Flat Roof

A flat roof is a roof with little or no visible slope. In practice, flat roofs are not truly level: they are laid to a small fall so water drains to gutters, scuppers or internal outlets, and they are covered with continuous waterproof membranes rather than the overlapping shingles, tiles or slates of pitched roofs. In North America they are usually called low-slope roofs.

History and geography

Flat roofs are among the oldest roof forms, traditional in arid and low-rainfall regions of the Middle East, North Africa, South Asia, the Mediterranean and the American Southwest, where packed earth, mud or lime over timber beams formed usable roof terraces. Modern waterproofing materials have since made low-slope roofs practical in wet climates and on large commercial buildings, and they are now common on single-storey extensions and contemporary houses.

Drainage falls

Codes and standards set minimum falls. British guidance commonly cites a minimum finished fall of 1 in 80 for flat roofs so that construction tolerances do not leave ponding; US codes likewise set minimum slopes for each type of low-slope covering. Tapered insulation and crickets are used to create falls and direct water to outlets.

Coverings

• Built-up roofing (BUR) with multiple bitumen plies.
• Modified bitumen (SBS or APP) sheets.
• Single-ply membranes: EPDM, TPO, PVC.
• Liquid-applied membranes.
• Metal (such as zinc or copper with batten-roll or standing seams on very low slopes).
• Asphalt and mastic asphalt, traditional in the UK.
• Green (vegetated) roofs over a waterproofing layer.

Warm, cold and inverted

Flat roofs are classified by insulation position: cold (insulation below the deck with a ventilated void), warm (insulation above the deck under the membrane) and inverted (insulation above the membrane). Many authorities now favour warm or inverted designs because ventilated cold flat roofs are prone to condensation.

Advantages and drawbacks

Flat roofs can provide usable terraces and space for equipment and solar arrays, and avoid the volume of a pitched roof. They rely entirely on the membrane for waterproofing, are sensitive to ponding and detailing at upstands, and need regular inspection.