Valley Framing: Calculating the Jack-Rafter Assignment
With the run measurements made from the site conditions, Warwick CTC students calculated the diagonals and made templates.
The valley between the north and east roofs is a common valley, with the intersecting roofs at the same 10-in-12 pitch. The challenging part is that the sub-ridge brakes the valley line into two sections. The lower section is a true valley, with a valley rafter between a rafter beam and the sub-ridge, and the upper part is a layover valley. I couldn’t figure out a simple way to pinpoint the center of the valley line between the rafter beam and the cross-gable ridge. I ended up extending the plane of the cross-gable rafters to locate where it met the main roof plane on the rafter beam. At the cross-gable ridge, I located the point where the main rafter plane met the edge of the ridge board. I ran a string between the points. The string pinpointed the position of the valley rafter between the sub-ridge and the rafter beam.
From there I could calculate and cut the valley to length.
Ordinarily a ridge-to-plate valley rafter has a top plumb cut with single or double bevels, and a seat cut. On this one, the top and bottom cuts are plumb cuts with single bevels.
I didn’t have time to calculate the valley jack rafters when working on the weekend, and Monday it’s supposed to rain. So here is a plan view with measurements that the students at Warwick CTC used to calculate the jacks.
The valley jack layout has three jacks. I marked measurements on the plan. The measurements are all runs (horizontal measurements). They are not diagonal measurements.
![A view from the spot where the cross-gable ridge meets the plane of the main roof and down the center of the overlay valley area and the true valley that runs between the sub-ridge and the rafter beam that supports the sub-ridge.The valley jack layout has three jacks. I marked measurements on the plan. The measurements are all runs (horizontal measurements). They are not diagonal measurements.](https://images.finehomebuilding.com/app/uploads/2016/09/10172625/a-view-from-the-spot--700x394.png)
Here’s what the students came up with for the diagonal lengths for each rafter:
The hangers used to mount the valley jacks to the valley rafter are skewable. An unskewed one is lying flat on the floor beneath one I bent and tack-nailed to the side of the valley rafter. Using the skewable hanger eliminates the need to make an angle cut on the valley jacks. They can just be square-cut to the short-point measurement.