Replacing floor underlayment and subflooring is a home improvement project that takes planning and attention to detail. Underlayment is a protective layer of sheeting, that is installed between the subfloor and the top floor covering. An effective and correctly-installed underlayment protects the floor covering from moisture and cracks, due to wear and tear. Subfloors provide a solid base for various floor coverings, and are often made from plywood panels. There are some factors to look for when determining if either of these need to be replaced.
Any type of floor underlayment should be smooth and level. If sections of your underlayment are wearing thin or becoming uneven, this is a good sign the underlayment needs to be replaced. If your floor underlayment includes an isolation membrane, check this for any visible tears or holes.
Inspect the subfloor for any chips, cracks, knots, or loose plywood panels. All but the largest cracks and dips can be filled with a floor leveling compound applied with a putty knife. Loose plywood with no other damage can be secured down with basic wood screws. If there are any signs of dry rot or water damage to sections of your subfloor, these need to be pulled up and replaced before installing new underlayment.