Underlayment

A layer that goes between the structure and the finished surface, and which means two different things depending on the trade. On a roof it is the sheet laid over the deck under the shingles, the second line of defence against water. Under a floor it is the layer over the subfloor that levels small imperfections, quietens the room and, in the right form, holds back moisture. Both are the part nobody sees and the part that decides how the visible work behaves.