Rafters

Anatomy of a Rafter

Basic Rafter Layout

Laying out the framing for roof involves four tasks:

1) Mark off the rafter locations on the top plate.

2) Determine the length of each rafter.

3) Make a plumb cut at the ridge-end and tail-end of each rafter.

4) Make a bird’s mouth cut in each rafter.

Terms Associated with Rafters

The following is a list of terms related to rafters. Click on each term to find out more:

Span

The horizontal distance from the outside of one exterior wall to the outside of the other exterior wall.

Run

The horizontal distance from the outside of the exterior wall to the outside of the top plate to the center line of the ridge board (usually equal to half of span).

Rise

The total height of the rafter from the top plate to the ridge. This is stated in inches per foot of run.

Pitch

The angle or degree of slope of the roof in relation to the span. Pitch is expressed as a fraction; e.g., if the total rise is 6’ and span is 24’, the pitch would be ¼ (6 over 24).

Slope

The inclination of the roof surface expressed as the relationship of the rise to the run. It is stated as a unit of rise to the number of horizontal units; e.g., a roof that has a rise of 5” for each foot of run is said to have a 5 in. 12 slope. The roof slope is sometimes referred to as the roof cut.

Determining the Length of a Common Rafter

The first step in determining the correct length of a rafter is to find the unit rise, which is usually shown on the building’s elevation drawing.

The unit rise is the number of inches the rafter rises vertically for each foot of run. The greater the rise per foot of run, the greater the slope of the roof.

A rafter framing square is a special carpenter’s square that is calibrated to show the length per foot of run for each type of rafter.

Determining the Length of a Common Rafter

The next slide presents the steps for determining the required length of a rafter.

Step 1

Measure the building span. Divide that in half to determine the run.

Step 2

Determine the rise. (Calculate the total rise by multiplying the span by the pitch. For example, 40’ span x ¼ pitch = 10’ rise. Look for it on the slope diagram on the roof plan as discussed previously.)

Step 3

Divide the total rise by the run to obtain the rise.

Step 4

Look up the required length of the rafter tables on the framing square.

Speed Square for Rafter Layout

The speed square, also known as a super square or quick square, is a combination tool consisting of a protractor, try miter, and framing square.

A standard speed square is a 6”-triangular tool with a large outer triangle and a smaller inner triangle.

The large triangle has a 6” scale on one edge, a full 90-degree scale on another edge, and a T-bar on the third edge. The inner triangle has a 2” square on one side.

A 12” speed square is used for stair layout.

To use a square, you need to know the pitch of the roof. When you buy a speed square, it usually comes with an instruction booklet. This booklet normally contains (among other information) tables that show the required rafter length for every pitch.

Laying Out and Cutting a Common Rafter

The following is an overview of the procedure for laying out and cutting a common rafter:

1. Start with a piece of lumber a little longer than the required length of the rafter, including the tail. If the lumber has a crown or bow, it should be at the top of the rafter. Lay the rafter on sawhorses with the crown (if any) at the top.

2. Start by marking the ridge plumb cut using the framing square. Be sure to subtract half the thickness of the ridgeboard. Make the cut.

3. Measure the length of the rafter from the plumb cut mark to the end (excluding the tail) and mark another plumb cut for the bird’s mouth. Reposition the framing square and mark the bird’s mouth seat.

4. Make the end plumb cut, and then cut out the bird’s mouth. Cut the bird’s mouth partway with circular saw; use a hand saw to finish the cuts.

5. Use the first rafter as a template for marking the remaining rafters. As the rafters are cut, stand them against the building at the joist locations.

builders should know the building span of rafters when building roof framing or rafts..good notes.