Ask the Builder: Know slope before building roof rafter

DEAR TIM: I’m now retired and starting to attack a huge list of projects. One of them is building a simple shed, and I have no clue how to cut and create a simple roof rafter. I know it can’t be that hard, but I’m confused. What’s the process, and do you think a rookie with little or no experience can cut simple rafters to create a basic gable roof? – Paul E., Canton, Michigan

DEAR PAUL: Roof framing is one of the aspects of construction I savored as a builder and carpenter.

I’ve got good news: Not only can you cut a simple common roof rafter, but you can also, with minimal guidance, cut hip and valley rafters along with all the jack rafters that connect to them.

You will need a framing square, a tape measure, a pencil and a saw. That’s it. It will also help you if you recall some of the basic geometry you hopefully learned in high school.

A roof rafter is just the hypotenuse of a right triangle. Make a simple line drawing of a gable and you’ll see that it is just two right triangles standing back to back, mirroring one another.

If you look at a standard carpenter’s framing square, you’ll quickly see you have two of the legs of the triangle in that tool. They form the 90-degree angle. The third side, or hypotenuse, of that triangle can be created by just connecting one of the numbers on each leg of the framing square. That’s the magic of creating a roof rafter in its most basic form.

The pitch of common roof rafters is designated by identifying the rise of the roof per foot of run. You may hear a carpenter say, “Your house has a 4/12 roof.” What he’s saying is the roof on your house rises 4 inches for every 12 inches of horizontal run. (In the United States, the run figure in the denominator is always given as 12.)

If you lay a framing square on a table and look at the numbers on the outside edge, you’ll see they start counting out from the corner. What your carpenter would do is go up the narrow leg of the framing square to 4 inches and go out on the fatter leg 12 inches. If you lay a straightedge between these two points on the framing square, you’ll create the sloped part of the triangle.

The first thing you need to do is determine your roof slope. How many inches of rise will your roof have per 12 inches of run? You might want to match the slope of your house roof, or you can go with a 4/12 roof slope, as that will shed water but it’s not so steep as to be hard to work on.

Then determine the width of the shed. You need this measurement because half of it will be the base of your big triangle. Let’s say you’re going to have a shed that’s 10 feet wide, and you’ve decided to go with a slope of 4/12. This means your simple line drawing of your roof will show the peak of the triangle to be 20 inches above the top of the walls. That’s because for each foot of horizontal run from the outside of the shed wall, the roof climbs 4 inches, and five times four equals 20.

In reality, the peak of your roof will be probably 23 inches above the actual top of the walls. This happens because you will make a little birdsmouth cut in the base of the rafter so it sits flat on the top plate of the wall. Most rafters extend down past the edge of the exterior wall, creating an overhang. Roof overhangs are good things because they keep rainwater away from the sides of the building.

You will have a heel above the notch in the rafter, and the heights of these heels vary with the size of the roof rafter, the width of the wall, and the size of the birdsmouth cut. This is the part of the job that will confound you.

Here’s how you use your framing square to measure and cut the lumber for a rafter: Starting at one end of the piece of lumber, lay the square on the flat board and use the outside edges of the square and align the roof slope numbers you’ve decided to use. You’ll notice the narrow leg of the framing square makes an angled line across the lumber. Draw that line with a pencil. That will become your plumb cut up at the top of the rafter where it touches the opposing rafter or a center ridge board.

Before you move the framing square, make a pencil line across the lumber along the bottom edge of the fatter leg of the framing square. 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. If your shed will be 10 feet wide, you’ll end up with five of these triangles and sets of marks as you march down the piece of lumber.

Making the birdsmouth cut where the rafter sits on top of your shed wall is not too hard. The framing square legs create both lines for you. Use the square to make one last final line that’s parallel with the first line you drew at the far end of the rafter. 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 this last line. It will form a 90-degree angle to the last vertical line. Trace along the square a line that extends from the last vertical line about two inches.

This tiny little triangle of wood, once cut out of the rafter, makes a tight corner allowing the rafter to sit flat on top of the wall and the other side of the notch fitting tight against the vertical face of the outside of the shed wall.

You may have to make a couple of test cuts to get this right. Good luck!