It has a life span of about 12 years, it is easy and cheap enough to replace.
It's mainly down to the weather conditions, the heat of the sun and the cold of the frost, that reacts with the felt over time.
soften it at times and making it brittle at other times, this takes it's toll and the felt starts to crack allowing ingress of water.
Dunno why its called felt, never have. Anyway, its easy to change and pretty cheap for a decent roll from Q&B etc. Get some galvanised clout nails too, then just cut roughly to lengths, nail on, and trim. You'll need to take off the trim at each end of the shed roof to get the felt off, so just nail it back on to tidy up the ends. Simples.
not sure how long it has been leaking but check the wooden roof boards are not shot before replacing the felt.
The are usually plywood and may have soaked up a bit of water before it started to leak.
No point replacing the felt if the boards are dodgy underneath. You would have to change boards too.
Still not to difficult a job though.
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