A closer look at built up Bituminous Membranes or ‘Felt Roofing’ systems.
These tend to be the lowest costing form of Flat roof covering and as a result they are also likely to be the most familiar to great many of you. They can be used on pretty much any kind of supportive structure and are common for domestic applications. As commonplace as they are, they have still advanced to very high standards over the years, having earned a poor reputation back in the 90’s due to a mixture of poor materials and substandard workmanship.
Modern felt membranes are reinforced with polyester, glass or a combination of the two and are impregnated and further coated with bitumen. Typically two layers of felt will be used to create the roof surface. These will be bonded together using hot bitumen. The thinking behind this approach is that each layer will act to reinforce the other, if there is damage to one, the other will act as backup reducing the risk of wear and tear becoming a much bigger problem.
Felt is purchased on a roll and is generally referred to by the unique ingredients used in its manufacture, which will in turn impact upon its application. AAP is made from atactic and polypropoylene and is usually used for torch on techniques for instance, while SBS is made from butadiene sandwiched between two laters of styrene and is generally used for pour and roll techniques.
There are various different types of felt which will be used at different levels in the development of a layered structure, all of which should confirm to BS EN 13707:
- Type 5
- Class 5B or 5U felt is generally used for base and intermediate layers. 5B can additionally be used for top layers but only when it has been provided with adequate surface protection.
- Class 5E comes with mineral granules which acts and surface protection and this makes it ideal for top layers.
- Type 3
- All type 3 felt is less durable than type 5 felt. As a result it is generally used for intermediate layers only. It can be used as a base layer if it has been partially bonded.
- Type 3G is perforated which, if partially bonded, can be used as a venting base layer if required.
- Type 1
- These are the traditional rag based felts that were used back in the day. They have been removed from British standards and should no longer be in regular use.