Posted: 15/12/2014 at 11:35
Sadly I didn't spot this thread until just now. I agree, the felt used in initial construction is feeble and probably only has a lifespan of a couple of years.
I recently had my summerhouse & shed/workshop re-felted. The process (although a little more expensive than normal felting) is far superior and I received a 25 year written guarantee with mine.
This process involves laying a base felt to the shed roof using heat (after stripping the old felt) to this base felt a heavy mineral felt is added using a blowtorch, the two then bond together....no tacks or nails are used.
Just another option.