Bitumen Felt Roofs

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Bitumen Felt Roofs

Laying bitumen felt roofs

Laying bitumen felt roofs

Bitumen felt roofs are probably the most common type of flat roof in the United Kingdom and though they are not recommended for use on habitable buildings, they quite often are. The processes involved in the cold construction of bitumen felt roofs (as opposed to hot construction of bitumen felt roofs where hot bitumen is used) in this instance are far more involved than for a shed or other outbuilding and should include the following stages: –

1. Preparation of the roof...

2. Fitting the first layer of underlay felt...

3. Fitting the second layer of underlay felt...

4. Dealing with gutters...

5. Laying the top layer of heavy duty mineral felt...

6. Dealing with the edging detail...

7. Finishing touches...

Useful tips for laying bitumen felt roofs

  • Bitumen felt roofs should not be laid in wet conditions as the timber deck should remain dry and the adhesive will be less efficient if damp. Quite apart from that, it is uncomfortable for the installers.
  • Bitumen felt roofs should not be laid in very cold conditions as the fibre based membranes are coated with oxidised bitumen and although the felt remains flexible down to around +5º centigrade, it is prone to cracking and damage at temperatures below that. Roofing felt should be stored at a temperature above +10º centigrade for at least 24 hours prior to use and it will be beneficial to unroll the felt at least 30 minutes prior use to improve its workability.
  • Only large headed clout nails should be used to fasten down the first layer of underlay felt and it would be unwise to cut costs by using ordinary nails which probably would not only fail hold the felt in place for very long but could cause tears if it were to move slightly. Normally 19 mm long clout nails should be used but you should ensure that when in place, they do not protrude below the timber deck.
  • Do not try to skimp on usage of the felt by reducing the amount of overlap below that recommended above.
  • Where an upstand has been created, such on the edges, apply adhesive to both surfaces to be fixed together.
  • All cuts to the felt should be on a cutting board and certainly not on the roof over a lower layer as it would be easy to cut through both layers at the same time.
  • Laying roofing felt can be hard work and generally two people will be required to carry out the work as the rolls of felt can be quite heavy to lift onto a roof.
  • Appropriate gloves should be worn to protect your hands when handling roofing felt and installing bitumen felt roofs, particularly the top layer which has a fine mineral or grit bonded to the surface.