Applying bitumen mastic to a very wet roof- how?

Anglichanin

Hello. I recently erected a wooden garden building, nailing the roofing felt as directed, but returned from being away at Christmas to find that I had a leak. It has barely stopped raining since and I have placed a tarpaulin over the building but the leak persists.
I have bought a tin of bitumen mastic to trowel onto the felt- the instructions say the felt must be dry first, so I'll wait for a dry spell.
My question is, if I seal the roof with bitumen mastic and there is still significant moisture in the roof timber below, will it cause damage? I have fairly powerful dehumidifiers which I can use to dry out the shed but I don't know if they will draw enough of the moisture from the roof, or perhaps even cause damage to the rest of the shed if I run them for too long.
Any advice much appreciated! Thanks.

DaftyDuck

Buildings - even sheds - are tough old things. I doubt the retained moisture will be a problem even if you have to wait for a month to get a good seal. There are some wet-applied sealants for flat roof situations, but I've never used them myself. If you've nailed the felt it'll like as not be there that it is leaking. If there are nail dimple holes, you can use bitumen filler to level it off first, so there's less pooling of water to seep in.

Best thing would be to wait for dry weather to get a good adhesion (and warmer too) and solve the problem in one go. I speak from learned ignorance. I applied bitumen to a balcony around wooden handrail base posts in the wet. Never set right, and it was a devil of a job to sort out in the drier weather... still not really right.

Anyway, that's an answer based on the experience of the ignorant. you may well get an informed answer soon.

Anglichanin

Thanks for the tips. I think I'll wait until the next dry few hours (forecast Friday...) peel back the tarp and seal up the edges around the tacks we hammered in, as there lies the problem. I intend to apply the rest when Global Warming kicks in and it stops raining.
One extra question, if anyone can help- could I then close the tin of bitumen mastic and use it at a later date or will it need using in full as soon as the tin is opened? I've never used it before.

DaftyDuck

Apologies for late response.

You can use a bit at a time. A good tip with all paint tins is, once started, seal the lid on tight, and put the tin upside down for a few hours... any slight air gap gets sealed by a small quantity of escaping gunk. Great for gloss and for bitumen! You can stick it on newspaper, or even in a bucket, if scared it will all go wrong.

Anglichanin

Thanks very much. I actually found a short-term solution by buying some waterproof sealant and applying that during the deluge, which has helped. I have ordered some more felt which I'll put over the top of the existing, sealed-up felt, and then I might apply the bitumen when the weather improves.
Thanks loads for your advice- it is much appreciated.

tony6403

If you have felted the roof starting from the bottom and overlapping as you go up using felt nails there should be absolutely no need for any sealants.
I've erected five or six sheds plus a large summer house with just felt and felt nails and none has leaked.

Hintza

tony6403 wrote: »

If you have felted the roof starting from the bottom and overlapping as you go up using felt nails there should be absolutely no need for any sealants.
I've erected five or six sheds plus a large summer house with just felt and felt nails and none has leaked.

I redid our shed roof two years ago and followed what the guy who built it did:-

12" overlap and nails every 11/2-2" apart so it will not lift.