Most roofs are fitted with fascia boards, which are the timbers fixed onto the edges of rafters, hiding them from view. Roofs and roof timbers overhang the walls of a house. Soffits are fitted to bridge the gap between the wall and the edge of the roofline.
The area covered by soffits is not exposed to the weather and so wood is often used to make soffits. However, wood will take in some of the moisture in the air, and over time will warp and distort. Replacing timber fascias and soffits with Upvc is a common job for roofers.