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...

The roof structure for bitumen felt roofs should consist of either water resistant plywood or Sterling board (which is made of softwood strands, compressed and glued together with water-resistant resins) fixed to treated timber joists with all nail heads flush with the surface and there should be no sharp splinters of wood capable of puncturing the roofing felt. The timber boards on bitumen felt roofs should be laid with a fall as little as 1 in 80 but it can be up to 1 in 40 (expressed as a percentage these are respectively, 1.25% and 2.5%) to provide for water run-off.

Where existing bitumen felt roofs are being refurbished, all the old felt, nails and adhesive should be removed and it may help to remove the gutters and gutter brackets. Any wet timber should either be allowed to dry out or be replaced if it is rotten and all remaining timber should be clean, dry and in good condition. Timber triangular fillets should be fixed around the perimeter where up-stands are required and timber fascia boards should also be fixed at this point.  Where new bitumen flat roofs abut a higher wall, the roofing felt should be dressed up the wall and covered with a suitable flashing fixed into a mortar joint – this joint should be grooved out prior to starting the roof so that the prepared roof can be brushed clean of all debris.

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

Felt for bitumen felt roofs is typically supplied in rolls of one metre wide and ten metres long. It should be fitted in dry weather and unrolled at least 30 minutes prior to being used to improve its workability. The first layer is sanded roofing felt of underlay quality and should be measured and cut to size prior to fixing.

The underlay felt should be laid at right angles to the direction of fall and starting from the lowest part of the flat roof, should be nailed to the wooden decking with 19 mm large headed clout nails at 150 mm centres. There should be a 50 mm overlap along the run (or long edge) of the felt and 100 mm where the roll comes to an end. Nails along joints should be at 50 mm centres and this procedure should be continued until the whole roof is covered.

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

With bitumen felt roofs, the second layer of roofing felt is also sanded underlay but this time it should be bonded over the first layer using felt adhesive and should not be nailed down. It is important that the joints of the two layers of felt on bitumen felt roofs do not coincide with each other and this can be avoided by starting to lay the second layer with a half width roll.

The surface of the first layer of felt in bitumen felt roofs is covered with adhesive but the felt should be measured and cut to size prior to laying, smoothing it down to avoid any air pockets between the two layers. A broom is useful for this part of the process. Again, a 50 mm overlap should be allowed along the run (or long edge) of the felt and 100 mm where the roll comes to an end. This procedure is continued until the whole roof is covered with particular attention given to the roof edges to ensure that they are bonded properly to the layer below.

4. Dealing with gutters...

On bitumen felt roofs, a gutter should be fitted along the low edge of the roof to take surface rain water, and to direct it into the gutter a welted drip should be formed along the edge of the flat roof. To form the welted drip a timber batten should be nailed to the vertical part of the structure, along the line of the roof edge.

A strip of roofing felt is then bonded with felt adhesive over the edge of the roof (with approximately 150 mm on top of the roof) and dressed below the timber batten, folded and then bonded to its face. This strip of roofing felt will direct surface rain water from the roof, away from the walls of the building and into the gutter. It is important that this detail is formed prior to the top layer being applied.

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

The third layer is heavyweight roofing felt with a mineral layer and should again be bonded on to the undelay layer of felt below, using felt adhesive – it should not be nailed down. Mineral felt has a 50 mm strip along the long edge which is not coated in mineral or grit, this is known as the selvedge and aids adhesion where the felt is overlapped. It is important that the joints of the second and third layers of felt do not coincide.

The surface of the second layer of felt is covered with adhesive, the felt should be measured and cut to size prior to laying, smoothing it down to avoid any air pockets between the two layers with a broom. This procedure is continued until the whole roof is covered, with particular attention given to the edge of the felt which should stop approximately 25 mm from the edge of the roof.

6. Dealing with the edging detail...

The low edge of bitumen felt roofs has been dealt with in the section on gutters above but you must also prepare the detail along the other edges, which include a welted drip onto a verge and where the flat roof abuts a vertical wall.

A welted drip onto a verge is created in the same way as that for a gutter but instead of directing surface rain water into a gutter, the purpose is to stop rainwater driving over the edge and direct it away from the vertical wall of the flat roofed building where it could cause a damp problem with water ingress into the building. To form the welted drip along a verge, a timber batten should be nailed to the vertical part of the structure, along the line of the roof edge.

A strip of roofing felt is then bonded with felt adhesive over the edge of the roof (with approximately 150 mm on top of the roof) and dressed below the timber batten, folded and then bonded to its face. This strip of roofing felt will direct surface rainwater away from the roof and from the walls of the building.

Where a flat roof abuts a higher wall a length of triangular timber fillet should be fixed onto the roof at the intersection of the roof and the wall and covered with a strip of felt which is fixed into position with adhesive, with 150 mm on the roof and 150 mm up the wall. This is then finished with flashing which is fed into a mortar joint (cleared out at the preparation stage) and re-pointed. The flashing should overlap the felt upstand almost down to the level of the flat roof.

7. Finishing touches...

Finally, the whole roof should be checked to ensure that surface water runs off as intended and adjustments should be made where required if it does not.

Additional adhesive can be applied at the joints on the top layer of roofing felt to help improve its performance in bad weather.

Once the flat roof surfacing has been completed the gutter and down pipes can be fitted but if it is an old roof which has been refurbished, new gutters and down pipes should be used rather than re-fitting the old ones, which may leak.

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.