Often the most difficult repairs to accomplish in historic barns are the repair or replacement of the wall plates where the rafters meet the wall frame. Unfortunately this is a common area of failure in historic barns where roofs have been allowed to leak for extended periods of time. Most timber frame barns have rafter connections that incorporate a rafter "seat". The cutting of the rafter seat joint typically involves the removal of a portion of wood on the top of the plate timber creating a perfect place for water to collect. The trapped water causes rotting commonly not detected until the plate has suffered severe deterioration. To make matters worse, the rot often extends into the rafters themselves and can travel into the posts, braces and tie beams that are connected to the plates.
Plate repair involves lifting sections of roof frame in order to allow the damaged timber to be removed and the repair timber to be inserted. Depending on the frame type this can be a predictable and controllable procedure or a precarious and dangerous one. The problem comes in how the loads are being transferred from the rafters to the frame.
In large barns the rafters are supported by timbers called purlins. These horizontal timbers typically are placed midway between the wall plate and the ridge. In early barns it is common for these purlins to be supported by vertical posts that transfer the load of the roof into the tie beams. The figure above shows this type of frame. Provided the roof frame has not been modified, the rafters will be "birds mouthed" at the purlins which provides a stable connection. If however the "straining beams" have been removed (a common practice when hay tracks were added) the frame may require stabilization before the work proceeds.