Increase the lifespan of the roof at a cost effective price buy upgrading the roof to Onduline. 
This lightweight roofing sheet is made from compressed felt, and formed into a corrugated profile to add exceptional strength to a lightweight cover. 
To add longevity, the felt is mixed with bitumen and coated with a waterproof resin.
Wind tested up to 75MPH, this is a perfect upgrade for the garden shed that needs to last a long time.