A layer of wood or other material designed to support a surface material; used under vinyl and other types of flooring as well as beneath shingles or other roofing material.

wood panels laid on the floor to provide a smooth surface to attach finish flooring.

Smooth panels of plywood or backer board used as a base for tile setting.

A material installed over the roof deck prior to the application of the primary roof covering. Usually consists of fifteen (15#) or thirty (30#) pound organic felt but can also be self-adhering such as an ice and water protection membrane.

A material, usually asphalt impregnated felt (which provides additional protection from the water) installed between the roofing and deck.

A material used between the laminate flooring and the subfloor (e.g. foam padding).

A layer of asphalt saturated {sometimes referred as tarpaper} which is installed over deck before shingles are installed, which provides additional protection for water intrusion.

The material that is laid underneath a laminate or ceramic floor. For example, plastic underlayment allows a laminate floor to float freely, while cement board provides a good surface for ceramic tiles.

Sheet material placed over a subfloor or old floor covering to provide a smooth, even surface for a new covering. Underlayment is usually sheets of hardboard, particleboard or plywood.

Top layer of plywood (or other material) in a two-layer floor. Provides a smooth base for carpet, tile or sheet flooring.

The material (usually roofing felt) laid on top of sheathing before shingles are applied.

Weather-resisitant material placed under vinyl siding panels.

Foam or other material such as Cork (which is also used as a sound barrier) and laid over an existing floor or subfloor. Floating floor planks or tiles are installed on top of the underlayment material.

A material, such as saturated No. 15 felt, placed on the roof deck to improve weather resistance.

A layer of wood between the sub-floor and the floor.

A material used between the laminate flooring and the subfloor. This material serves as a means to keep moisture away from the laminate. Certain varieties of underlayment are designed to cut down on sound.

A 1/4" material placed over the subfloor plywood sheeting and under finish coverings, such as vinyl flooring, to provide a smooth, even surface. Also a secondary roofing layer that is waterproof or water-resistant, installed on the roof deck and beneath shingles or other roof-finishing layer. [Go to source

Layer of wood between the floor and the subfloor.

A layer of asphalt saturated (sometimes referred to as tar paper) which is laid down on a bare deck before shingles are installed to provide additional protection for the deck.

Material laid between a sub-floor and a finish floor.

felt paper, base sheet, vapor barrier

Wood panels as used under a finished floor to provide a smooth finish.

thin sheet or building paper placed between two building Components to ensure that slip (relative movement) can take place. A concrete beam cast on top of a brick partition needs to be separated from it because otherwise the partition would crack. A damp course may act as an underlay. Clay flooring tiles need an underlay below the screed they are bedded on, so that the slight expansion of the tiles does not result in the disastrous failures by arching or lifting that have happened in the past. There should also be an expansion joint at the skirting. The reasons for an underlay below asphalt or flexible‑metal roofing are similar and below slates or tiles it ensures a watertight roof. Hardboard or plywood placed over a rough floor to make a smooth surface suitable for laying floor vinyl, cork tiles, and parquet flooring. Etc.

A material placed under finish coverings, such as flooring, or shingles, to provide a smooth, even surface for applying the finish.

an asphalt-saturated felt or other sheet material (may be self-adhering) installed between the roof deck and the roof covering, usually used in a steep-slope roof construction. Underlayment is primarily used to separate the roof covering from the roof deck, to shed water and to provide secondary weather protection for the roof area of the building.

Moisture resistant material, such as asphalt-treated paper, applied over roof and wall sheathing and under roof and exterior finish to prevent water from entering the structure. Also used between sub-flooring and finish floors.

A layer of wood between the subfloor and the floor.

One or more layers of felt, sheathing paper, non-bituminous saturated felt, or other approved material over which a steep-slope roof covering is applied.

A flooring layer over the base sub-flooring, over which tile or resilient floor covering is laid.

Asphalt saturated felt used beneath roofing to provide additional protection for the deck.

Weather-resistant material placed under vinyl siding panels.

A material used to cover sub-floor irregularities and to absorb thte movement of wood subfloors prior to the installation of the finish flooring materirals.

An asphalt saturated felt applied over the roof deck and under the roofing material.

A layer of plywood or other manufactured board used as a base material under finished flooring. Underlayment is often used as a substrate to increase the strength and/or smoothness of the flooring.

A layer of hardboard, particleboard, plywood, mastic with latex binders, mastic with asphalt binders, mastic with polyvinyl-acetate binders, etc., placed to cover subfloor irregularities, to absorb the movement of wood subfloors and to provide a smooth surface for the finish flooring material.

refers to the material that is laid underneath the laminate floor. Generally a plastic material which allows the laminate floor to float freely. Some underlayments are designed to prevent moisture from directly contacting the backing of the flooring and some underlayments help reduce sound levels generated when walking over a laminate floor.