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Why Roof Trusses Are More Popular Than Rafters

Posted April 23, 2015

Roof framing has undergone a quiet revolution over the past half century. Trusses are gradually replacing the conventional rafters, to the point that nowadays more than 80 percent of new residential construction makes use of trusses to support the roof. Why has this change occurred and what are the advantages and disadvantages of each roofing system?

Rafters

The traditional means of supporting a roof is rafters (otherwise known as "stick framing"), cut, built and installed on site by an expert carpenter. Rafters are made from sloped framing boards of dimensional lumber -- usually 2x8s or 2x10s -- connecting the roof peak, known as the ridge board, to the plates of the exterior walls. They are secured at the bottom by ceiling joists. Collar ties, placed horizontally halfway down the rafters, hold the rafters and roof planes together.

While rafters are less and less frequently used today due to their higher expense and labor costs compared to trusses, there are certain advantages to the former. Stick framing is flexible and may be altered without compromising the structure. As a result, you can create open space under the roof, valuable for either good looks, as in the case of vaulted or cathedral ceilings, or practical purposes such as building an extra bedroom, office, or storage space. It is also simpler to accommodate a home addition with rafter roofing.

Although the actual installation process is slower than for trusses, rafters require a shorter lead time to prepare, so if you are in a hurry, rafters will be the quicker way to go overall. They are also more suitable for remote locations, where transporting a pre-assembled truss system to the site would be overly complicated, or for small roofs, when going through the process of site-specific truss design would not be cost-effective.

Trusses

Trusses are lightweight engineered units consisting of top and bottom chords supported by a webbing of lumber in a triangle shape. Because they are pre-assembled to your building specs offsite, often using automation such as computer-driven saws, they are much faster and more economical to install than rafters. Although they make use of smaller (and therefore cheaper) pieces of lumber linked with tooth plate connectors, trusses are very strong and stable, capable of spanning great distances of as much as 60 feet. This reduces the need for interior load-bearing walls and allows for a more open floor plan, in tune with today's trends, on the stories below.

Another plus offered by truss framing is that it saves an important natural resource by utilizing smaller amounts of lumber and producing less wood waste.

However, truss systems do come with some distinct disadvantages. The pre-built truss system is heavy to transport and lift into place. You are likely to need an 18-wheeler and a crane -- together with adequate road access -- for the job.

The webbing which supports the chords tends to impinge on the space directly underneath, a pronounced disadvantage if you would like to convert your attic or the area above the garage into usable living space. Scissor trusses allow for somewhat of a cathedral ceiling, unlike other styles, for example, cambered trusses.

Once a truss system has been installed, it will be difficult and even dangerous to try to modify it. Cutting any part of the webbing or even drilling a hole to accommodate electric wiring can undermine your truss roof's integrity. You'll need to consult a professional building engineer if you must make alterations.

In addition, the National Fire Protection Association has raised concerns about the fire safety of truss-framed roofing made of engineered wood.

Laura Firszt writes for networx.com.

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