Floor Underlayment

Underlayment is a layer of sheeting that's screwed or nailed to the subfloor to provide a smooth, stable surface for the floor covering. The type of underlayment you choose depends in part on the type of floor covering you plan to install. For example, ceramic and natural stone tile floors usually require an underlayment that stands up to moisture, such as cementboard. For vinyl flooring, use a quality-grade plywood since most manufacturers' warranties are void if the flooring is installed over substandard underlayments. If you're using your old flooring as underlayment, apply an embossing leveler to prepare for the new installation (see opposite page, bottom). Most wood flooring carpeting do not require underlayment and are often placed directly on a subfloor.

Plywood is the most common underlayment for vinyl flooring and some ceramic tile installations. For vinyl, use 14" exterior-grade, AC plywood. This type has one smooth side for a quality surface. Wood-based floor coverings, like parquet, can be installed over lower-quality exterior-grade plywood. For ceramic tile, use 1% AC plywood. When installing plywood, leave A' expansion gaps at the walls and between sheets.

Fiber/cementboard is a thin, high-density underlayment used under ceramic tile and vinyl flooring in situations where floor height is a concern.

Cementboard is used only for ceramic tile or stone tile installations. It's stable even when wet and is therefore the best underlayment to use in areas likely to get wet, such as bathrooms. Cementboard is more expensive than plywood, but it's a good investment for a large tile installation.

Isolation membrane is used to protect ceramic tile installations from movement that may occur on cracked concrete floors. It's used primarily for covering individual cracks, but it can be used over an entire floor. Isolation membrane is also available in a liquid form that can be poured over the project area.