How To Frame Hip Roofs 11

Basic Construction And Carpentry Techniques Home

Hip Jack Rafter Line Length Calculation

Jack rafters have the same rafter spacing as the common rafters. See figure 11. If the rafters are spaced at 400 mm oc then the distance of the first jack from the corner of the building is 400 mm. The run of the jack is then 400 mm. The line lengths of the jacks can be calculated. The units of run for the first jack = 400 mm divided by 250 mm = 1.6. 1.6 times the unit line length of 291.5 mm for a 150/250 slope = 466.4 mm rounded to 466 mm. The line length of the first jack is 466 mm. The run of the second jack is 800 mm and its units are 3.2. 3.2 times 291.5 mm = 932.8 mm rounded to 933 mm, the line length of the second jack. The "difference in jacks" formula can also be used to determine the line lengths for each jack. If the jacks are spaced at 400 mm oc and the slope is 150/250, then the line length for the first jack is 1.6 times 291.5 mm = 466 mm. 466 mm is subtracted from the common rafter line length to give the line length of the jack closest to the common rafter. 466 mm is then subtracted from this jack to give the line length of the next smaller jack and so on.

Jack Rafter Cheek Cut Layout

The jack rafter cheek cut shortens the length of the jack rafter and enables it to fit against the hip rafter. See figure 11. To make a cheek cut square the top edge of the jack rafter at the working point at location 1. Use the common rafter units of rise and run and draw a plumb line at the working point at 1. Mark the plumb line along the unit of rise. Measure back a distance equal to half the distance of the diagonal thickness of the hip rafter, 90 degrees to the plumb line, and draw another plumb line on the jack rafter at 2. Extend and square the line across the top of the jack rafter.  Measure 1/2 the thickness of the jack at 90 degrees to the plumb line, ahead and back of the new plumb at 2. Mark the two new locations at 3 with plumb lines that are extended and squared across the top of the jack rafter. Draw a cutting line connecting the opposite corners of the two plumb lines at 3.

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Note: If you are having trouble fitting the hip roof components together, check to make sure the roof is plumb, level and square before you redo your measurements and cuts.

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Written and maintained by
Ronald Hunter

All images and text are copyright Ronald Hunter 2005 to 2011