Roof Deck (Low-Slope)

The roof deck is the structural layer that spans between beams, joists or purlins and carries the roofing assembly, snow, rain and equipment loads. It is the only structural component of a typical low-slope roof; everything above it, from vapour retarder to membrane, is non-structural. Deck type affects how insulation and membranes can be attached, fire ratings, drainage and condensation control.

Steel deck

Cold-formed corrugated steel decking is the most common deck for commercial low-slope roofs in North America. Insulation and membranes are mechanically fastened into the steel ribs or adhered over a board. Steel decks are light and quick to erect but flexible, so deflection must be considered in drainage design. The US Steel Deck Institute, formed in 1939, publishes design and manufacturing standards.

Concrete deck

Cast-in-place or precast concrete slabs are heavy, stiff and non-combustible. They are standard for inverted roofs, podiums, green and blue roofs and multi-storey residential buildings. In the UK, zero-fall inverted roofs under one warranty provider's policy are accepted only on in-situ reinforced concrete or rib-deck. Falls are often formed with screeds. New concrete releases moisture, so vapour control and drying time matter.

Wood deck

Plywood, OSB or solid timber boards on joists or glulam beams are typical on houses, extensions and smaller buildings. In the UK, falls are often formed with firring strips. Timber decks must be kept dry; BS 6229:2025 advises avoiding cross-laminated timber (CLT) roof decks where possible and sets conditions, including a minimum finished fall of 1:40, where they are used.

Poured gypsum deck

Gypsum concrete poured over formboards on bulb-tee sub-purlins was used on many North American roofs in the twentieth century. It is lightweight and fire-resistant but sensitive to prolonged wetting, and fastener pull-out must be tested in reroofing.

Cementitious wood fibre deck

Panels of wood wool or wood fibre bound with cement or magnesite have been made in Europe since the early twentieth century and were widely produced by 1930. They serve as roof deck, insulation and acoustic ceiling in one, and are common on schools, gymnasiums and pools. Fasteners and adhesives must be specifically approved for them.