Although the roof covering of a mediaeval house may have changed, say, five or six times the structure
remained forgotten and comparatively unaltered, This is not so elsewhere in a building: mullioned windows are glazed, open halls floored over, doors changed, and whole new brick fronts added.
The type of roof truss used for a building was determined, to a certain extent, by the form of heating to be used. Open hall houses have crown posts or windbraced side purlin roofs for "show", queen post trusses facilitated the formation of smoke bay walls while butt purlins could be used as trimmers for dormer windows in a floored attic space. Thus the roof structure which has traditionally escaped the "improver's" hand is to us a key pointer to the age of a property and deserves early and comprehensive study.
Vernacular roof trusses of Surrey may be divided into four basic types, depending upon the position of the only longitudinal timbers in the roof structure - the purlins. These long timbers in the roof structure support the rafters between the wall plate and the apex.
The purlin is found in all mediaeval Surrey roofs except the earliest, the COUPLED RAFTER ROOF. In this type of roof every rafter couple is framed as a freestanding truss, the tile battens are the only longitudinal members. It is likely that these early roofs were originally thatched, with fully hipped roofs with gablets. As a coupled rafter roof does not have purlins an alternative name is a SANS PURLIN ROOF.
A coupled rafter roof must have formed a most impressive sight over a mediaeval hall, and some examples are still to be seen in Surrey churches. An unusually elaborate example is that over the Bishop's Camera at Farnham Castle.