When remodeling an old house, what can be more tempting than to vault the ceilings or add a large dormer? The attic joists don't seem to be doing much. But before you grab the chain saw, here are a few things to remember:
Attic joists typically form a tie beam, which prevents the rafters from spreading outward. This is very important. In fact, the International Residential Code (IRC) even specifies the number of nails required to fasten the joists to the rafters. The lower the roof slope or greater the snow load, the more nails required to tie the rafters and joists together.
The effects of rafter spreading often don't show up for many years. This is primarily caused by the nails biting into the framing lumber due to the snow load or the weight of the roofing. Slowly, the walls start pushing out and cracks begin to appear. The roof ridge begins to sag and develop a swayback appearance. Once the process is under way, the horizontal thrust can be enormous and inexorable.
Collar ties alone can't handle the spreading, and neither can ceiling joists that are raised too high above the bottom of the triangle. If you still want to raise the ceiling this way, it's best to have an engineer design the connections and ties. The stresses involved are complicated, and nails alone may not be sufficient to make the connections work.
Using a ridge beam at the peak of the roof is a safe and simple way to allow the removal of a few rafters or joists.
The beam must be capable of supporting half of the total load on the roof without significant deflection. Sometimes this means a pretty big beam, but it sure beats having your walls bowed out.
— Dick Kawalek, a registered architect for more than 30 years, is founder of Kawalek Architects, Cleveland; firstname.lastname@example.org.