For example, the width of the building may be 72. 75 inches (184. 8 cm). Use a pencil to jot the width down somewhere. Write on a spare sheet of paper or a section of exposed wood. Making these measurements 2 or 3 times is a good way to ensure accuracy.
So the new width measurement will be 71. 25 inches (181. 0 cm). Jot down this “adjusted” width measurement as well.
So, the example halved width measurement will be 36. 63 inches (93. 0 cm). The final measurement is referred to as the “run” of a building. Write the run measurement down alongside the other measurements. If you’re using a roofing calculator, press the “Run” button to input the run measurement in the calculator’s memory.
If you’re using a roofing calculator, press the “Pitch” button to input the pitch ratio in the calculator’s memory.
If you’re not using a roofing calculator, you’ll need to perform some complex trigonometry to calculate the rafter length. You’ll also need to purchase a copy of a roofing ready reckoner: a manual with rafter length tables. Roofing calculators are commonly used by professionals. If you’d rather not purchase one, there are many available for free online. For example, there’s a free roofing calculator at: https://www. roofcalc. org/roof-rafter-calculator/. Keep in mind that, if you want your rafters to overhang past the edge of the building wall, you’ll need to add that measurement in separately. The calculator (or math equation, if you’re writing it out longhand) will only calculate the diagonal measurement to the edge of the building.
If you’re not using a roofing calculator, simply subtract the height of your walls from the total height of the roof at its peak. The example rise will be 17. 81 inches (45. 2 cm). Jot down this number alongside the others you’ve already written.
You can purchase both stair gauges and a framing square at your local hardware store.
Since you’ve attached stair gauges to the framing square, the line you’ve drawn will be at the correct angle for the rafter to lay flat against the ridge beam.
Clamp the rafter to a workbench or sawhorses before cutting to ensure stability. C-clamps would work well for this. If you’re using a handsaw, grasp the handle firmly in your dominant hand. Saw back and forth using the full length of the blade.
The diagonal measurement is the full length of the rafter, minus the overhang. Measuring the diagonal will allow you to make the tail and bird’s mouth cuts in the correct locations. In this example, the diagonal measurement that you found earlier was 39. 81 inches (101. 1 cm).
Use your pencil to trace the full length of the tongue along the width of the rafter.
Sheathing is the term used for the board or panel that forms the outer surface of a building.
If your bird’s mouth cut is not 4 inches (10 cm) deep, slide the framing square to shorten or lengthen the length of the cut accordingly. [16] X Research source
Overhang is the portion of the rafter that will extend past the edge of the building’s wall. This will lengthen the total length of the rafter by 6 inches (15 cm), bring the new total length to 45. 81 inches (116. 4 cm).
Now all that’s left is to cut the rafter accordingly.
For added stability when making the birdsmouth and tail cuts, use C-clamps to clamp the rafter to a workbench or sawhorses.
The number of rafters needed will be determined by the length of your roof. Rafters are commonly spaced about 24 inches (61 cm) apart. Refer to this chart for more detailed rafter-spacing information: https://www. mycarpentry. com/rafter-span-tables. html. Keep in mind that you’ll always need an even number of rafters.
