Flooring underlay is a product which is installed between an existing structural sub-floor, and the non-structural floor. In the case of floating floors, the underlay is a rubbery material, usually 2mm thick (some products are thicker), which cushions the floating floor, provides sound absorption, and may also provide a moisture barrier. In the case of solid overlay flooring, the underlay, such as non-structural plywood, serves as an intermediary between the overlay floor and the structural floor. The underlay is attached to the structural floor such as joists or concrete slab, whilst the overlay floor is attached to the underlay