I am building an 2 store extension. This unfortunately runs into some problems with foundation as it is near trees. We decided to have a piled foundation with ring beams and in situ suspended slab.

The in situ slab and the ring beams will be supported by Cordek etc to allow for soil heave.

There is a DPM layer above the slab wit the insulation layer and the screed above it.

My question is: do we need to have a damp proof or water proof layer under the slab? I saw some reference have it and some do not have it. My slab will be only about 200 mm. As we have use plastics for that water proofing we need to use expensive voltex to do the job. I would rather not to have it if I can. If we use card board instead of cortek, we need to have water to dissolve it anyway.

thank you for your advise.

  • Peter Eade

    I’m not too sure I understand what your problem is, I assume you are building your extension with Building Regulation approval. In order to get the project approved your plans must not only show where the DPM is to be positioned but also indicate what it is to be made of, just follow what is shown. If you are undertaking the work under a Building Notice, which is not a good idea I would suggest you consult the person who draw up your detailed plans or possibly discuss this with the Building Inspector.

  • Wangwen Zhao

    Peter. Thanks. The original design has a layer of voltex under the in situ concrete. But the builder thought that he can use normal plastic DPM. I read somewhere that you cannot use such as it will be damaged by concrete swelling. And also someone told me that he did not use DPM for his job. So the question would be, do we really need DPM? secondly, is there a cheaper option of DPM than voltex.

  • Peter Eade

    To conform to the building regulations you will require a DPM and Voltex is by far the best, hence the cost. On a normal concrete floor slab there would be a DPM which is usually 1200g polythene (Visqueen… check their website). The DPM would be under the concrete and turned up the walls to connect with the DPC. Don’t forget you will also require thermal insulation either under or over the slab, if it’s under the concrete the edges would also turn up to the DPC in order to achieve a fully insulated floor. I would have thought if you have engaged a builder on a fixed price quotation he would have included for the Voltex in his price why not stay with what’s been approved by Building Control.

  • Wangwen Zhao

    I found the answer in


    If we have a concrete cover or at least 40 mm, we do not need DPC as in clause 4.18. Otherwise clause 4.19 apply which needs DPC and a 150 mm ventilation space. This is much more troublesome.

    Am I right?

  • Peter Eade

    I’m still just a little confused by your problem, surely you don’t have (or intend to have) 400mm of concrete for a residential floor, even if you do why not still lay a 1200g polythene DPM it’s not expensive. My confusion is not knowing if you have started the project or are still at the stage of deciding which way to go. I feel you should be talking to your designer or perhaps the building inspector, its very difficult to fine-tune any advice without seeing your plans. Assuming you end up with a normal concrete slab, it can either be sitting on the ground usually with a hardcore sub-base or suspended as in beam and block with a ventilated air gap under, either way the depth of the concrete would not exceed 150mm and you will need a DPM and insulation.

  • Peter Eade

    I’m sorry I misread ( 40mm and not 400mm) your question, surely you can’t be planning to lay an over site to a depth of 40mm (under two inches in old money…) the minimum accepted depth is 125mm and YES you will require a DPM…

  • Wangwen Zhao

    sorry. I am still planning or design the project. It was messed up by the architect at first, who did not foresee the problem to build near the trees, hence we got a piling contractor to put some piles. This problem was compounded by the structural engineer who works for the piling contractor but has little experience in designing the slabs. They designed a 350 mm slab coupled with all sorts of cordek and DMC. I am a structural engineer myself but not on house building. I found taht it is very odd to have 350 mm slab and I checked code and did some analysis and decided to have only 200 mm thick slab and the engineer agreed. The problem with DPC is still unsolved. SO I checked building reg, if you have 40 mm concrete "cover" to the reinforcement inside the slab, you dont need to have DPM. Is that clear?

  • Wangwen Zhao

    this is building reg 4.18 and 4.19
    4.18 One solution for a suspended concrete floor could be:

    a. in situ concrete at least 100mm thick (but thicker if the structural design requires) containing at least 300kg of cement for each m3 of concrete; or

    b. precast concrete construction with or without infilling slabs; and

    c. reinforcing steel protected by concrete cover of at least 40mm if the concrete is in situ and at least the thickness required for a moderate exposure if the concrete is precast.

    4.19 A suspended concrete floor will meet the requirements if it incorporates:

    a. a damp-proof membrane (if the ground below the floor has been excavated below the lowest level of the surrounding ground and will not be effectively drained); and

    b. a ventilated air space. This should measure at least 150mm clear from the ground to the underside of the floor (or insulation if provided). Two opposing external walls should have ventilation openings placed so that the ventilating air will have a free path between opposite sides and to all parts of the floor void. The openings should be not less than either 1500mm2/m run of external wall or 500mm2/m2 of floor area, whichever gives the greater opening area. Any pipes needed to carry ventilating air should have a diameter of at least 100mm. Ventilation openings should incorporate suitable grilles which prevent the entry of vermin to the sub-floor but do not resist the air flow unduly.

  • mike selwyn-hughes

    I’ve followed with interest your questions and the answers. I’m a little mystified why you posted the question in the first place as you now give the impression that you already know all of the answers. You appear to have got rid of your architect and seem to be unhappy with your engineer. My suggestion would be you discuss all of the issues you have regarding your project with the Building Inspector, I’m sure he will be able to advise.

  • Wangwen Zhao

    Mike, I did not know the answer mid way through the process. I am not confident with the architect as he misled us to this mess in the first place. He should have advised on an anti heave solution in the first place so we avoid delay and have a more clearer plan. I am equally not confident about the engineer from the piling contractor as he designed a 350 mm slab solution together with a lot of expensive gears. I went through a hard learning process and redesign the slab to only 200 mm thick.( I am a structural engineer but not for domestic buildings) I will certainly discuss with building inspector.

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