On stone tenements, asphalt roofs are found on:

  • ‘high backs’ – raised backcourt areas above ground floor commercial premises
  • the top floor roof which may have been designed as a clothes drying area
  • deck access walkways

Asphalt is applied hot, usually in two coats onto a felt membrane, which isolates it from the roof. It is finished with small white chips to reduce solar radiation which can soften the asphalt. If it’s not possible to cover with chips (the preferred solution), an appropriate reflective paint may be used.

Asphalt flat roof. The white reflective coating has faded. Upstands are most at risk as the asphalt can slump in the heat.

Problems
  • asphalt needs a reflective covering to stop heating and cooling which leads to cracking
  • protective chippings may need to be re-spread
  • if the asphalt is bonded directly to walls or concrete decks, any movement will lead to cracks
  • insulation may be poor and if there is no vapour barrier below, condensation can develop on the underside of the deck causing bubbles which can pop and lead to leaks

If there is shallow ponding on the roof, it is not an urgent problem unless the roof is leaking. However, over the long term, ponding can lead to problems. Large or deep ponds add weight to the roof and problems can occur at the edges, so keep an eye on the situation.

Solutions
  • if cracks are no deeper than 3mm, then painting on bitumen may be sufficient to seal cracks
  • slumping asphalt upstands may need to be replaced, using a stainless steel mesh to support the asphalt on vertical surfaces
  • polymer-modified mastic asphalt is now available which is less prone to heating and cooling problems

Slumping asphalt upstand. A stainless steel mesh backing should be used here and a compatible solar reflective paint should also help.

Professional help recommended?

If the flat roof is covering large part of your building and needs major repair or replacement, get a professional survey and specification before proceeding.

Who pays?

Roof repairs are normally a common responsibility.

Further information

Mastic Asphalt Council Technical Guides (see ‘Roofing’ sub-section)

406 305 Under One Roof

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