Definition
Underlayment is a thin layer of material installed between the subfloor and the finish flooring. It provides smoothness, moisture protection, sound reduction, thermal insulation, and cushioning depending on the type. Common underlayment materials include foam (for laminate and floating floors), felt or rubber (for carpet), cement board (for tile), and plywood (for hard surfaces over existing floors).
Formula
Underlayment typically comes in rolls covering 100-400 square feet. Add 5-10% for seam overlap and waste. For foam underlayment, rolls are often 3 feet wide by various lengths. Butt seams together (no overlap) for thin foam, or follow manufacturer instructions for overlap requirements. Tape seams for moisture barriers.
Example
Underlayment Selection Example
For a floating laminate floor: Use 2-3mm foam underlayment with attached vapor barrier for concrete subfloors. For tile installations: Use 1/4 inch cement board underlayment screwed into the subfloor, with thinset mortar between layers. For hardwood over concrete: First install a moisture barrier (15lb felt or epoxy), then 1/2 inch plywood underlayment.