Two layers of top felt on shed?

[beatts]

I am felting my summerhouse.

I am aware of the best practise of at least one Layer of underlay nailed to the deck before a cap sheet is glued to the top with cold adhesive.

However.

I have been supplied with way too much felt. It didn't come with any underlay but I have worked out I could felt the roof twice with the cap sheet felt they have supplied. It's quite thick with a gravelly / stone chipping coating.


Could I just put two layers of this stuff on instead, nailing the first then bonding the second?

Can you envisage any problems with this? I'm thinking the folded corners will be a swine.

 

[alan333]

All you're trying to achieve with the first layer is to stop movement so it doesn't really matter whether it's underlay or cap sheet.

However, especially if you're using cold adhesive, layer 2 probably won't stick well to the mineral surface of layer 1. It's not the usual way but you would probably get better results if you laid layer 1 upside down, ie with the bitumen surface upwards. Obviously the mineral grit could penetrate/fall downwards tho, so if there's no ceiling you might get some grit on your head for a while.

You also might encounter problems with the thickness of the laps on layer 1, ie your cold adhesive for layer 2 might not like the 4mm height differences of the laps below.

Doing the corners is much easier if you can heat the felt so it bends.

 

[beatts]

Cheers Alan,

I've since thought sod it and been out to buy a few rolls of underlay so I can do the job properly. I'll keep the spare cap sheet for my other shed.