What is a built up felt roof?

A built up felt roof is an established method of weather proofing roofs, normally flat roofs, by using more than one layer of bituminous roofing felt which is bonded to the previous layer.

Thee layers are usually laid with chippings which is layed on top to protect the felt from the sun, or a mineral cap sheet is sometimes used instead.

What is felt and are there different types?

Felt is what keeps the rain out; it comes in rolls of 1 metre wide and in various lengths from 8 meters up to 20 metres depending on the type and manufacturer.

The felt is made up of a mat of fibres coated on both sides with a bituminous coating. The felt surface is then finished with a light sand which stops the felt from sticking to itself.

There are lots of different types of felt for various types of applications; the main differences are lay in the various fibre mats which are used as well as the finished surface.

Modern felts generally use either a glass fibre mat or for more high performance polyester mats. The finished surfaces are either made of sand or slate granules for a mineral finish.

The aforementioned mineral finish can also come in a variety of colours, usually green, blue/grey, red/brown or white. These colours, other than green are only available on the high performance felts.

The mineral finished felt is commonly used for edging detail work and as a cap sheet if chippings are not required.

Advantages of High Performance felt

The main advantage of High Performance felt is it’s life span. Any well designed, well carried out felted roof should last roughly about 15 years. But for an extra quarter if the price, the same roof with High Performance felt should last twice as long! (roughly 30 years).

On that basis, the actual cost per year during the life of the roof will be greatly reduced.