Roof framing: how to measure, cut, and build roof rafters

Jack Wisniewski

By Jack Wisniewski

May 26, 2021

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Roof construction starts with framing.

Not only does it provide vital support for the weight of the roof, but framing also determines the roof design, attic space, cathedral ceiling, and dormer opportunities.

You have two main options when it comes to roof framing – trusses and rafters.

While many seasoned framers prefer pre-built roof trusses for their convenience and cost-effectiveness, piece-by-piece rafters arguably make more sense for smaller and custom builds.

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Rafters can save you on delivery costs and require minimal space for assembly. Furthermore, individual framing members weigh less than fully built trusses, which means you need less muscle for the job. Rafters also allow for maximum attic space once the structure is finished.

The roof framing process can get complicated for the uninitiated when it comes to complex roof shapes like the hip, mansard, and gambrel. However, building rafters for basic types of roofs like a gable or shed is relatively straightforward with some geometry and carpentry know-how.

This article provides step-by-step instructions for measuring, cutting, and erecting basic gable roof rafters.

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Roof framing basics: what is a rafter?

Roof framing rafters

A singular rafter frame typically includes three components:

  • Rafters
  • Ridge board
  • Rafter ties

Roof framing: key terms and definitions

Rafters

The rafters are the diagonal members that directly support the roof deck.

Ridge board

The ridge board serves as a nailing surface for the opposing rafters and provides no structural support.

Rafter tie

The rafter tie resists the outward forces imposed on the load-bearing walls. As the rafters sit on top of the wall plates at an angle, they exert horizontal forces on the exterior walls. Rafter ties, which often double as ceiling joists, prevent these horizontal forces from causing the walls to "pancake."

Ridge beam

Some framing systems use a ridge beam combined with gable-end posts instead of a ridge board, which provides structural support to the rafters. For roofs with less than 3:12 pitch, codes require ridge beams because they reduce the extreme outward forces on the walls. A ridge beam can eliminate the need for rafter ties, which is often paramount for the cathedral ceiling aesthetic.

Collar ties

Some applications call for collar ties, often confused for rafter ties. Although they run parallel to the rafter ties, they sit towards the upper-third of the rafter to resist separation at the ridge – not the walls. Collar ties mainly come into play where codes require improved ridge strength, such as in high-wind climates.

Tools needed to build roof rafters

To build a roof rafter, gather the following materials equipment:

  • Lumber stock
  • Framing square or speed square
  • Pencil
  • Saw
  • Measuring tape
  • Hammer and nails or a nail gun
  • At least one helper
  • A construction calculator (optional)

How to build roof rafters

1. Know your measurements

Measurements of the roof

Before you cut anything, you need the span, rise, run, and line length of your roof frame.

  • Span: the length between the outside supporting walls
  • Rise: the height of the roof ridge from the center of the span
  • Run: half the span
  • Line: the "hypotenuse" of the triangle, as in the diagonal distance from the outer wall to the center of the ridge along the rafter

The span and run measurements hinge on wall dimensions. Before you can install a roof, the walls must be in place. In turn, you should already have your span and run measurements to work off of.

The rise and line measurements, on the other hand, depend on your roof pitch.

2. Determine roof pitch

How to determine roof pitch

The roof pitch dictates the angles at which you need to cut your rafters as well as their length.

Fortunately, you can figure out roof pitch with a simple equation – rise divided by run.

For instance, if your rafter rises 6 inches over every 12 inches of run, your roof pitch will come out to 6:12.

You may need to consult building codes to ensure your roof pitch meets the requirements for the planned roofing material. For example, the International Residential Code requires roof slopes no smaller than 2:12 for asphalt shingles. Any lower and you will need to use a flat roof material to meet standards.

Rise and line measurements

Once you zero in on your roof pitch, you can determine the rise and line measurements.

Remember that you should already know the run of the roof. By hand, you can create a proportion between your fixed roof pitch (rise over run, like 6:12) and actual roof run to solve for the missing value (actual rise).

Pythagorean Theorem equation

With your rise and run figured out, you can then calculate your line measurement (the triangle's hypotenuse) using the Pythagorean theorem.

If you're not confident in your math skills, a contractor can use a construction calculator with a rafter function to speed up the process.

Regardless of line length, you must account for the width of the ridge board that will go in between the opposing rafters. If your ridge board is a 1 1/2-inch board, subtract 3/4 inches on our line length from each rafter.

3. Buying lumber

A stack of lumber

Image source: Bloomberg

There are two things to consider before buying lumber for roof framing:

  • How long do the boards need to be?
  • How many frames do I need?

Board length

With our measurements all figured out, we know not to purchase lumber shorter than our required lengths. We recommend using lumber at least a foot longer than your final measurements plus rafter overhang to provide ample space for cuts.

How many frames

There are two extreme methods to go about planning frames with a designated rafter length.

  • Maximum amount of frames, weakest allowable lumber
  • Minimum amount of frames, sufficiently strong lumber

For example, the widest conventional rafter spacing is 24 inches on-center, which means installing a new rafter course every two feet along the walls. Since you have fewer rafters supporting the roof, the lumber needs to be stronger.

On the flipside, rafters every 12 inches on-center, the shortest distance required by codes, can support a roof with weaker wood because the system consists of more frames.

Lumber strength stems from the species, grade, and sawn dimensions. Instead of weighing all of these variables yourself, the American Wood Council provides tables that outline the maximum allowable spans of softwood lumber in standard sizes under normal loads depending on rafter spacing.

Once you vet out your lumber, you can calculate how many boards you need.

Let’s say you need rafters 24 inches on-center over a 16-foot building length. Every frame requires two rafters. If there are rafters every two feet, we need 8 frames, which means we need 16 pieces of lumber for rafters.

Then, you have to account for your ridge board, which should measure one size higher than your rafters to provide ample space for nailing. If you use 2-by-6 rafters, your ridge board should be 2-by-8 that runs the length of the building plus overhang. If your building is too long, you may need to splice two pieces of ridge board together.

Don't forget about rafter ties and collar ties, if needed.

As always, you should order more lumber than you need to account for errors.

4. Outline the cuts

We have our lumber. Now it’s time to cut our rafters.

Birdsmouth rafter cut

There are five cuts you need to make for a common rafter.

  • Plumb cuts: where the rafter meets the ridge board
  • Tail cut: the lower end of the rafter that establishes the roof eaves
  • Bird’s mouth: where the rafter meets the wall top plate
    • Seat cut
    • Heel cut
  • Fascia cut: shortening the lower tail-end of the rafter to accommodate fascia and soffit, if necessary

Start by outlining the plumb cut. To do this, you need to pencil in the cutting angle by following your predetermined roof pitch. Professional carpenters pinpoint the required angle by using a framing square or a speed square.

We explain the process using a framing square. With this tool, it helps to use stair gauges, which keep measurements consistent.

Start by picking out the straightest lumber you have. If there is a curve or “crown,” make sure it faces up to prevent the roof from sagging following installation.

Using a framing square

Framing square

The long arm of a framing square is called the “blade.” The short arm is called the “tongue.”

In this example, we’ll outline cuts for a 6:12 pitch roof.

  • Place the blade (long arm) of the framing square on top of the rafter board so that the tongue (short arm) sits upright. It should look like an elongated "L."
  • Push the end of the blade up until you reach the 6-inch mark on the tongue and the 12-inch mark on the blade at the upper edge of the rafter board.

Plumb cut

  • Draw a line following the outside edge of the tongue – this marks the plumb cut.
  • Starting at this line, measure down the rafter until you reach the line length.

Bird's mouth

  • Once you reach the necessary rafter length, make the same angle with your framing square as you did for the plumb cut – this marks the heel cut of the bird’s mouth.
  • Turn over your framing square, holding it perpendicular to the bird’s mouth line.

Seat cut

  • Match the blade measurement at the bottom of the rafter with the width of your wall plate and draw towards the heel cut line – this marks the seat cut of the birds’ mouth.
  • Beyond the bird’s mouth resides the rafter tail. Measure out your overhang.
  • At this point, mark your tail cut parallel to your plumb and heel cuts.

5. Make the cuts

Circular saw cutting lumber

Image source: The Family Handyman

Now that you’ve marked your lines, you can make the cuts with your saw to form your first rafter.

You can easily make duplicates by tracing the same cuts with your original rafter.

To ensure their rise, run, and span is correct, place a rafter pair on the ground with a piece of ridge board between them and check your measurements.

You may wish to test the rafters on your building to verify that they sit correctly at both the wall plates and the ridge board. If your initial rafters meet requirements, you can go ahead and cut the rest.

6. Install the rafters

For larger projects, it helps to recruit a helper or two to arrange the initial rafter courses with the ridge board.

  • Create a brace for your ridge board by nailing a 2-by-4 board up the center of the gable-end wall. The board should measure taller than the wall and roof rise combined.

How to install the rafters

  • Place your ridge beam across the walls or rafter ties perpendicular to your rafter layout.
  • Lean your rafters along your outside walls with the ridge-ends facing up. This will give you easy access to them up on the roof.
  • Bring up your gable-end rafters and drive one nail through them into the rafter ties. Make sure the heel cut meets the wall plate.
  • Lean the nailed in rafters against each other. Nothing is supporting them from falling, so your helper can hold them in place.
  • Head to the other end of the ridge board, nail two opposing rafters to their respective rafter ties, and lean them against each other just as you did with the gable-end rafters.
  • Hoist one end of the ridge board up to where the two rafters meet.
  • Nail the rafters to the ridge board.
  • At the initial rafter course, slip the ridge beam between the two rafters nail them off.

At this point, these two common rafters have sufficient support that they will stand on their own. Making sure they're spaced correctly, nail in the remaining rafters to the ridge board.

rafters fastened securely, you can install collar ties, purlins, sway braces, and other supports as needed or required by code.

Roof framing is not as hard as it looks

Circular saw cutting lumber

Image source: The Spruce

The whole process is rather complicated, but when you break down designing, measuring, cutting, and hammering everything together into small, digestible parts, framing a roof can be DIY.

Of course, this guide merely scratches the surface of roof framing. We don't get into the nitty-gritty of building codes, gable studs, and roof sheathing. However, these fundamentals ideally provide a solid foundation for a gung ho roof framer.

Keep in mind that you can skip most of these steps by opting for pre-fabricated trusses. For those who would rather not spend on delivery, recruit additional labor for installation, and seek to hone the waning craft of roof framing, rafters are the way to go.

With enough experience, framing even a complex hip roof with hip rafters, jack rafters, and the like should come somewhat intuitively.

Get in touch with a local contractor today.

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