Not an easy answer. Many factors play a part in the roof’s life, from the direction the roof faces, if it’s in a valley or on a hill or whether there are trees over hanging the roof. The pitch of the roof and quality of material and craftsmanship all play a part. So as you can see it’s not a clear cut answer but generally speaking 20 to 30 years for the main roof and 8-10 years for the ridge. If the ridge is left too long it will affect the top layers of thatch. Water reed will last longer than wheat reed in most cases.