Q: A client wants to create a vaulted ceiling in an addition with a shallow, 4:12-pitch roof. The rafters are 16 inches o.c., with no structural ridge. Can I raise the ceiling joists to create the vaulted ceiling?
A: Darren Tracy, PE, owner of West Branch Engineering, in Saratoga Springs, N.Y., responds: Yes, you can raise the ceiling joists, but with restrictions. Because you do not have a structural ridge, you must utilize rafter ties. Ceiling joists can serve as rafter ties to resist outward thrust on the walls from the rafter loads if they are installed parallel to the rafters and in accordance with code.
Figure R802.4.5 of the 2018 IRC states that a rafter tie can be raised a maximum distance of “HC” above the top of rafter support walls. HC is determined by a simple formula in which that height is a function of the ridge height (HR): The ratio of HC/HR cannot exceed 1/3 (see illustration). For example, in a roof structure where the ridge height is 9 feet above the top of the support walls, the maximum height that a rafter tie can be raised is 3 feet (3/9 = 1/3).
Additionally, be sure to adhere to the specific fastening requirements in the 2018 IRC Table R802.5.2 for the rafter-to-rafter-tie (ceiling joist) connections. That table provides the number of 16d nails at each connection based on rafter slope, rafter spacing, and snow load.
Section R802.5.2 also states, “Where the ceiling joists are installed above the bottom third of the rafter height, the ridge shall be installed as a beam.” In other words, to raise the ceiling joists more than one-third of the ridge height, a structural ridge would be required. With a properly engineered structural ridge, rafter ties can be eliminated completely. Also section R802.4.4 states that a roof with a pitch less than 3:12 requires a structural ridge.
As a final note, do not confuse rafter ties with collar ties. Collar ties, which are designed primarily to resist wind uplift, are required in the upper third of the attic space. Unlike ceiling joists, collar ties can be made of less-substantial material (1x4 minimum) and can be spaced up to 4 feet apart. Ridge straps can be used in lieu of collar ties to resist uplift.