Getting back to basics in order to lay a solid foundation for the next generation of builders, JLC is launching a new fundamentals series for Training the Trades. We’re focusing on some of the skills a well-rounded carpenter should acquire during his or her first year on the job. Our goal is to provide a primer for people wishing to enter the industry as well as a refresher for veterans and a tool for business managers to use when training employees. We’ll be covering both frame and finish skills, from laying out and cutting common framing members to hanging doors and running trim. We’ll talk about the technical aspects of each component and cover the basic terminology that’s crucial to clear, concise on-the-job communication.

In this first installment, we'll start with roof framing:

Objective: Demonstrate the basics on calculating, laying out and cutting a common rafter for either a full gable roof, or for a shed roof on a porch or small addition.

Prerequisite Skills:

  • Ability to read a tape measure/ruler.
  • Familiarity with using a Speed Square.
  • Ability to safely operate a circular saw.

Tools: There are several approaches to laying out rafters. I’ll cover the most efficient method I’ve found, which uses a Construction Master calculator* and a layout square. The only other tools necessary for layout are a tape measure and a sharp pencil.

To cut the rafter, you will need:

  • Circular saw
  • Hand saw or jigsaw
  • Safety glasses

*For an overview of the Construction Master calculator, watch the video here.

° 2x6 (or larger) rafter stock

Rafter Terms
A common rafter has three basic parts you need to become familiar with:

  • Plumb cut, typically called the ridge cut, at the top end of the rafter.
  • Birdsmouth, which consists of two cuts – a horizontal seat cut and a vertical shoulder cut.
  • Tail cut, which may be cut plumb or square, depending on how the eaves are detailed.

Locate these rafter terms in the illustration below:

Rafter Layout
To lay out a rafter, you need to know the roof slope and calculate the rafter run:

Roof slope is the incline of the roof, expressed as a ratio of the number of inches of rise (vertical travel) for every foot of run (horizontal travel). In our example, we have a 7:12 roof—7 inches of rise for every foot of run. On construction calculators, slope is often called “pitch.”

Rafter run is the horizontal distance the rafter travels. On a full gable roof, this is equal to half the building width minus half the ridge thickness.

Calculating Rafter Length

The calculation sequence to find rafter length on a Construction Master is shown below. Note that rafter length is measured from the tip of a plumb cut at the top end of the rafter (ridge cut) to the building line (shown as a dotted red line in the Section/Elevation below). Rafter length does not include the rafter overhang.

For a detailed explanation of how to run this type of calculation on the Construction Master, watch the video here.

On a shed roof, rafter run is the distance from a ledger on the main building to the outside of an addition wall or porch carrying beam. We need to adjust the total run (73 inches in our example) by subtracting 1 1/2 inches, the full thickness of the ledger (adjusted run = 71 1/2 inches).

Next page: Rafter layout and cutting sequence.