Waterproofing under felt?

[beatts]

Hi,

I'm in the process of building a 5x3m summerhouse. It is in kit form and I am just at the roof stage. The roof is constructed of 20mm tongue and groove planks which is then to be coated in felt.

The slope of the roof is 1 in 50 along the length (5m) and I am wondering would it be a good idea to waterproof the T&G planks before applying the felt due to the slope being quite shallow and therefore not allowing for rapid water drain off.

If so how would I do this? Liquid bitumen along the T&G joins perhaps?

Thanks

 

[alan333]

The object is really to stop the water reaching the boards. What type/layers of felt are you putting on?

 

[beatts]

Hi,

The summerhouse came with 4 big rolls of felt. I paid extra for their premium felt but I couldn't tell you what that means

 

[alan333]

How are you laying the felt?

 

[beatts]

I've just been thinking about that. I was originally going to lay it lengthways in 4 long pieces with overlaps between each, bonded with felt adhesive.

However I now think laying it across the direction of flow would be better but would mean more joins

Should I lay under felt ?

 

[alan333]

I've not used felt adhesive so can't really comment on how good it is, but I have my doubts. The proper way to lay a felt roof onto t&g boards is to nail a layer of underlay (to allow for movement, not to waterproof it), then lay a further two layers stuck with bitumen, each layer also having edge flashings done. Stagger the laps of each layer, ie don't put a lap on top of a lap. Torch-on felt is widely used cos the bitumen is already stuck to the felt and only needs heating up to lay, thus saving messing about with tar boilers etc. The top layer is normally mineral surfaced felt. Three layers could be a bit overkill for a summer house tho, so maybe the middle layer could be skipped. It doesn't really matter whether you lay it across or along the roof.