Adam Funk wrote:
I have a shed covered with tongue-and-groove cladding, which I've
repainted on the outside since it was built.

The roof is the same material but covered with roofing felt. I'd like
to make the roof last a long time and I happen to have some bitumen
paint lying around --- would putting a few coats of this on the
roofing felt every few years be a good way of extending its
durability?

Thanks,
Adam

If you want it to last a long tiime buy a Butyl pond liner and use that
as a roofing membrane, it'll go for 30 years or more !! I have two
sheds/utillity room done like this with ply underneath, works a treat
!!
got mine from www.pondlinersdirect.co.uk