How do I Build a 24-Foot Roof Rafter?

Checking standard rafters in a house.

Rafters form the framework of a roof. They can be independent rafters that combine with joists to form a roof bottom and peak or part of a truss, which is built with internal supports. Rafters afford more space in an attic or open space above a ceiling than trusses, although long rafters also need internal braces. The length of a rafter is determined by its run; that is half the span or distance between outer walls of a house. The run is from the wall to the roof peak.

Things You'll Need

• Framer's square
• Tape measure
• Circular saw

Instructions

1. Determine what is meant by a 24-foot rafter. A rafter 24 feet long will not cover 24 feet of roof; that depends on the run and pitch, or slope, of a roof. A typical 5/12 house roof slopes up 5 inches for every foot of run. A rafter covering that run needs to be 26 feet long, plus any distance for eaves or overhang outside the wall. An actual 24-foot rafter covers only 22 feet of run.

2. Check local building codes for regulations about the length of rafters and size of lumber required. A typical code requires a 23-foot rafter to be a 2-by-12-inch board. Many codes do not specify for 24-foot rafters but require an engineer's inspection. Some require a double rafter or internal support between the rafter and the ceiling joists. Codes will vary with local conditions, such as the amount of snow and wind.

3. Build the rafter, once all requirements are satisfied and questions answered, using a framer's square. This has a thin side and a wide side with tables showing rafter lengths and angles. Use a 2-by-4-inch board to figure the angle for the top or plumb cut of the rafter. Put the heel or point of the square at the bottom of the board with the 6-inch mark on the thin side and the 12-foot mark on the wide side at the bottom of the board. That shows an angle at the end. Cut the rafter to that angle, whether it's a 4-inch or a 12-inch board.

4. Calculate the length by looking at the table under the 5-inch mark on the wide side. That shows 13, meaning a rafter must be 13 inches long for every foot of run. With a 12-inch overhang, a 24-foot rafter will cover 21 feet from peak to wall. Measure that distance from the top cut down the rafter and cut a 1-by-4-inch triangular notch to fit over the wall cap board. Cut a pattern rafter and test fit it to make any minor adjustments.

References

• Vance Hester: How to Cut Roof Rafters
• Article Alley: Rafters and Roof Work
• Builder Bill: Roof Framing
• Bloomington, MN: Building Code
• APB Pole Barns: Building Rafters