Thursday, September 5, 2013

Cutting Jack Rafters on Irregular Hip Roofs

Cutting Jack Rafter Side Cut Angles on Irregular Hip Roofs

For an example of cutting the jack rafter side cut angles we'll use a roof with an 90° Eave angle and a 9:12 & 12:12 pitched roof.

Plan Angles:
9:12 -- 53.13010°
12:12 - 36.8699°

You can cut the jack rafter side cut cheek angles using the plan angle as the saw blade bevel angle along the miter/plumb line on the side of the jack rafter. In this example, the 9:12 jack rafters would use a saw blade bevel angle of 53.130100° to cut the jack rafters side cut cheek angles. Unless you have a swing table that tilts over to the  53.130100° you need to use a different approach, besides using a hand saw.

To cut the correct jack rafter side cut cheek angle you can set the saw blade bevel angle to the complementary angle of  53.130100°, which is 36.8699°.  The complementary angle, or sometimes referred to as the Bisected Foot Print Angle, is 90° minus the plan angle. For an equal sided Octagon roof the plan angle is 67.5° and the complementary angle is 22.5°.

First layout the jack rafter side cut angle using a framing square or use the shiften method.


Cut the plumb cut of the jack rafter square.


Set you saw blade bevel angle to the complementary angle and run the saw down the face of the plumb cut.



The correct jack rafter side cut angle in this example is 30.96376°






2 comments:

  1. One thing I've done is clamp some more complementary cut boards to the rafter to give the saw foot a broader surface to follow.

    The other way I do them is just cut the pitch with a square head cut and add ridge blocking to bolster the Jacks square face cut. (Ok if hidden in the attic, not for exposed stuff.)

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    Certified Precision Roofing

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