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Then, using my framing square (some carpenters choose to use a speed square, but speed squares aren’t as precise, especially on fractional pitches), I draw the parallel plumb line across the rafter, marking along the tongue of the square.
This line represents the plumb line on the rafter at the edge of the building.
I generally go with 4 in., and it works well with most roofs. seat cut, I rotate the square 180 degrees from the plumb cuts I’ve marked so far—this way the stair gauges will be referenced against the bottom edge of the rafter.
I then slide the square along the bottom edge until the 8 in.
If you have any two of those values, the calculator will quickly figure out the rest of the right triangle—which means it will tell you everything else you need to know about a rafter. Each rafter only spans half the width of the building, and they start at the face of the ridge beam. The result on my Construction Master calculator is 2 ft. Next, I press the Run key, instructing the calculator to use that dimension as the ‘run,’ which is the first element of the right triangle I am working with.
Most of the time, the two values I have are the run of a building and a specified pitch, which is why I used these values for the example in this online tutorial: Looking at our model roof, I need to find the two elements that will give me all the information needed to frame the roof. For simplicity, and to prevent error, the first thing I do is deduct the full width of the ridge beam from the building width: in this example 6 ft. I need one more element, and in this case I know the pitch of the roof, which is 6/12.
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There are also two laudable software versions available as smartphone apps: one from Calculated Industries, and one from Build Calc.All I have to do is press the Diag key, and the calculator displays the measurement: 3 ft. I write this measurement down on the template rafter, too.Next, I press the Rise key, and write that number down: 1 ft. Be sure to go through all the calculations a few times, clearing the calculator in between.The seat cut (or “bird’s mouth”) is referenced from this line. I like to keep the seat cut the same width as the wall, including the sheathing.If you are framing from scratch, and not matching rafter heights (which will be explored in a future article), you will need to decide on what size the seat cut should be. In my model here, and on most of my jobs using 2×4 walls, the seat measures 4 in. With wider plates, you cannot cut into the rafter more than a third of its overall width—this would weaken the structure too much.