Reconstructing a Roof
A Project for the Pros
By Matt Weber
Our mission at Extreme How-To is to cover professional-grade home improvement projects for the do-it-yourselfer—but we have our exceptions. Roof construction, for example, is definitely a job for a professional, and it takes a professional many years of experience on the job to become a knowledgeable roof builder. A smart homeowner won’t enlist just any random contractor to alter the structure of their home; they’ll research their options and choose a reputable professional with verifiable references. This is one of many reasons why even the most savvy DIY’er should leave a project as complex as roof framing to someone who knows what they’re doing.
Nevertheless, DIY homeowners are often interested in how a job is done, even if they aren’t going to do it themselves. This article looks at the complicated nature of roof remodeling and shows just why this area of the home is best left to the experts.
Engineering & Design
Just like floor joists, roof rafters must be selected according to joist span. The larger rafter size of a 2×10 will have a longer allowable span than a 2×8. Unlike floor joists, however, the rafters are installed at an angle, which also affects allowable span. A low angle (closer to horizontal) places more strain on a rafter than a rafter of a steeper pitch. Also, distances between rafters affect allowable span. Plus, the grade of the lumber affects allowable span; No. 1 grade lumber will have a greater span than No. 2 lumber. All these factors must be calculated when designing a roof to ensure structural integrity and to satisfy local building codes.
And the variables of design don’t stop there … The builder must determine the pitch or slope of the roof, and use that measurement to figure the total rise from the wall’s cap plate to the roof ridge. With the total rise they can calculate rafter length and the total number of rafters needed, accounting for any overhang along the eaves.