The existing felt was old and had started to leak into the shed. The left hand side roof timbers were rotten and had to be replaced. I applied heat bonded felt with a blow torch and no nails were used so it can’t leak! Cheaper roof felt (like the type bought from B&Q) is traditionally nailed and glued on and doesn’t have as long a life span. Heat bonded felt is a bit more expensive but will last a lot longer – should be in the region of 10 years!