Mastic Asphalt Roofing

Mastic asphalt is a dense, void-free mixture of bitumen and finely graded mineral aggregate, typically limestone, that is heated until it is fluid and then spread by hand with wooden floats. On cooling it forms a hard, seamless and fully monolithic waterproof layer. It has been used for roofs, balconies, car parks and tanking in Britain and continental Europe since the nineteenth century, when natural rock asphalts from sites such as Seyssel in France were first used for paving in the 1820s and 1830s.

How it is laid

The material is delivered as blocks or hot in mixers, heated on site in cauldrons or mixers, and carried to the roof in buckets. The asphalter spreads it in successive coats over an isolating underlay, usually a loose-laid sheathing felt that separates the asphalt from the deck and lets each move independently. Upstands and skirtings are formed in the same material so the roof, edges and details are one continuous piece. A solar-reflective finish such as chippings or a coating is usually applied to reduce heat gain and surface crazing.

Characteristics

• Seamless and monolithic: no laps or seams, which suits complex shapes and detailing.
• Hard-wearing: can take foot traffic and, in paving grades, vehicles; widely used on podiums and car-park decks.
• Durable: long service lives are reported where well maintained.
• Heavy and rigid: needs a stable deck, typically concrete; less able to accommodate movement than flexible membranes.
• Hot work: application involves heating to high temperatures.

Current use

Mastic asphalt remains an established UK flat roofing method for new build and refurbishment, often in inverted and podium roofs, and in balconies and walkways. Its market share has declined as reinforced bitumen membranes, single-ply and liquid systems have grown.