When misalignment in structural support occurs, roof sag can result. There are two primary roof framing systems used by builders: rafters and trusses, as well as two primary roof shapes: gables and hips. Rafter-framed roofs consist of individual rafters (sawn lumber members), usually spaced from 12 to 24 inches on center, which span from the exterior walls or roof-eaves up to the roof top or ridge, or into the sides of the main hip rafters. This style of roof construction is often called “stick-framing”. The most common rafter size is a 2×6; unfortunately, this small-size member cannot span very far and must typically be braced near mid-span. A structural analysis will usually show that the roof-bracing system picks up most of the roof load (weight). Hence, it is very important that the roof braces land (rest) on only designated interior “load-bearing” walls. Unfortunately, it appears as though many builders/framers do not realize this since they often support the roof bracing systems on the closest or most convenient interior room partition wall. This can lead to long-term floor and roof sag because most floor joists are not sized for roof loads.
The ridge board located along the peak of the roof typically does not provide any structural support for a rafter-framed roof system; it simply serves as a convenient bearing-plate or nailing plate for the opposing rafters. However, it is important (and a Code requirement) that the ridge board be deep (tall) enough to provide full contact to the cut face of the mating rafter and that opposing rafters meeting at the ridge directly align with one another. Whenever the ridge board is non-structural, it is absolutely necessary that the roof rafters be lapped alongside and connected to the underlying ceiling joists at the exterior wall plate; moreover, the ceiling joists that extend across the home must be properly lapped and connected to one another because they provide a critical “tension-tie” across the home. If the ceiling joists do not provide this critical tie or if they span transverse to the roof rafters (which is often the case), the roof ridge will likely sag and the exterior walls will likely lean outward.
The latter is a common problem with stick-framed cathedral roofs/ceilings. In this type of roof system, the rafters and roof decking also serve as the interior ceiling. Since a conventional rafter-framed roof exerts outward forces on the supporting exterior walls, a structural ridge beam (board) is required for cathedral ceiling construction because there are no ceiling joists to provide the normal cross-tie. In other words, if the ridge beam is capable of providing vertical support to the rafters, then they do not exert an outward thrust on the supporting exterior walls. A structural ridge beam normally consists of either glue-laminated timber (glu-lam), laminated veneer lumber (LVL) or structural steel. Seldom will a solid-sawn lumber beam or built-up lumber beam suffice-especially for long ridge beam spans.
Failure to provide a structural ridge beam in cathedral ceiling construction always results in roof sag and corresponding outward lean in the exterior walls which support the rafters. Although I have never witnessed the collapse of an improperly constructed cathedral ceiling, I suspect that it can happen. The reason why complete failures don’t occur is probably because the distortions that slowly develop inside the home, from leaning walls and sagging roofs, gives the homeowner plenty of time to hire a professional to develop a corrective repair. Hence, this condition should never be ignored, no matter how slight the distortions.
Truss-framed roofs consist of pre-engineered, light-gage-metal-plate-connected sawn lumber members which are fabricated inside a controlled environment, per some proprietary engineering design and delivered to a construction site on flat bed trucks or special “cradle trailers.” These structural units/frames are designed to withstand numerous Code-specified structural load combinations and, because of their engineering and close fabrication tolerances, often provide the best solution to complex roof shapes or difficult roof framing configurations. Trusses usually transfer all of their load to the outer bearing points (exterior walls); because of this, they do not need any support from interior room partition walls. Because roof trusses are designed to span across the entire width of a home, the top chord of the truss and oftentimes many of the interior web members are placed into compression. The compressive forces try to make the truss members buckle or distort out of plane and, unless they are properly braced and held straight during erection, the entire truss may warp and distort. This can lead to reduced load-capacity, increased deflection, and the possible transfer of roof load to interior room partition walls. Once the roof decking is nailed to the truss assembly, the top chords become permanently braced. The very long compression web members (interior diagonal members between the top and bottom chords) still require some sort of permanent bracing to prevent them from distorting. Many home builders and framers do not realize this. Another problem with trusses is that very long/large trusses are usually flimsy and somewhat fragile and hard to handle at the job site. Unless the trusses are handled and stored very carefully in the field, and later during actual erection, the metal-plate connections may detach from the wood members. If this occurs, the truss has been compromised and will no longer behave as designed unless repaired properly/immediately. Also, if any truss member(s) are cut or damaged during construction or later by other trades (electricians, plumbers, HVAC), the truss will be compromised and rendered somewhat ineffective. In either case, the end result is usually roof distortion, ceiling distortion or the creation of isolated floor sag.