Roof Framing

Most new homes use factory-built trusses, but stick framing with rafters still shows up on additions, dormers, porches, custom homes and repairs. Either way, the roof is the most dangerous and the most mathematical part of a house frame. A correct rafter comes from a formula and a careful layout. A safe truss installation comes from following the bracing plan.

Numbers here come from common practice and the International Residential Code (IRC). Rafter sizes, spans, connections and ties come from your plans. Never change roof framing without approval, because the roof holds walls together as well as keeping rain out.

What You Will Be Able to Do

• Read roof pitch and find span, run and rise.
• Figure the length of a common rafter.
• Lay out and cut a bird's mouth.
• Describe what ridge boards, rafter ties and collar ties do.
• Install trusses with temporary bracing and sheath the roof.

Roof Terms

Figuring Rafter Length

The rafter is the long side of a right triangle. The run is one side and the rise is the other. For each foot of run, a roof has a fixed rafter length, which you find from the Pythagorean theorem or from the rafter table stamped on a framing square.

Worked example. A gable roof spans 24 feet at a 6/12 pitch.

• Run = 24 / 2 = 12 feet.
• Rise = 12 x 6 = 72 inches, or 6 feet.
• Rafter length = 12 x 13.42 = 161.04 inches, or about 13 feet 5 inches.

That is the theoretical length, measured along the top from the outer plate to the center of the ridge. Two adjustments give you the real rafter:

• Subtract half the ridge thickness. For a 1-1/2-inch ridge, subtract 3/4 inch measured square to the ridge plumb line, not along the rafter.
• Add the tail. Lay out the overhang past the wall line separately.

Many crews use a construction calculator or an app. Knowing the math lets you catch a bad entry before you cut 40 rafters.

Laying Out a Common Rafter

1. Pick a straight rafter to use as a pattern. Mark the crown. Crown goes up.
2. Set stair gauges on a framing square at 12 on the body and the rise (6, for a 6/12 roof) on the tongue.
3. Mark the ridge plumb cut at the top by drawing along the tongue.
4. Measure the length along the rafter edge from that line, after deducting half the ridge.
5. Mark the bird's mouth.
6. Mark the tail with the plumb cut for the fascia.
7. Cut one pair and test it on the wall and ridge. Only then use it as a pattern for the rest.

The Bird's Mouth

A bird's mouth is a notch in a rafter where it sits on the top plate. It has two cuts:

• The seat cut is level and rests on the plate.
• The heel cut is the plumb cut against the outside of the wall.

Make the seat wide enough to rest on the wall. The IRC requires at least 1-1/2 inches of bearing on wood or metal. Do not cut too deep. A deep notch weakens the rafter at the point where it carries the most load. Notches at the end of a rafter are generally limited to one-fourth of the rafter's depth, and many framers keep at least two-thirds of the depth above the seat. Your plans control this.

Finish cuts with a handsaw instead of overcutting past the corner. An overcut creates a weak spot where the rafter can split.

Ridges and Ties

Gravity pushes a rafter down and out. Without something to resist that, the walls spread and the ridge sags.

• Ridge board. A non-structural board that gives rafters something to meet. It must be at least 1 inch nominal and as deep as the cut end of the rafter.
• Ridge beam. A structural beam, with posts at each end, that carries the load. It is what lets a vaulted ceiling skip ceiling ties. Use only what the plan calls for.
• Rafter ties or ceiling joists. Horizontal members low in the triangle that stop the walls from spreading. These are often the ceiling joists, nailed to each rafter.
• Collar ties. Horizontal members in the upper third of the attic that help hold the ridge together. They do not replace rafter ties.

Rafters and walls also need connections that resist wind uplift. Hurricane ties or other metal connectors go in the exact spots the plan shows.

Trusses

A truss is a triangulated frame built in a factory. It is designed as a complete unit.

Warning: Never cut, notch, drill or repair a truss in the field without a written fix from the truss engineer. A cut web or chord can cause the whole truss to fail.

Trusses are strong in their final position but very floppy until braced. Installing them with too little temporary bracing has caused domino collapses that killed workers. Follow the truss manufacturer's bracing plan and the industry's Building Component Safety Information (BCSI) guidance.

1. Read the truss layout. Each truss is marked. Check the plan for location, direction and spacing, commonly 24 inches on center.
2. Store trusses flat or upright and braced on blocking, keeping them dry. Never lift them by the webs.
3. Set the first truss and brace it to the ground or to the structure so that it cannot tip. All other trusses depend on it.
4. Lift each truss with the right equipment. Long trusses need a spreader bar. Only a qualified signal person and rigger direct crane lifts, and no one stands under the load.
5. Fasten at the plate with the specified connectors.
6. Install lateral bracing along the top chord as you go, at the spacing in the bracing plan. Diagonal braces tie the set together.
7. Install permanent bracing shown on the truss drawings. Do not remove temporary bracing until permanent bracing and sheathing are in place.

Never stack heavy loads such as bundles of sheathing on trusses that are only temporarily braced.

Sheathing the Roof

Roof sheathing locks the frame together. Common panels are 7/16-inch OSB or 1/2-inch plywood, depending on spacing and loads.

1. Start at the eave, with the long side across the rafters or trusses.
2. Stagger joints from row to row, and land ends on framing.
3. Use H-clips at unsupported edges where the plan or panel calls for them.
4. Leave a 1/8-inch gap between panels.
5. Nail at 6 inches on edges and 12 inches in the field, unless the plan calls for something tighter. High-wind areas often do.
6. Leave a slot at the ridge if a ridge vent is planned.

Staying Safe on the Roof

Residential framing needs fall protection at 6 feet. Roof framing puts you near open edges, on narrow top plates and on steep slopes, so work out the plan before you climb.

• Your employer must set up guardrails, safety nets or personal fall arrest for work 6 feet or more above a lower level. Anchors must hold 5,000 pounds per worker or be designed by a qualified person.
• Do as much as possible from the ground, or from inside the building, on a scaffold or on a lift.
• Do not walk the top plate or truss chords without protection.
• Use roof brackets and planks on steep slopes.
• Stop work in high wind. Sheets and trusses act like sails.
• Keep tools tethered or stored, and keep people out from under the work.

Troubleshooting

Check Your Understanding

1. A house is 30 feet wide with an 8/12 roof. What are the run and the rise?

Run = 15 feet. Rise = 15 x 8 = 120 inches, or 10 feet.

2. Using the same roof, how long is the rafter before adjustments?

15 x 14.42 = 216.3 inches, or about 18 feet.

3. Your foreman says to install a ridge board and skip the ceiling joists for a vaulted ceiling. Is that correct?

No. A ridge board cannot carry the load alone. A vaulted ceiling needs a structural ridge beam or a designed tie system. Check the plan and ask.

4. A plumber wants to cut a truss web for a vent pipe. What is the answer?

No. Trusses are not cut without a written repair from the truss engineer.

5. The first truss is set but not braced, and the crane is ready with the second. What should happen?

Stop. Brace the first truss before lifting another. Every other truss depends on it.

Key Takeaways

• Run is half the span, and rafter length comes from run and pitch.
• The bird's mouth has a seat cut and a plumb cut. Do not cut too deep.
• Rafter ties and ridge beams keep walls from spreading.
• Never cut a truss, and follow the bracing plan.
• Sheath from the eave up, nailing 6 inches on edges and 12 inches in the field unless told otherwise.
• Plan fall protection before you go up.