Why do flooring manufacturers recommend not smoothing true expansion joints with any type of underlayment product?
True expansion joints are those placed between separate pours of concrete. These joints are designed to absorb the movement of the concrete and are normally filled with elastomeric fillers which absorb the movement of the separate pieces of concrete. If an underlayment is placed across or in these joints, the movement of the concrete will cause the underlayment to break up or will push it out of the joint. Additionally, any flooring placed across these joints will break or crack with the movement of the concrete. Expansion joint covers, which are designed to span these joints, should be used in these areas.
