![]() (Total span - ridge) / 2 = individual rafter span You might need a long tape measure to get the span, then The first thing to do when working out the actual length of a common rafter is determine the horizontal span of the rafter as discussed in the previous page.Įach common rafter span is calculated by subtracting the thickness of the ridge from the total span, then dividing the answer by two. We'll cover the purple overhang separately and deal initially with the green triangle in the picture that takes the rafter to the wall plate. That way you can get a perfect straight line for the wall and your fascias will look right even if the brickwork is less than perfect. If the brickwork is not so great and the walls are not perfect, it may be safer to cut the rafters with longer overhangs at the bottom, and then mark the tails for cutting later. If your walls and wall plate are all perfectly straight and square, you are probably safe to cut the complete rafters in one go on the ground and then take them all up and assemble the roof. The difference between the green span and the purple span below is the addition of the horizontal overhang shown above to allow a fixing for the fascia and soffit boards. You can see in the picture above where the two right angle triangles are that we use to make the calculations for our rafters. One for the actual length from plate to ridge, the other for the added overhang that will take the fascia and Soffit boards if your roof has these. If you use the center line for your Rafter Length(L) calculation, remember to subtract half of the thickness of the Ridge from the Rafter Length(L) result from the Roof Pitch Calculator.There are two quick calculations to determine the two lengths as shown in the picture above. Note that the diagram below shows the Rafter Run extending from the outside edge of the top plate (location of the inside of the bird's mouth) to the center line of the Ridge. The Ridge Height can be calculated using the same formula as the Ledger Height on a Shed Roof. Once you have verified the first rafter, you can use it as a template to lay out the remaining rafters.Ĭalculating rafter sizes and spacing is the same for a gable roof as it is for a shed roof. Roof framing is easy, right? Please verify all of the measurements before you start cutting rafters. Ledger Height (from the top of the slab) = Wall Height + Rafter Rise(M) + Y Wall Height: 96 inches (assuming an 8-foot wall).Now that the 'Y' height is known, you have the data you need in order to calculate the Ledger Height. Using the Rafter Span Tables on, you can see that 2圆 rafters (Southern Pine #2 Grade), spaced 16 inches apart, will span 8-5 (8 feet - 5 inches). ![]() Rafter Run: 8 feet (horizontal distance between my Ledger and outer wall).Rafter Material: Southern Pine #2 Grade (because it is the most widely used framing lumber in my region).Design Load: 70 PSF ( snow load of 50 PSF and a dead load of 20 PSF).Rafter Run: This is the horizontal distance between supports.įor this example, let's say you have the following design criteria:.Rafter Material: Select an available species of framing lumber in your area. ![]()
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