I wish to convert my internal garage into a living space
I shall be installing a wooden floor (chipboard) on top of the current concrete slab
I believe there is a DPC in the floor but am not sure
Do I need to lay a PVC DPC under the timber joists that will be fixed to the floor (it is only being raised by 2") or a DPC above the insulation
under the chipboard that is to be laid on top or do I need both?
You really must be certain if there is a damp-proof membrane under the concrete otherwise after laying the new floor, the humidity may increase, joists may rot and the chipboard could suffer. If there is no membrane, consider laying the floor on top of Delta FM floor membrane or similar. Alternatively, apply water-based epoxy floor membrane (e.g. Remmers Viscacid BS3000).
If existing concrete is quite dry, sensible precaution to put DPC under joists but do not cover joists with membrane because of possible humidity problems.
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