Question

Under what conditions might you specify a slip sheet under a new concrete slab? Would you require a granular bed above or below the slip sheet? And would you accept a slip sheet if it becomes punctured?

Answer

Slip sheets are sometimes specified for use under concrete slabs to diminish friction between the subgrade and slab bottom as the slab shrinks during curing. This helps minimize cracking particularly where control joints are widely spaced or the slab is post-tensioned. A slip sheet should not be confused with a vapor barrier, although if so designed, it can sometimes also serve as a vapor barrier. The slip sheet is usually punctured to allow moisture to escape, and no granular fill separates it from the slab. A double layer of slip sheets is the most effective.