Felt underlay - Why?

[beatts]

Why would you use underlay underneath shed felt?

just a curiosity as a few people have suggested it, and not said why.


Also if applied properly, why won't one layer of felt do the job on a large shed roof? Some people are saying there should be 2 or 3 layers. That would make the folds at the edges / corners very difficult I would imagine.

 

[noseall]

One layer of decent green mineral will do for a shed. Stop listening to people who haven't got a clue.

 

[alan333]

If it's the big summer house from your other thread then IMHO it needs underlay cos it's on t&g boards. The boards expand and contract at a different rate from the felt so the felt will crack and/or blister over time. Nailing a layer of underlay allows the boards to move about underneath it, and the top felt layer(s) are unaffected. Obviously this underlay layer with nail holes all over it won't be waterproof, so you need at least one layer over it for protection. Also with your fall of 1 in 50 over a 5m run your laps (on the top layer) will need to be waterproof, hence my suggestion of torching (or you could pour and roll) a cap sheet. If it's decent quality mineral felt you have, often it can be torched on, carefully and using less heat, even tho it doesn't have the layer of bitumen on the back.

On the other hand if it's a shed with a traditional pitched roof made of ply sheets or similar then one decent layer will be plenty for years to come, as per noseall's suggestion.

 

[beatts]

Thanks for your informative reply Alan.
It now winds me up that after spending 2 grand on a summerhouse with quick gardens, they only supply it with roof felt and don't mention what you have suggested.

Also I have had to add firrings to give it the 1in50 fall otherwise there would have been no fall whatsoever. Don't buy from quick gardens!

 

[alan333]

This is the type of underlay I'd use. Glass fibre based type 3b underlay. It'll be expensive from the diy shops but should be cheaper from your local roofing merchants. Nail it using 3/4 inch galvanised clout nails at about 9 inch centres. Allow minimum laps of 2 inch at the sides and 4 inch at ends.

http://www.jjroofingsupplies.co.uk/anderson-3b-glass-fibre.html

The technique for torching is to have the strip of felt rolled up, and apply heat using a torch to the front/bottom of the roll as you roll it, thus melting the bitumen on the bottom of the felt. This also melts the top surface of the layer below, thus bringing it's bitumen to the surface. If your premium felt isn't torch-on then it won't have the bitumen layer on the bottom, so it can't take too much heat before it becomes floppy or burnt. However if you "burn up" the surface of the underlay first by torching it until the bitumen comes to the surface...then the cap sheet doesn't need so much heat to apply. Therefore you can probably torch your existing premium felt if you wish. Be careful to have the laps well stuck together tho.

BTW (for all the roofers reading this saying "that'll never work") I doubted whether it would work effectively myself but I had a customer years ago who had non torch-on felt (3E IIRC) and wanted it laid onto an existing roof. It went on well, after burning up the surface, and easily lasted approx 12 years until the building was demolished.