Common Roof Support Problems

roof support
March 10, 2017 Roof Support Problems

To know what kind of roof support problems are most common, we should first find out where they originate. There are two main types of roof framing systems used worldwide by home-builders. These are rafters and trusses. Despite their popularity and advantages in architectural design, they cause more roof support problems than you might think.

Rafter framed roofs

  • Rafter-framed roofs consist of individual rafters which span from the exterior walls or roof eaves up to the rooftop (the ridge) or into the sides of the main hip rafters. They are typically one or two feet from the centre. Unfortunately, rafter framing cannot span very far and must typically be braced somewhere near mid-span, and as a result, the roof bracing ends up holding most of the roof weight. The roof braces must, therefore, rest on designated interior walls to prevent them from collapsing under the stress of the roof weight. However, many builders do not realise this and, instead, support roof framing systems on the closest or most convenient room partition wall. This, in turn, leads to roof support problems such as roof sagging since most floor joists are not sufficiently sized for roof loads.
  • Along the peak of rafter framed roofs is a ridge board that some builders assume provide structural support to the system, but this is not the case. The ridge board simply serves as a convenient bearing plate or nailing plate for opposing rafters. This doesn’t imply it doesn’t serve any important function, however, and it is, in fact, a Code requirement that the ridge board should be tall enough to provide sufficient contact to the cut face of the mating rafter and that any opposing rafters that meet at the ridge directly align with one another. If, however, the ridge board isn’t included in the structural design of the roof, the rafters should be lapped alongside and connected to the underlying ceiling joists at the exterior wall plate. Additionally, the ceiling joists that extend across the home must be properly connected to each other because they provide a critical tie across the home.  A common mistake made by builders is spanning transverse roof rafters, in the process failing to provide this critical tie. As a result, the roof ridge is likely to sag and the exterior walls will likely lean outward.

Stick framed roofs

  • Stick-framed roofs, a type of raft framing, are the most common victims of leaning outwards. In this roof system, the rafters and roof decking also serve as the ceiling to the interior. Rafter framed roofs usually exert outward forces on the exterior walls, and thus a structural beam is usually required for stick frame construction (notably used in most cathedral ceilings and roofs) because there are no ceiling joists to provide the conventional cross-tie. Simply put, if the ridge beam is capable of providing vertical support to the rafters all on its own, then the rafters do not exert an outward thrust on the supporting walls. A structural ridge beam should, on normal occasions, consist of either glue-laminated timber (glu-lam), laminated veneer lumber (LVL) or structural steel. Solid-sawn lumber or built-up lumber shouldn’t be used for the beam since it rarely holds for long ridge beam spans. Failure to provide the crucial structural ridge beam in cathedral ceiling construction almost always results in other roof support problems such as roof sag and a corresponding outward lean in the exterior walls.

Trusses

  • Truss framing consists primarily of sawn lumber which is fabricated inside a controlled environment, according to the proprietary engineering design chosen, and delivered to a construction site on special flatbed trucks. These structural units are designed for consistency with numerous Code-specified structural load combinations and, due to their proprietary engineering and close fabrication tolerances, nine out of ten times offer the best solution to complex roof shapes or difficult roof framing configuration tendencies.

 

  • Trusses usually transfer their entire load to the exterior walls of the buildings, and thus do not need any support from the interior room partition walls. Roof trusses are designed to extend across the entire width of a home, and the top chord truss, and most of the times many of the interior web members, are subjected to stress and forced into compression. These areas of the roof subjective to compressive forces oftentimes face roof support problems such as the truss members buckling or distort out of a plane, and unless they are properly braced to hold them tight during erection, the entire truss may warp up and distort out of a plane. Other roof support problems consequently experienced as a result include reduced load capacity, increased deflection and the likely transfer of roof load to room partition walls.

 

  • Once the roof decking is nailed onto the truss assembly, the top chords become permanently braced. The compression web parts (which consist of interior diagonal members between the top and bottom chords), however, still require some form of permanent bracing to prevent them from distorting out of shape. Sadly, many home builders do not realise this crucial fact. Another problem with this type of framing system is the fact that if they are extremely long, they are also likely to be flimsy, fragile and increasingly difficult to handle at the job site. Oftentimes they are mishandled out in the field and the metal plate connections end up detaching from the wood members. The truss will from then henceforth not behave as designed unless repaired immediately. This is also the case if any of the truss parts are damaged during construction or at any stage of transportation. Either way, the end result is going to be one of the following roof support problems: roof distortion, ceiling distortion, walls leaning outwards or the creation of isolated floor sag.

 

These problems aren’t as simple as straightening up your house again. In fact, they pose a serious danger and if home suffers from these common roof support problems, you should immediately contact a roof restoration company to fix the problem. It will save you lot of money, a lot of time (maybe even a dream home, a life or two and leave your hard-earned work intact) in the process.

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