No Plastic Under Concrete Slab

Almost everywhere except california that slabs are placed on the ground a plastic sheet a vapor barrier is located directly under the concrete slab and on top of the ground photograph 1.

No plastic under concrete slab.

This water can take literally years to dry out through the top of the concrete slab. This plastic sheet is in direct contact with the concrete as it should be in a rational world. Why do concrete slabs need black plastic membranes under them. The structural why fil.

So it is common to use crushed stones with no fines as a capillary break and then add a plastic sheet over the top of the stones in contact with the concrete as a vapor barrier. What is a vapour barrier under a concrete slab and why do you need one. So why not just use plastic and not stones. If the plastic is right under the concrete the bleed water is forced to rise to the surface.

Thick concrete samples having a w c 0 50 and sealed from below required 82 days for the vapor emission rate to reach 3 0. Or just use the plastic sheet to do both be the vapor barrier and the capillary break. Although it may seem water tight this grade of material allows a lot of water vapor to pass through. One laboratory drying study indicated that 4 in.

One contractor says to put plastic under the concrete it prevents moisture from coming up into the concrete. In fact most professional shed slab builders will install a polyethylene membrane under a shed concrete slab even if it is not specified. Recently though research has shown that the old traditional layer of 6 mil visqueen polyethylene plastic under the slab is seldom effective for two main reasons. The reason is that the vapour barrier stops moisture from the wet concrete escaping into the ground under the concrete.

A concrete vapor barrier is simply a sheet of polyethylene plastic visqueen placed directly on top of the sub grade before the concrete floor or slab is poured to help keep moisture from the soil from passing up through the concrete.