Joists, Rafters and Trusses Part 2

Welcome back to the Professional Roofer’s blog, where we are discussing all the important structural elements and materials that make up roofs. In our last blog, we discussed the importance of roofing joists, ceiling joists, rafters, and trusses; and explained what roofing joists and ceiling joists were. If you haven’t already read that blog, we highly recommend you go back and read it now, as we will be building off of that information for this blog piece, where we will be introducing rafters and trusses.

From last week’s blog, we know that roofs with a slope of less than 2 in 12 have parallel planks of wood called roof joists that are spaced at specific distances apart for load-bearing purposes. These planks are called joists because they are relatively horizontal to the ground, and are therefore categorized within the “joist” family of building terms. However, since steeply sloped roofs of more than 2 in 12 are raised up too high for the parallel planks to be considered joists, they go by a different name, and are instead called “rafters”. Roof rafters are typically joined where the two slopes converge along the ridge of your roof, running all the way down to the eaves. They too are spaced at specific intervals apart for load bearing purposes, and are parallel to one another. The only difference between joists and rafters are the amount of load they must be designed to carry, and their angle – that is generally it. Raftered roofs have a great deal of attic living or storage space beneath their planks, as insulation is generally fitted in between individual rafters, leaving the rest of the space open for personal use.

If you have a sloped roof, but your attic space is nonexistent or uninhabitable, you likely have trusses, the most common roofing support system used in North America today. Trusses are lightweight, pre-fabricated, and specifically engineered roofing support systems. Trusses are created from straight structural components (like pieces of wood) that are interconnected into a roughly triangular shape. They primarily use lighter, smaller pieces of wood in their construction that allow them to be cheaper than traditional rafters. Trusses cannot be disassembled or altered in any way without drastically reducing their structural integrity, so they do limit construction to a certain degree and prevent a home from having usable attic space. However, since they come pre-made, are typically cheaper than rafters, and are much easier and faster to install than regular rafters they tend to be the preferred choice for builders. They can also span long distances, by as much as 60 feet, which allows for a more open housing floor plan and a reduction in load bearing walls for most homes.

We hope that you’ve enjoyed learning about the various plank-like structural elements that make up your roof, and feel a little bit better informed about your home and some of the technical jargon your contractor or roofer may use. Until next time Toronto!