Bitumen Felt Flat Roofs

Most short lived of the flat roof covering materials, unlikely to last much more than 15 years.

Normally made of 3 layers of bitumen felt bonded together with hot bitumen and finished with reflective grit. Ponding on the roof is not an urgent issue (as long as there are no leaks) but does indicate underlying problems which may further reduce the length of life of the roof.

Bitumen felt roofs need a reflective covering to reduce damage from heat. Reflective paint can lead to longer term damage to the material and stone chippings are preferred.

bitumen flat roof crop lite.jpg
Bitumen Flat RoofJohn GilbertNote the refelective paint used on the far roof and the total lack of reflective material on the nearer roof.

Problems

  • blisters that develop between the layers because the applied bitumen may not have been hot enough or because damp has got trapped underneath during laying
  • condensation on the underside can rot timbers
  • crazing of the surface - may be because some felt layers were not dry when installed
  • expansion and contraction may cause cracking of the membrane

Solutions

  • if there are bulges or cracks in the felt but the roof is not leaking there is no need to repair it immediately -think about how the work can be coordinated with other repairs.
  • trapped water in the membrane can sometimes be relieved by the installation of special vents
  • otherwise localised felt repairs may be needed

Professional help recommended?

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

Who Pays?

Roof repairs are normally a common responsibility.

Further information

Historic Scotland Inform Guide 'Bituminous Sheet Flat Roofs: Their Repair and Maintenance'