Since I published the first review of the Swanson Speed Square, many people have been wondering how to use it. There are actually a large number of things you can do with it, but the most common is probably to use it the same way a framing square is used - to determine the correct angle to cut rafters.I'll show you how to determine the angle you need to use.
But before we get there, a couple words on how the pitch or slope of rafters is stated. In the US, rafter slope is typically given in terms like "4 in 12" or "12 in 12". The first would be close to the minimum slope for a shingled roof and refers to a vertical rise of 4 inches for every horizontal run of 12 inches. The reason that 12 is used as the denominator is that by doing so it is easy to do the math. If you have a 10 foot run, and a pitch of 4 in 12, you simply multiply the 10 by 4 and arrive at a vertical rise of 40 inches.
Now, calculating the correct angle for a common rafter is simple with a little trigonometry, but the speed square takes away the need to drag out a calculator. Just set the pivot at the edge of the board and then rotate the square around that point until the edge of the board aligns with the correct mark on the "Common" scale - marked in red in the picture. Then draw a line along the ruled side of the square (the one parallel with the yellow mark). You can see that at "12 in 12" the edge will make a 45% line.
For hips and valley rafters, the angle is slightly different because the ridge itself is at an angle. This is a bit more difficult to do the math for, but once again, the speed square takes away the need to do math. Just do the same procedure as for common rafters, but instead use the "Hip-Val" scale. You will see that because the boards meet at an angle the cut is not as steep.
One thing the speed square won't do for you is determine rafter lengths. However, simple geometry will tell you that the square root of (rise-squared + run-squared) will give you the length of the other side of the triangle. For this you may need a calculator.
One other feature which I've found useful at times is the scribe feature. In the area marked in yellow in the picture you see a number of notches a quarter of an inch apart. You can set a pencil in the notch and slide the square along the board and have a mark parallel to the edge.
As you can see, the speedsquare is rather coarse in its markings. That isn't to say it is not accurate, but rather, the fineness of the markings is better suited to wood framing rather than fine woodworking. So buy one of these and throw it in your toolbags. It should last a long time.