Roof and Ceiling Construction
Roof and ceiling construction
Roof framing questions test the load path from roof to wall and the difference between members that tie a roof and members that support it.
Concepts
- Rafters are sized from span tables and are notched at the bearing only within limits; the remaining heel must be adequate.
- A ridge board is not a beam; it needs rafter-to-rafter connection and proper support.
- Rafter ties resist thrust at the eave and are placed low; collar ties resist separation near the ridge and are placed high. They are not interchangeable.
- Ceiling joists support ceiling finishes and, where designed, storage; they also serve as rafter ties in many assemblies.
- Roof sheathing, clips, and fasteners create a diaphragm; panel orientation and nailing patterns matter.
- Attic ventilation, access, and draft stopping affect performance and fire safety.
- Engineered trusses require bracing per the design and cannot be field-cut.
Traps
- Calling a collar tie a rafter tie, or moving a tie too high.
- Over-notching a rafter at the wall.
- Adding storage or a heavy ceiling in an attic not designed for it.
- Cutting truss webs for ductwork.
Worked example A cathedral ceiling has rafters but no ceiling joists. The plans rely on a structural ridge; if the ridge is only a 1x board, the rafters will spread. The correct question is whether the ridge is designed as a beam-if not, rafter ties or a truss system is required.
Rule of thumb Follow the load path: roof to rafter to wall, and keep the triangle from spreading.