every hour then after a year it has turned on and off, pressurised and depressurised, nearly 10,000 times and it will break and cause a flood.

Traditional formworks need a kicker and holes for threaded bar - all of which cause difficult, on-going problems of waterproofing. However there are proprietary systems that can be used successfuly with FRP instead of steel.

ICFs are permanent. No one can see what voids remain inside; and because polystyrene is never strong enough to always properly compact concrete inside it without movement or damage, there will be voids that will leak. We will not use ICF.

Waterproofing products applied to the outside down an excavation do not work because they get wet or dirty during installation plus they get damaged during backfilling and when the soil settles over time.

Outside you can have as much insulation as you want but only if closed cell and waterproof. Extruded polystyrene (XPS) for instance BASF Strodur, Knauff Polyfoam ECO XPS and Dow Styrofoam. Note. Many of these boards are now filled with carbon dioxide, to lock some carbon dioxide up, and are not as thermally efficient as the old Pentane gas which is still available. Pentane boards cost less because you need less thickness.

A sprayed closed cell polyurethane foam is suitable outside but not too thick else it cracks. Trials may be needed first.

Door and Window Openings.

In these photos you see that the window opening is formed in a piece of wall the thickness of a concrete block and side steps are 300mm away from the opening to allow the brickwork to come down and look a seamless part of the brickwork for the whole house.

These are before, during and after pictures of the same window.

form window opening in icf  form window opening in icf

form window opening in icf

form window opening in icf  form window opening in icf

form window opening in icf

The ledge beneath the opening is 3 brickwork courses down so the upstand in waterproof concrete creates a cavity tray and some height in case the window well should flood temporarily.

The window frame could be fixed to the concrete or the brickwork in front, as detailed by the architect.

Below here is detail from our founder's days constructing basements before this company formed. You might look at the photos and come back to the detail later.

Some of these are ideas that did not work so well, such as the window well being no lower than the window opening and the wall that misjudged the bank.

Completely Waterproof Basements and Basement Waterproofing.

BS 8007:1987; 'Code of practice for design of concrete structures for retaining aqueous liquids' is quite clear stating that if the concrete and workmanship are good then the concrete at the bottom of a wall will bond well enough with a slab beneath not to require any tapes or strips in the joint to make it watertight.

I have found many times that by taking great care to thread a concreting pipe down inside formwork only half wall height to release the concrete within 1m of the bottom and to direct all the concrete past all the reinforcing steel, that my joints were sealed immediately without any further attention. The steel starter bars grip the wall concrete preventing it shrinking as it cools.

No leaks through concrete since 2008.

Without any BBA certificated products. Just excellent supervision.

tremmie   Perfect concrete.

Actual size.

perfect concrete
  But most workforces, especially those with experience such as when I was a site engineer, do not take care. The wall at the bottom is usually missing its fine sand and cement because that was left sprayed all over the steel and formwork when the first concrete went in released at the top. Hydrophylic strips and water bars are usually specified but, unfortunately, they do not work if surrounded by stones and not concrete. So, although they may be of some value overcoming slightly imperfect workmanship, they are of zero value when the workmanship is the usual poor sub contractor standard, or if they get wet early or get left dirty or damaged before the formwork is complete.

Indeed, April 2012 I had a phone call from a hydrophylic strip sales person, on a site where he had sold the strip as specified by the architect. He had been called in because water was all over the slab and could not be removed because the slab was cast against sheet piling - and could I help? I told him to throw it away.

BBA certificates state such products need to be used with care, kept dry, kept clean and kept protected. So they are worthless down a typical excavation.

void waterproof basement   segregation   hydrophylic tape problem

basement walls without internal insulation    There is another issue and that is cutting chases for services inside.

Waterproof concrete is too strong. Burying conduit needs planning and adds difficulty.

Sticking insulation inside and cutting chases into it before sticking plasterboard to the insulation seems simple and a good solution.

The choice of internal insulation is yours. A balance between thermal mass, living accommodation and insulation.
   basement walls with internal insulation

Here the non-retaining wall around the doorway, within the stairwell, should have been thinner so brickwork off the slab joined brickwork on the ledge above seamlessly. They got over it with a cant brick.

waterproof basement construction build

waterproof basement construction build    waterproof basement construction build    waterproof basement construction build

You can have any step, any corbel and any timber ledge cast in place that you specify.

waterproof basement construction build        waterproof basement construction build   waterproof basement corbel

Ground levels outside can vary, so your step needs to vary with it.

waterproof basement construction build

waterproof basement construction build

Think very carefully, at the design stage, where your soil line will be. The team building the walls will be down an excavation so final soil levels will not be apparent to them. This architect got caught out by the client not getting the expected permission to build a retaining wall alongside the road.

waterproof basement construction build

To sum up. Please draw in detail all the wall thickness changes. Or note on the drawing to be decided on site.


Window and door openings

Preventing water getting in around basement windows is the same as above ground but finishing around them can be quite different.

waterproof basement construction build 

This window seemed to have everything until it was found water got in under the window frame. Note that insulation formwork is not being used any more so your structural opening dimensions need to allow for insulation to be added later.

I suggest you allow for 3 courses of brickwork under the frame.

If you can get it right, site can get it right.

With the new formwork it was easy to create a narrowing for face brickwork around the window as well as the corbel for the plinth detail in the brickwork round this basement where the ground level was a little lower.

Where the inner skin of cavity wall is actually the waterproof concrete wall narrowed where it is is not retaining, your basement waterproofing, DPC and cavity tray are all sorted.

waterproof concrete basement window brick ledges waterproof concrete basement window opening brickwork



The biggest enemy of waterproof concrete has to be a kicker, a small upstand on the slab to clamp traditional formwork to.

It takes great care and added expense to cast a waterproof kicker on the slab because, if it was cast at the same time as the slab, the fresh concrete would sink into the slab and disappear when properly compacted.

More often, kickers get made out of old concrete or the wrong mix bought in for something else. That is why kickers are notoriously un-waterproof.

The poker is obviously being allowed to dwell in one place. This will knock the largest stones away first and eventually the sand as well so that when the poker is withdrawn there will be just a column of water through the concrete.
  waterproof concrete basement kicker

This photo is from a competitor's BBA certificate for a water-resisting admixture.

As a practitioner, I think they, and I presume BBA as well, are absolutely clueless.

We have a number of other pages with information.

waterproof basement construction   You Must Have a Soil Report Before Design Starts   Please Ask Your Architect To Read This Page   Please Ask Your Structural Engineer To Read This Page   construction contract

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