Saturday, Apr 21, 2018
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Gable roof rafters part of triangle

Q: I have one last project before winter sets in, building a small shed. I'm not sure how to frame the standard gable roof. Can you give me a crash course on how to get the length of the rafter just right?

A: All you really need is a framing square with crisp dimension markings and the tables that one normally sees printed on the body of the square.

The two roof rafters are two of the three sides of a triangle. The base of the triangle would be the straight line that connects them where they pass over the outside surface of the exterior walls.

The numbers on the framing squares are the multipliers that will give you the exact length of the rafter. All you need to provide is the "run," which is one half of the width of the shed that the roof covers. If your shed is to be 20 feet wide, the run would be 10 feet. I always measure the total width to the outside face of the wall sheathing.

The only other thing you have to decide is the roof pitch. That's what the inch markings on the body of the square represent.

Let's say that you want a 4-12 pitch on the 20-foot shed. Using the number from the table on the square, you would multiply 12.65 times 10 to get the length of the common rafter.

The length of the rafter is 126.5 inches. This is the true mathematical length assuming no ridge board.

If you cut two of these rafters exactly the same and set them up on the wall plates, the plumb cut lines at the top would fit perfectly with no gap.

The same would be true for the heel cut where the rafter sits on the wall.

But in practice, there's almost always a ridge board, usually a piece of lumber.

If this is what you're going to do, you must shorten the length of the rafter by half the thickness of the ridge board. It's half because the ridge board is centered between the rafters that touch it.

But that this doesn't mean your rafter length is 3/4 inch shorter than the 126.5 inches. It's close to that, but not exactly.

What I always do is make the accurate plumb cut line on the lumber as if there were no ridge board. Then I measure out from this plumb cut line perpendicular the 3/4 inch shortening dimension.

Next I draw a second parallel plumb cut line next to the first one.

The plumb and heel cut lines are made conveniently with a framing square because a square is a 90-degree angle. The tongue of the square, which is almost always 16-inches long, is oriented 90 degrees to the body of the square.

Since the plumb cut is absolutely the vertical line of a right triangle and the heel cut is the base of a right triangle, you can see how the framing square creates these lines as you slide it up and down the rafter with it set at the proper pitch.

This is why it's important to use the small brass gauges on a square so its orientation doesn't vary as you slide it up and down the rafter.

Tim Carter is a former builder of custom houses. For free home-improvement information go to and sign up for Tim's free newsletter. To ask Tim a question, click the Ask Tim link on any page of the Web site.

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