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Planning your basement floor slab.

To some extent your architect and your structural engineer will design your slab. However, you might want input regarding
  1. Any slope or fall, or do you want to make it flat and level?

  2. Any sump and if so what size and depth?
Typically, your architect will say you have to have internal drainage and that means you have to have a whopping great big sump. And all that can be extremely expensive.
  self build basement


It can also be a big, white elephant if your basement does not leak - and built my way it won't.



But you might be a developer and you might feel you have to have a waterproof guarantee to market the new property. I can't help you much, I'm afraid. Only the people who put in expensive internal drainage systems give a guarantee your basement won't be wet inside.



The text box to the right and the text box below are reproduced from the page about guarantees up near the beginning of this web site. So you might have read them.



They try to explain how the industry got itself into the situation where an awful lot of money is wasted
  1. On poor workmanship

  2. On a lot of expensive waterproofing products (that don't work due to poor workmanship)

  3. On internal drainage, a sump, a pump, a backup pump and a backup power supply.
In my opinion, it is because work hasn't been supervised since the introduction of Design and Build Contracts introduced about 1980. By 1995 or so, the Latham report had lambasted construction for not delivering clients value. But Clerks of Works and Resident Engineers were almost a thing of the past. No one then or since checks to see whether the right materials have been used correctly.

Companies like Carillion realised that expecting problems later allowed them to hold back huge retentions from sub contractors, so sub contractors learned to cheat the main contractors so they made money without the retention. Often by cheating on materials and quality. In the end, Carillion lost out and closed.

It served the industry's purpose for architects to increase the cost of delivering projects with extra products that claimed to overcome poor workmanship. Poor workmanship and not using the materials specified proliferated.
Due to past court cases your design team, including your architect, feel they are 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 empty the basement of rainwater that will have collected in it.

    Get it dry and show them, after some heavy rain, that your workmanship was very good and you have no leaks.

    Only 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.


Walls at new schools built in Scotland falling down, Grenfell and other towers clad in flammable material, Carillion going bust after projects turned sour - all because the industry lost sight of doing work properly decades ago.

So you might want to build your own basement, do the work properly and use fewer and less expensive materials as a result. Understand this page and hopefully you can stand up to your design team and win.

* 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. or

  3. 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.

  4. or

  5. 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.

But supervising the concrete, then not covering over the concrete till you have had a chance to search for any leaks after heavy rain, then repairing any leaks with a tub of stuff costing under £100, satisfies the judge's summing up just as well.



self build basement formwork   I would recommend a sump in your basement plant room so that you can sweep plumbing spills somewhere and suck them up with a Wet-Vac. So 2 litres capacity, not 200 litres.

You might also want a very slight fall over your basement floor to the sump as a precaution.


It does depend on what you intend to do with your basement. Many customers used their basement to store stuff, play drums and hide heating equipment. Uses for which they did not need floor insulation or a floor screed.

To my mind, if these people have a floor sloping very slightly to a very small sump, they have internal drainage even without any membrane. Even if their only pump is a Wet-Vac, it is still a waterproofing defence to BS 8102.


self build basement formwork   self build basement formwork   Here you can see a choice of deep sumps. The blue sump is part of a plastic barrel. The yellow sump is from the Wykamol catalogue.

There is another important question about the choice of sumps. I mentioned it at the top of the page. If you need a 10 year waterproof warranty so you can sell your new house then I'm afraid you have no choice. You need the Wykamol, Newton, Delta type system installed by one of their teams.



To get your slab either level or falling very gently to a sump you need guides.

If your concrete is supposed to be waterproof then those guides mustn't create a leak. They must also be accurate, and guide rails often aren't.

I have found that fixing in my FRP rod on 2m centres and either cutting the top off accurately or adjusting a nut carefully is best.

self build basement formwork

You will find more to read about the slab levelling system on this page



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