Basement Construction. Our page for architects' FAQs.
Whether to build properly or usually?
The truth about waterproof concrete.
How a floor over a basement is different to a normal house.
Door and window openings in a waterproof concrete wall.
This first section explains why we tend to build basements only for private clients and not for developers or as a sub contractor to a main contractor.
I explain to clients on a different page that there are 3 types of client requirement.
- The Developer. Two distinct extreme situations and many in between.
The developer needs a latent defects warranty. That usually means he wants internal drainage and a sump and a pump because he can get those installed as soon as the roof is on, the windows are all in, he can pump the rain out the basement and get the first fixes under way quickly.
It also means that the developer will get his warranty without question. None of the latent defect insurers (NHBC, Premier and so on) include below ground waterproofing in their cover so they are delighted if that risk has been insured elsewhere. Then they can allow you to believe they include it.
If there is no other information available, the internal drainage designer will provide a system that can cope with this:
About 50 litres a minute for ever and ever and ever switching the pump on and off every 40 seconds. The problem with this property (see the video here) is that every few months a pump would fail and the basement flooded in about 8 minutes. So it flooded many times over the 7 years and in the end the owner couldn't rent it out any more.
The trouble is, no one insures the pump or the damage caused if it fails.
But very often I see that architects have specified the most comprehensive and expensive systems available (E.g. Delta or Newton) long before they know how much water will be leaking into the basement.
The first extreme.
The developer with a site close to the Thames, in gravel with a tidal water table just a metre or so down.
He gets the basement built by the cheapest gang he can (or he employs a main contractor who gets the basement built by the cheapest gang he can - and probably has no intention of paying them in full so they do the job as fast as possible with no regard to quality).
After all. No one supervises or inspects these days.
Note the pile in the corner in the wrong place so that the 'waterproof concrete' specified doesn't cover it.
This next photo is a link to a very short video
The other extreme.
The developer whose basement is being built into the side of a hill, or on chalk, or with a very effective land drain.
This developer, no matter how badly the basement structure is built, is not going to have any leak anyway.
So why have 3 pumps, backup battery power and a water level alarm?
In fact, why have internal drainage?
It is up to the architect to recognise a situation where a basement could not leak, usually from the soil investigation report, and make a sensible judgement.
Visit the property after the roof is on, the windows are in and the basement has been cleared and dried and assess then just how comprehensive an internal drainage system does his client need?
Unrestrained internal drainage designers will specify £45,000 of internal drainage and other items if they feel the architect has given them free rein.
The Grenfell Tower disaster is going to change everything, including clients' belief that they cannot sue their architect for wasting money.
The private client and self builders.
People who are going to live in the new property have an entirely different requirement.
They want a basement that will not leak.
This is what we do. We build basements and we guarantee that if there is any visible ingress of water I will fix it.
How do I manage this amazing feat?
Good old fashioned training.
Good old fashioned supervision.
Some lessons I learned along the way, such as how to pour concrete properly.
The architect needs to decide, before work starts, which sump needs creating in the basement floor slab?
We would recommend the middle sump because the first could be fitted inside if needed or the second could be formed above it later, ignoring it.
I don't build basements for developers selling the property for a profit because we cannot agree the price or payment terms.
However I have built many basements for the house a developer wants to live in himself.
This second section explains the truth about waterproof concrete.
Also, why paying for any big-name brand, such as Caltite or Sika, is a waste of clients' money. After Grenfell clients will look to specifiers to get wasted money back if they learn that the BBA certificate the specifier relied upon (but did not read) provides the evidence that these products don't do anything of value.
At the moment specifiers and inspectors don't fully understand these certificates because they don't read them in full. But I think they soon will because a Grenfell enquiry will condemn this practice as negligent. At the moment, most specifiers and inspectors just tick a box if a product has BBA. No further questions or research.
It is concrete that can be made waterproof. There is not any admixture that waterproofs concrete used below ground for basement construction.
BBA certification is completely inappropriate, being little more than a licence for the supplier to print money if he invests in some meaningless testing by BBA.
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).
My evidence is from The UK's Concrete Society report 2013* as well as BBA certificates.
The proper certification for waterproof concrete is concrete made to BS EN 206-1, and concrete tested to BS EN 12390: testing of hardened concrete.
Thousands of test cubes are tested to BS EN 12390 part 3 every week. This is the test for compressive strength.
BS EN 12390 part 8 is the test for depth of penetration of water under pressure.
The cladding and insulation for the Grenfell Tower both have BBA certificates. The insulation certificate got taken down. The cladding certificate states: "For resistance to fire, the performance of a wall incorporating the product, can only be determined by tests from a suitably accredited laboratory, and is not covered by this Certificate."
How could the cladding have been allowed? Unless everyone in authority saw it had BBA and ticked their box?
BBA certificates for water-resisting admixtures for concrete are just as bad.
Caltite's does not prove it makes concrete any more water resistant than a structural concrete without it.
Sika's improvement is barely measurable.
Most of the rest might at first appear to have made a small improvement but, actually, the small improvements can be explained by a reduction in water cement ratio* rather than any clever chemical.
All this from analysing the various BBA certificates*.
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.
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.
Every BBA certificate for a water-resisting admixture for concrete tells the user to buy an already water-resistant mix of concrete. This is 325 to 350kg of cement per cubic metre with reduced water.
C35A in BS 8007 is a concrete mix with 325kg of OPC and 55% water by weight of cement and this is a watertight mix.
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.
We always buy concrete over-sanded, with 350kgs of OPC and water a maximum 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.
However such a mix would be too stiff to pump or compact without a more powerful plasticiser than those usually available. So we have our own.
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
The influence of integral water-resisting admixtures on the durability of concrete. P36. Concrete Society. 2013.
The authors of this report had the bright idea of plotting all the BBA data on to one chart. Here is the same data plotted on to a chart we produced. It includes our educated guesses where data is missing, see the legend to see which.
Concrete needs to be the right consistency to be pumped into formwork and compacted properly without bleeding. 120mm to 140mm slump would be fairly usual.
So any BBA testing on much drier, stiffer concrete has not tested concrete that would be used on site. Site concrete would have to have more water, which would massively affect test results.
Seemingly, a very similar situation to Grenfell Tower cladding. The cladding resisted a flame in a lab but they (BBA) stated that they didn't know if it would work on a tower, on the certificate.
That is what the cladding BBA certificate states and every admixture certificate also warns that BBA didn't really find out if any of the admixtures would work on site.
Slumps of tested concrete
Kryton KIM 45mm
Caltite no information
BASF 135mm. Hooray. Site concrete. But BASF performed very badly.
Only BASF has a certificate for concrete with the consistency used on site.
All these admixtures have to include a water-reducing plasticiser.
Apart from plasticiser what seems to be in them?
With a little more cement or cement replacement:
With liquid plastic absolutely useless below ground where it won't dry
Others with polymer.
There is no evidence anywhere that anything other than extra cement and less water can waterproof concrete used beneath ground where it won't dry. The concrete will need a particularly powerful plasticiser as well to make it workable.
If you specify any of the famous brands, you risk litigation after Grenfell enquiries rule that not reading and fully understanding a BBA certificate is negligent.
None of the BBA certificates provide any evidence that any of these products make enough difference to site concrete to worth any money at all.
The proper procedure is to specify concrete made to BS EN 206-1, and concrete tested to BS EN 12390: testing of hardened concrete.
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.
We use waterproof concrete.
We cast fibreglass threaded rods in walls instead of leaving holes through plastic tubes.
We pour walls to only half height or so. This means we get concrete down to the bottom in better (perfect) condition and we can poker the concrete right at the bottom properly.
We don't use kickers. Kickers are impossible to form to be waterproof.
All joints are waterproof to BS 8007 (instead of using swellable strips or carpets that swell before the concrete is poured and don't overcome poor workmanship anyway).
I train my own workers. I don't use experienced workers experienced at doing the work badly.
We supervise all our concreting.
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.
The floor over a basement is completely different to the ground floor of a normal house
because water getting in under a normal house settles harmlessly on the earth beneath.
Please do not picture in your mind sticky-back membrane as the solution.
The ground floor over a basement must prevent a horizontal ingress of water beneath it.
The retaining wall must prevent a horizontal ingress of water over it.
Sticky-back membranes stick fabulously well on the south side with the sun shining.
They will not stick to a primed surface or to a neighbouring sheet on the north side where just the atmosphere has to be moist for these products not to work.
After they are fitted the bricklayers or the scaffolders will damage a sticky-back membrane and just push it back into place without a repair.
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.
Our business 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.
top chord supported
floor joists in 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.
Nu Heat underfloor heating for timber floors.
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.
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 for their not to benefit from propping by a concrete 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 insitu is flat and absolutely waterproof all round.
Our rods and nuts make forming an insitu concrete roof very easy indeed.
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.
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.
We have a number of other pages with information.