Thatch is naturally air and moisture permeable; when constructed traditionally it provides a weather resistant, breathable and highly durable roofing material. However, the introduction of modern impermeable plastics, the requirements of building regulations for additional insulation and reduced air leakage, can increase the risk of condensation within the thatch, so affecting its durability. This Guide explains how to reduce that risk by introducing cavities and ventilation below the thatch. It applies to new roofs and where the thatch of an existing roof is to be replaced completely. The local planning authority may require a consent application for the removal of thatch or if the insertion of new materials would change the behaviour of the roof. 4 pages.