How to Cut a Roof Rafter

What You'll Need
Framing square
Meter tape
Worm drive circular saw

The laying out and cutting of a rafter for your roof are not the simplest of tasks; they require both careful calculation and precise execution. This brief tutorial shows the steps you should follow when carrying out these operations.

Step 1 - Take Measurements

Purchase a framing square in case you don’t have one already. This device is an L-shaped measuring tool, outfitted with plastic or metal stops, which is used to mark the desired length. Then, place the rafter you are to cut on sawhorses. Set the stops of the framing square at the desired values, making sure you use only the inside or outside parts of the square and not an inside and an outside part.

Step 2 – Determine the Plumb Cut

Choose the top side of the rafter and place the square tongue at the angle between the rafter length and width. Draw a line with a pencil. This will be your plumb cut or the cut at the top of the rafter where it joins the contact board of the roof.

Step 3 – Mark the Birdsmouth

Place the tip of a meter tape at the plumb cut and mark the desired length of the rafter. At the marking, draw a line using the square tongue. This line is where the birdsmouth will lie. Here, the birdsmouth is the notch in the rafter that will rest on the top plate of the wall.

Measure the width of the wall plate. Mark it on the framing square and slide the square down the line you marked in the previous step until the distance between the line and the side of the rafter becomes equal to the width of the wall plate. When it does, draw a second line which will enclose the triangle of the birdsmouth.

Determine the Overhang Line

You need to also make an overhang cut (a.k.a. tail cut) which is the cut at the bottom of the rafter that will hang over the exterior wall of the house.

To lay out the overhang, take the meter tape, place it at a straight angle to the line you made in Step 3 and mark the desired overhang length. At the marking, draw another line parallel to the first. This line will be your overhang line.

Rafter Cutting

To cut the rafter thus laid out, you have a variety of tools available, all of which can be of good use. However, provided that you can afford it, a worm drive circular saw would be the most appropriate cutter for this operation. Whichever instrument you choose to use, be sure to do the cutting with the utmost caution and full concentration.