The roof is a fundamental part of any building and as such needs to be well constructed to keep water out and heat in. Most traditional roofs were built with the natural ability to allow water vapour out without condensing and causing major problems. For example, thatch was (and still is) very efficient at keeping water out and heat in. But through the inappropriate use of modern (mostly impermeable) materials many buildings have lost the ability to breath.
When it comes to the reconstruction of an ancient roof, at The Organic Building Company we take great care to conserve as many of the original timbers as possible, as a lot of vernacular roofs were constructed not of uniformed sized timber, but of green boughs. If roof space allows, organic material such as sheep's wool, hemp or flax can be used for insulation whilst allowing water vapour to leave the building without causing condensation problems.
For owners worried about potential fire hazards there are modern thatch treatments and fire barriers, all of which can be incorporated into the structure. Tiles or slates will accommodate insulation equally as well and materials consistent with both modern building regulation and the desire to produce a building that is breathable and warm can be achieved through the use of the correct materials.