Joists, Rafters and Trusses Part 1

In today’s Professional Roofers blog, we have chosen to go “beneath the shingles” a little to discuss some important structural terms that relate to your roof, but that you certainly won’t see on the outside of your home: roof joists, ceiling joists, rafters, and trusses. Depending on the style and slope of your home roof, you will likely have one or two of these structural elements integrated into your house. These structural components are all a little bit different, which we will explain in more detail soon, and yet they all share the same jobs: carrying the live and dead loads of your roof, supporting the walls of your home, preventing the walls of your home from spreading or racking (shaking), and creating a space for ventilation and insulation. In other words, they’re incredibly important and completely necessary for any modern house.

No matter whether you have a sloped roof or a flat roof, you will need some sort of structural support to hold up the weight of the roofing materials you or your contractors choose, and to connect the walls of your home and prevent them from spreading or racking. This structural support comes in the form of roofing joists, ceiling joists, rafters, or trusses and can be made from a variety of materials including wood, metal, reinforced concrete, etc.; however, in Canada the most common material used in residential framing is wood, so that is the material we will discuss here. Assuming that your builders have chosen to use wood for your house’s frame, your roofing joists, ceiling joists, rafters or trusses will be made from long planks of wood placed parallel to one another at evenly spaced intervals along the length of your roof.

If you have a low-sloped roof (or flat roof) of less than 2 in 12, these evenly spaced planks will be called roofing joists. “Joist” is a term used in construction for load bearing planks that run horizontally or nearly to horizontally to ground. So the only reason these planks are called roofing joists is because they run horizontally, or nearly horizontally, to the ground. Similarly, “ceiling joists”, are the planks of wood that create the flat surface of your attic floor, or top story ceiling. Ceiling joists can be on both low-sloped roofs and high-sloped roofs, and help to prevent racking and spreading of your rafters or walls, which could lead to sagging. They are important load-bearing structural elements that hold up both the weight of the ceiling materials for the floor below, as well as the weight of any objects or people that may be housed within an attic space.

Now, it’s likely that you will be able to guess what roof planks are called when they have a steeper slope than 2 in 12; however, we are going to keep you hanging a little since we’ll be discussing that topic in our next blog! Join us next time when we discuss what trusses and rafters are and some of the important differences between the two.