How Do I Calculate the Ridge and Rafter Size for a New Gable Roof?

Answered by Brett ~ September 2, 2011 ~ 1 Comments » | Respond to this question

How do I determine the size of ridge and rafters I need to build a gable roof on a 16x24 foot, small cottage?

Felicia S. ~ Newport, VT

Brett Kulina

Felicia, before you can determine the size of the roof framing members for your cottage's gable roof, you will first need to decide if you are going to use dimensional lumber, I-joists, engineered lumber or some combination of these typical roof framing materials. Once you select the type of rafter and ridge material that you want to work with, then you can follow the manufacturer's specifications for size and span limitations. You can also visit the contractor's desk at most any lumberyard and ask them to size the framing members based on your blueprints or drawings.

One possible advantage of using I-joists instead of dimensional lumber for your rafters is that I-joists are readily available in longer (and straighter) lengths, which might be important if your cottage is going to have an open spanning cathedral ceiling. Likewise, engineered I-joists are available in widths greater than 12 inches, which allows for plenty of room in the ceiling cavity for insulation. Although I'm not sure what would be considered a minimum R-value for a ceiling by Vermont's standards, I would suggest that you strive for an insulating value of at least R-49. Remember, the increased cost of a super insulated ceiling can pay for itself over the long haul by helping to decrease your home's overall energy consumption.

If you will hang the rafters and create a cathedral ceiling, then you will probably need engineered collar ties and/or a sizable ridge beam to support the weight of the roof. If your cottage will not have a cathedral ceiling, but rather an attic space over a flat ceiling, then you can use smaller dimensional lumber or possibly even prefabricated roof trusses. One advantage of using roof trusses is that you don't normally need to install a ridge beam as well, plus setting roof trusses can be much simpler than laying out, cutting, and hanging rafters the traditional way. Good luck with your project!

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