Rafter Ties and Ridge Beams

Cathedral ceilings are a popular addition to many homes today. But they pose special problems in dealing with the downward force of the rafters that push the exterior walls out. Proper placement of rafter ties and use of structural ridge beams can solve these potential problems, which often result in cracked walls or ceilings and walls out of plumb. In a conventional wood roof truss, the bottom chord creates a tension tie between the outside walls. For a cathedral ceiling, open rafter ties, or collar ties, can serve the same purpose, provided the ties are placed within the lower 1/3 of the rafter span. The higher the ties go, the less leverage is available to counteract the forces pushing out.   Placing a rafter tie in the middle of a rafter also causes complications. Hanging a drywall ceiling from the rafters will add dead load to the rafter at its maximum bending point. This additional load can cause the ceiling to sag and creates more outward thrust on the exterior walls.   The most effective way to reduce outward thrust is to use a structural ridge beam. The ridge beam must be supported on both ends (and, if necessary at intermediate point(s), along its length) and must be sized to carry the load that will be imposed on it.   Properly connecting the rafters to the structural ridge beam is critical. Given the forces at work, a toe-nailed connection is not adequate. Joist hangers, attached with proper nails, provide a strong connection. Or, cut a birdsmouth notch in each rafter and attach them as they rest on the top of the ridge. Remember, for a birdsmouth, cut no deeper than 1/4 the depth of the rafter. Rafter ties