A batten is a thin strip of solid material (usually wood). Battens are used for various purposes in building construction, as well as other various fields.

Roofing battens

Battens are used to provide the fixing point for roofing sheet or roof tiles. The orientation and spacing of the battens depends on the type of roof. Battens may be oriented at right angles to the trusses or rafters of a roof, like purlins. They may be parallel to the slope of the roof, as on a batten seam roof, where the battens cover seams in the roofing material and are themselves covered by metal caps. Some roofs may use a grid of battens in both directions, known as a counter-batten system.

Batten trim

Batten trim, or batten molding, is a thin strip of trim, typically with a rectangular cross-section, used to cover seams between panels of exterior siding or interior paneling.

Board and batten

Board-and-batten siding is an exterior treatment of vertical boards with battens covering the seams.

Other uses

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In sailing, battens are long, thin strips (usually fiberglass, or some similar material, nowadays, but historically wooden) used to support the roach of a sail.
Also used on tall ships to form the ladders up the shrouds, same as ratlines.

In cabinetry, battens may be used to strengthen panels made up of multiple boards, as in a batten door, or to cover joins.

In stagecraft, a batten or rail is a long pipe, usually metal, affixed to the ceiling or fly system in a theater.

In the steel industry, battens may also be referred to as "Top hats," in reference to the profile of the metal.