The minimum pitch for a thatched roof is 45o, old properties with multi layers of thatch can lose pitch with the consequence that water is not easily shed and the roof remains wet and may deteriorate more rapidly. Some new build properties are designed by architects who are not familiar with the special roof construction requirements for thatch; often problems in this type of property are exacerbated by a conventional design of house with thatch on top almost as an afterthought, leading to difficult slack pitched dormers.
It is a well-established fact that the north side of a thatched roof will outlast a south or west facing aspect. It is the effects of the movement of moisture and frost triggered by early sunlight that reduces the life of a south facing roof. In seasons of normal rainfall and prevailing weather patterns the north side is not subjected to such environmental extremes, but because sunlight is often restricted may support the growth of algae and moss. This may become more of a problem if we experience cooler, moister summers and milder winters.