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Two Layers On Mastic Asphalt

Two layers on Mastic Asphalt


The roof to be refurbished must be surveyed by a H&J technical representative to determine if the existing mastic asphalt (B) will provide a suitable substrate. If the asphalt has extensive blistering this indicates trapped moisture vapour which may be relieved by cutting the blister and covering with a breather vent will help to relieve the problem but consideration should be given to stripping off the asphalt in its entirety.


The entire surface (B) to be waterproofed must be thoroughly cleaned, blisters and cracks cut out and filled with bitumen. Large blisters should be cut open and covered with a breather vent. Particular attention should be paid to upstands and drips which may have broken away from the substrate. If this has happened it is probably better to remove them.

  1. The asphalt (B) should be painted with bitumen primer (C) and if the surface of the asphalt (B) has become chalky it may require two coats.
  2. Apply over the outlet a flange made of a piece of membrane (50 cm by 50 cm).
  3. Place down the roof drain (E).
  4. The first layer should be Dermabit 30160 (F) and this should be fully torched on with 80mm longitudinal laps and 150 mm transverse laps when jointing to sheets.
  5. The cap sheet (G) should be either Casali Dermabit Extra 4170 smooth or Casali Dermabit Extra 43170 mineral. This should be fully torched to the underlay ensuring that the laps are staggered. This can be achieved by splitting the first run down the middle to give a 50 cm wide sheet.
  6. If Casali Dermabit Extra 4170 smooth is used as the cap sheet it should be painted with 2 coats of Aluminium Reflective Roof Paint (H).


Many asphalt roofs suffer from ponding and this can be difficult and expensive to eliminate, but if left it will tend to shorten the life of the felt.

Asphalt roofs which are 15 or 20 years old were often laid without insulation and this may be the time to upgrade by including insulation.

Please contact us if you would like to receive a quote from one of our Approved Contractors.


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