Sand under DPM to raise floor

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We had a shed built last year which is leaking at the base. The builder has agreed to raise the concrete floor and install a DPM lapping into existing DPC. We asked for another builder’s advice and he said that the DPM should have sand under it to stop it puncturing. However I thought I saw on see on some sites that you can put a membrane directly onto concrete but not hardcore?

Which is correct??

Also is it fair to insist the original builder specs out the repair in detail eg materials and methods? He is refusing to do so and just says he will fix it as noted above, but I prefer more detail ( not least to check it won’t be a bodge job).

Many thanks
Paul.
 
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Yes a sand blinding is recommended to stop the DPM being damaged - particularly over hardcore as any larger sharp stones can puncture the DPM if they are walked over.
If the concrete floor is level and smooth and whoever puts the DPM down is careful not to damage it then the sand blinding isn't a necessity.
 
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You could apply a liquid DPM product on the surface of the concrete slab.

Placing a polythene barrier on top now means that what is placed above that is limited and needs careful thought.
 
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There are other options to protect the DPM, e.g. insulation, pond liner underlay, old carpet (man made fibres only), but old carpet might smell eventually.
 

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