How To Figure Out Roof Rafter Length

There is no standard universal pitch of roof.

How to figure out roof rafter length.

Determine the camber along the timbers length. Place the board on a set of sawhorses with the crown away from you. The angle or pitch of a roof is calculated by the number of inches it rises vertically for every 12 inches it extends horizontally. Calculate the length of a rafter from the roof slope ratio of inches per foot and a building width measurement.

If you add an eaves overhang dimension then the amount the rafter sticks passed the wall will be added to the rafter length. Divide this in half and the result is 7 feet. Includes result for hip valley factor slope factor and the roof slope in degrees. Mark the plumb cuts.

Use a framing square to mark a plumb cut as close to the end of the board as possible. For example if the ridge beam is 1 5 inches thick subtract 3 4 of an inch from the overall rafter length. Use the pythagorean theorem to calculate the rafter length. Before you can mark the rafter lengths and layout check the.

With the tape measure hooked onto the top end transfer your two rafter lengths onto. In this example the shorter walls are 14 feet long. Rise and run means that a 6 12 pitch roof has 6 of rise vertical for each foot of run horizontal. Marking out the roof rafter lengths onto the timber before cutting step 1.

Using the appropriate roof pitch table in a rafter cutting guide such as 4 12 determine the length of each rafter based on the correlating measurement shown for 7 feet. I prefer using the right hand side of the board for this. Our roof rafter calculator tools are handy for calculating the number of rafters needed rafter length calculator lineal feet of rafter board feed in ridge and sub facia and the total board feet in the roof. If you are calculating rafter length from the center of the ridge beam as shown below you will need to subtract half of the thickness of the ridge beam from the top end of the rafter.

Laying out a calculated rafter. For example a roof that rises 6 inches for every 12 inches of horizontal run has a 6 12 pitch. Rafter length using pitch v r 2 sqrt 1 p 2 where v overhang r roof span p pitch.