Q.: While we were getting ready to build some slabs that required protection against moisture intrusion, we searched for all the information we could find on vapor barriers. Looking through the American Concrete Institute booklet Slabs on Grade (2nd ed.), we noticed slip sheets mentioned in the same section that discusses vapor barriers? What are slip sheets? And do we need them too?

A.: A slip sheet is a layer of material placed between the slab and the subgrade to reduce the friction between them. When a concrete slab dries it shrinks. As the concrete begins to shorten, some cracking can occur because the subgrade resists slab movement. Slip sheets reduce this frictional resistance and help reduce cracking. The greatest friction reduction is obtained when two sheets of plastic are placed on top of the subgrade directly beneath the slab. Puncturing the sheets before placing concrete allows water to escape from the bottom of the newly placed slab.

Slip sheets might be used under slabs with extra-long joint spacings or under post-tensioned slabs. A force applied to post-tensioning tendons induces a compressive prestress in the concrete, slightly shortening the slab. However, because of frictional resistance to this shortening, especially in long slabs, all the force in the tendons isn't transferred to the concrete. Thus, the compressive prestress may not be as great as desired. Slip sheets reduce friction so more of the tendon force is transferred to the concrete.

Don't confuse slip sheets with vapor barriers (or vapor retarders) just because they may be made of the same material. Unlike vapor barriers, which shouldn't be punctured, slip sheets are usually punctured purposely to allow water to escape from the bottom of the slab into the subgrade, and there is never a layer of granular material (blotter layer) on top of them. However, if slip sheets must also serve as vapor barriers, they aren't punctured.