Rafters are closely associated with traditional lumber-framed houses, but prefabricated trusses often replace rafters in modern residential construction. Trusses are popular for many reasons, but it is important to recognize that the popularity of trusses increased as home sizes also increased. The advantages of trusses are less pronounced for smaller homes.
A truss consists of multiple lumber-framed triangles assembled into a unified load-bearing structure. Through the use of careful calculations and specific fastening techniques, trusses can support heavy loads even when built of small-dimension lumber such as 2-by-4s. The term "rafter" refers to a diagonal piece of lumber that extends from the top of a wall to the ridge of a roof. A rafter typically requires wider lumber, such as a 2-by-8, because it is a single structural member supporting a roof load.
Ease of Construction
Perhaps the dominant reason for the popularity of trusses is that truss roofs are simple and quick to construct. Prefabricated trusses can be ordered from a home improvement store, delivered on a truck, lifted by machine up to roof level and fastened directly to the top plate of the wall. Rafters require more design time, custom sawing and careful adjustments. In a small house, however, the difference in construction time is less significant because it requires fewer rafters.
Compared to rafters, trusses can span longer distances without a supporting beam. In general, rafter span is limited by the length and rigidity of the collar tie -- a single piece of lumber -- attached between two opposing rafters. In contrast, public truss plans provided by Iowa State University can span up to 60 feet. Once again, this advantage is less significant for a small house because a rafter roof could certainly span a 20-foot-wide structure.
A major disadvantage of a conventional truss roof is the loss of attic space. The triangular sections that make up a truss cut through the space between the roof and the bottom of the truss. In many cases, the entire attic becomes unusable, even for storage. This is particularly problematic for small houses, which need to make the most of limited space. Alternatively, a roof can be framed with "attic trusses," which feature a central opening in the truss webbing to provide space for living area or storage. A rafter roof potentially opens up even more attic space for storage because the area under the eaves is uninterrupted by vertical framing members.
Reduced cost is an advantage of trusses that applies to both small and large homes. Trusses must be carefully engineered and manufactured, but they still offer cost savings because labor expenses are reduced and because manufacturers can use short lengths of small-dimension lumber. This is especially true for standard trusses that do not require custom design. Rafters require long lengths of wide lumber, and skilled carpenters often spend a significant amount of time designing, assembling and adjusting a rafter roof.
- Independent Builder: Designing and Building a House Your Own Way; Sam Clark
- Iowa State University: Truss Plans
- West Coast Lumber Inspection Bureau: Simplified Span Tables
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