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.