Q: I want to build a simple shed. How do I cut a roof rafter? A: To cut a roof rafter, you'll need a framing square, tape measure, pencil and saw. It will also help if you recall basic geometry.

Q: I want to build a simple shed. How do I cut a roof rafter?

A: To cut a roof rafter, you’ll need a framing square, tape measure, pencil and saw. It will also help if you recall basic geometry.

A roof rafter on a doghouse or a mansion is just the hypotenuse of a right triangle.

If you look at a standard carpenter’s framing square, you’ll see two of the legs of a triangle. They form the 90-degree angle.

The third side, or hypotenuse, can be created by connecting one of the numbers on each leg of the framing square. That is the magic of creating a basic rafter.

The pitch of a roof is designated by identifying the rise per foot of run. If a carpenter says, “ Your house has a 4/12 roof,” that means the roof rises 4 inches for every 12 inches of horizontal run.

The first thing you need to do is determine the slope your roof will have. Then determine the width of the shed.

You need this measurement because half of it will be the base of your big triangle.

To cut the lumber, lay the square on the flat board and use the outside edges of the square and align the roof slope numbers you have decided to use. Draw that line with a pencil.

Before you move the framing square, make a pencil line across the lumber along the bottom edge of the fatter leg.

Slide the framing square down the lumber and align the narrow leg on this mark and repeat the marking process so you create a cascade of small triangles that represent your roof slope.

Making the birdsmouth cut where the rafter sits on top of your shed wall isn’t too hard. The framing square legs create both lines for you. Use the square to make a final line that is parallel with the first line.

This line extends down from the tip of the last small triangle you created. Align the framing square one last time with your roof slope numbers and be sure the wider part of the square falls across the last line. Trace along the square a line that extends from the last vertical line about 2 inches.

This tiny triangle makes a tight corner allowing the rafter to sit flat on top of the wall.

Tim Carter is a columnist for Tribune Content Agency. He can be reached via his website, www.askthebuilder.com.