How to Cut a Compound Roof Rafter
Compound rafter cuts occur at roof valleys, on hip roofs or anywhere two sections of roof meet at an angle. The compound cut has two components -- the pitch of the roof and the angle at which it meets other rafters. A framing square makes marking the plumb line straightforward and a circular saw with the foot set at an angle makes the compound cut on the rafter.
Hold one end of the rafter with the faces vertical and look along the top edge to see if it bows up or down. The rafter has a natural bow and the peak of this bow is called the crown. Mark the crown edge as the top of the rafter.
Set the rafter across the sawhorses with the crown edge facing you. Lay the framing square on the rafter near the end with both legs on the face of the rafter and pointing away from you.
Align the 12-inch mark on the long leg of the square with the front edge of the rafter, and the mark that represents the roof pitch on short leg of the square with the front of the rafter. Draw a line using the short leg of the rafter as a guide from one side of the rafter to the other.
For example: On a roof pitched 4-in-12, the mark used on the short leg will be the 4-inch mark.
Set the circular saw foot to the angle at which the two roof sections meet. The foot only angles in one direction, therefore the cut will begin at either the top or the bottom edge of the rafter depending on which side of the hip, ridge or valley rafter to which the rafter you are cutting attaches.
Take the uncut rafter onto the roof and temporarily place it. Mark the angle at which it joins the opposing roof sections on the top edge of the rafter. Verify the roof pitch is laid out correctly -- the cut line marked on the rafter will be vertical when correctly marked.
Place the rafter on the sawhorses. Set the saw on the rafter and match the angle of the blade to the angle marked on the edge. Make the cut through the rafter with the saw.
Mark the rafter length on the opposite end of the rafter and lay out the pitch as before. The cuts at the rafter ends are parallel to each other. Set the saw foot angle to zero degrees and make the cut along the line.
Things You Will Need
- Framing square
- Circular saw
- The roof pitch and the angles at which rafters meet are shown on the building plans.
- Most compound rafters have a compound cut at one end and a simple pitch cut at the other. If your rafter requires two compound cuts, mark the cut angles on both ends and match the saw angle to the cut angle when cutting.
- Double check your angles and pitches before cutting. With experience, you'll easily visualize the cut, but it takes practice; double checking your measurements and layouts will prevent mistakes.
- Always wear appropriate safety gear when using power tools. Take extra care when working on roofs.