How To Figure Out Roof Rafter Length

Place the board on a set of sawhorses with the crown away from you.

How to figure out roof rafter length.

The angle or pitch of a roof is calculated by the number of inches it rises vertically for every 12 inches it extends horizontally. Marking out the roof rafter lengths onto the timber before cutting step 1. For example a roof that rises 6 inches for every 12 inches of horizontal run has a 6 12 pitch. Determine the camber along the timbers length.

Rise and run means that a 6 12 pitch roof has 6 of rise vertical for each foot of run horizontal. Divide this in half and the result is 7 feet. In this example the shorter walls are 14 feet long. 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.

For example if the ridge beam is 1 5 inches thick subtract 3 4 of an inch from the overall rafter length. Includes result for hip valley factor slope factor and the roof slope in degrees. A2 x b2 c2 your rafter length 14400 20736 35136 the square root of which 187 446 your rafter length in inches step 2 lay out your rafter align your framing square so the 10 mark and 12 mark meet the edge of your rafter stock. 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.

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. Calculate the length of a rafter from the roof slope ratio of inches per foot and a building width measurement. I prefer using the right hand side of the board for this. Use a framing square to mark a plumb cut as close to the end of the board as possible.

There is no standard universal pitch of roof. Rafter length using pitch v r 2 sqrt 1 p 2 where v overhang r roof span p pitch. 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. If you add an eaves overhang dimension then the amount the rafter sticks passed the wall will be added to the rafter length. Mark the plumb cuts.