Mastic asphalt is a dense, voidless waterproofing and flooring material made from suitably graded limestone aggregates bound together with an asphaltic cement, primarily refined bitumen. Unlike rolled asphalt, it cannot be compacted — it is applied by hand float, which gives it the seamless, monolithic finish that makes it suitable for roofing, flooring, tanking and paving applications alike. British Standard mastic asphalt is manufactured to BS 6925:1988, a specification covering roofing grades, flooring grades, tanking and several other application-specific mixes. For practitioners on site, understanding which grade and coat thickness applies to which substrate is the difference between a membrane that lasts for decades and one that fails prematurely.

What exactly is mastic asphalt made of?

The core material is a mix of limestone aggregate bound with bitumen-based asphaltic cement, producing a compact, void-free compound. Because it cannot be rolled or compacted like road asphalt, it is laid using a hand float — a detail that shapes how it behaves on sloping, vertical and horizontal surfaces alike. The system functions as part of the broader family of waterproofing and sealing products (Abdichtung) used across roofing, flooring and below-ground tanking.

Which BS 6925 grades should you specify?

British Standard mastic asphalt comes in distinct grades depending on the application. Knowing the grade designation avoids mismatched specification on site:

  • Roofing: grades R988-B and R988-t25
  • Flooring: grades F1076-1, F1076-2, F1076-3 and F1076-4
  • Tanking: grade T1097-t25

Supply is typically available either as palletised blocks or delivered hot in a tanker ready for installation.

How many coats and what thickness does each surface need?

Coat count and thickness depend on the slope, exposure and whether the finish will be buried. The number of coats must suit the waterproofing requirements and traffic conditions of the roof, with detailed application laid out in BS 8218:1998. As a practical reference on site:

  • Falls of 1:80 or steeper: two coats to a thickness of 20mm
  • Falls below 1:80 or buried specification: three layers to a total thickness of 30mm
  • Horizontal surfaces up to and including a 10° pitch: two coats, 20mm thick, laid over a separating membrane of sheathing felt
  • Sloping and vertical surfaces over 10° pitch: three coats, 20mm thick, without a separating membrane
  • Timber or lightweight concrete slopes: three coats, 20mm thick, over expanded metal lathing and a separating membrane of sheathing felt
  • Roof gardens, reservoirs or buried waterproofing: three coats to a 30mm thickness over a separating membrane of GRP tissue
  • Vapour barrier applications: a single coat at least 10mm thick on GRP tissue

How are skirtings and upstands specified?

Skirtings and upstands up to 300mm high are applied in coats to a thickness of 13mm. Above 300mm, the specification calls for three coats to a thickness of 20mm, though two-coat work may be acceptable on vertical upstands over 300mm in areas not exposed to the elements, such as tank rooms or mechanical services areas, where appearance is not a priority. Where timber or lightweight concrete forms the substrate, three coats of 20mm are applied over expanded metal lathing and a black sheathing felt separating membrane.

How do you protect and finish the surface?

All asphalt roofing, including upstands, needs protection against static point loading and mechanical damage. On inverted roofs, ballast and insulation provide this protection and should be installed as soon as practicable after the mastic asphalt laying is complete. On horizontal and slightly sloping surfaces, a finishing step involves rubbing clean, sharp sand evenly into the still-warm surface using a wooden float immediately after laying.

What about vapour control on warm deck roofs?

Any provision to control interstitial condensation should follow the guidance in BS 6229:1982, with the calculation method adjusted to align with BS 5250:1989 (1995). On warm deck roofs, insulation boards are bedded in hot bitumen onto the vapour control layer, with joints close-butted and movement joints staggered — leaving a fair margin between insulation slabs and all skirtings and abutments to allow for infill.

Practice take-away

Before ordering material, match the BS 6925 grade to the application — roofing, flooring or tanking — and confirm the fall of the surface, since this dictates whether two or three coats are required. For any system interfacing with thermal insulation under insulation standards, check the separating membrane requirement carefully: getting the membrane and lathing detail wrong at skirtings and upstands is one of the more common causes of early membrane failure on refurbishment projects.