Flat roof in torch-on felt.

If you are in need of a new flat roof a new, three layer torch-on felt system is can be good solution. Bitumen has been used as a flat roof covering since the time of the Romans. There is a reason for this, when installed properly bitumen is completely water tight, flexible and very long lasting.

Recently there have been a number of new solutions to covering a flat roof which hit the market, notably EDPM rubber flat roofing systems and GRP fiberglass flat roofing systems. It has been interesting to see the manufacturers and installers of these flat roofing systems launch smear campaigns against the trusty torch-on felt systems. Yes, there are bad 3 layer felt jobs out there, they are installed by insufficiently skilled tradesmen, it is not the materials that are at fault.

> 20 year plus life expectancy.
> No need for new boards unless rotten.
> Can be a less expensive option.

These smear campaigns are quite understandable from a marketing point of view as bitumen flat roofs still service then lions share of the market. The truth of the matter is that if any of the three mentioned flat roofing systems are not installed properly they are all likely to fail prematurely . There will be stories of bad rubber roof and fiberglass roof installations too unfortunatly.

A well installed 3 layer felt system will last in excess of 20 years which is not quite as good as the EPDM rubber of GRP fiberglass systems but, if the boards of your flat roof are free from rot, they can be reused with a 3 layer felt system, this is not possible with either EPDM rubber or GRP fiberglass and can mean that the bitumin felt option is upto 50 percent cheaper. If you do need to reboard the cost becomes comparable to the other two systems.

Cap sheet on the left, underlay on the right.

It is usually advisable to remove the old flat roof covering. (there are times when it may be advisable to cover the old flat roof with a fiber board or insulation and go over the top) Once the old covering is off the boards can then be inspected and and decision can be made as to whether any boards need replacing due to water ingress causing rot. Ply boards are preferable to OSB in my opinion.

Once the flat roofing boards are ready to work on you would look for any right angles where the flat roof meets an obstacle or termination point, such as a wall. Right angles cause weak points in the felt and the angle should be reduced with arris rail.

Now the first layer of underlay is ready to go down, the first layer should be tacked down into the flat roofing boards with aluminum clout nails rather than torched down to avoid air bubbles, Next the drip edges are formed and torched down to the first layer of underlay.

Once the drip edges are completed the second layer of underlay is torched down, it is important to lay the underlay upto the drip edges but not over the top of them as this would create a raised edge and an obstacle to water flow. Finally the cap sheet can be torched down and any detail work completed.

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Service	Price
Replace Flat roof with 3 layer system	approx £50 per square meter
Replace boards with new ply boards	approx £50 per 8ft x 4ft board