Pro Guide to Roof Framing

Roof framing begins with choosing the roof design, such as a gable or hip roof. Then builders should decide whether rafters or trusses make the most effective frame for the design. Make such measurements as the rise, run and pitch of the roof to size rafters and trusses when building them on site.

These are some of the most popular types of roofs.

• Gable roofs are the most common type of roof. Gable roofs have two sloping sides that meet at a top ridge. The design creates end walls with gables, or triangular extensions, at the top. They can be more complex to build but offer more attic space. Gable roofs are popular in cold, snowy climates but are not recommended for areas with high winds.
• Gambrel roofs have a design similar to gable roofs but with a familiar barn shape. Gambrel roofs have two sides, each with a shallower slope above a steeper one.
• Hip roofs, also called pyramid roofs, have four sloping sides that meet at a point, or roof cap. They may require more rafter sizes than gable roofs and offer less attic space. They are effective in areas with high winds.
• Mansard roofs are four-sided roofs that, like the gambrel roof design, have two slopes per side, with the bottom slope steeper than the top slope.
• Flat roofs are rarely perfectly flat as they need a slope for drainage that reduces the chance of water damage. Flat roofs are effective in hot, dry climates but are not recommended for areas with heavy rainfall.

A key decision when framing a roof is whether the design will use rafters or trusses. Rafters and trusses are both structural components that frame the roof.

Rafters are framing boards that extend from the ridge to the wall plate and are usually built on site. Trusses are usually prefabricated off-site in a triangular webbing frame to provide support for the roof and exterior walls.

Rafter frames include rafter boards, a ridge board and ceiling joists, also called rafter ties, built in a stick framing pattern. Rafters offer the following advantages:

• The boards can be cut on site, saving delivery costs.
• Individual rafters weigh less than assembled trusses, making them easier to raise when constructing the frame. Assembled trusses often require heavy equipment such as cranes during construction.
• Rafter design makes insulation easier to install and provides more open space in the attic for potential storage or living areas.
• Rafter design offers more flexibility for future adjustments than trusses.

Trusses are built from wooden pieces called top chords, bottom chords (also called ceiling joists) and interior posts that are also called “webbing.” Truss design offers the following advantages:

• Trusses are usually made from lighter materials and can be bought in quantity, which can make them more cost-effective than rafters.
• Buying preassembled trusses saves time and reduces the costs of skilled labor compared to making rafters.
• Truss framing design distributes the load to the exterior walls. This reduces the need for load-bearing interior walls, giving the home more options for open floor plans.

The difference between rafters and ceiling joists is that rafters are usually diagonal boards that form the structure of the roof, from ridge to wall. Joists are horizontal boards form the structure of the ceiling and transfer the flooring weight load to support beams.

Knowing how to cut roof rafters gives Pros an edge when framing a roof, from small sheds to large homes. (See the roof framing diagram above for the parts of a rafter frame.)

Begin gable roof framing by making the following calculations for sizing the rafters.

• The roof span is the width between the exterior walls. Also called the building width, it is measured from the outside surface of the exterior walls, including the sheathing. If the sheathing has not been installed when framing the roof, add a precise estimate to the measurement.
• The roof rise is the distance from the roof ridge to the center of the span and is the highest point from ceiling to roof.
• The rafter run measures the horizontal distance a single rafter must travel and is the roof span divided by two.
• The roof pitch is the angle of the roof. The rise divided by the run determines the roof pitch. Use the roof pitch to determine the length of the rafters and the angles of the cuts. (“Pitch” and “slope” are similar terms but not interchangeable.)
• The line of the roof is the diagonal distance from the outer wall to where the rafter meets the ridge. This is the same as the length of the rafter and can be determined with the rise, run and pitch.

Rafters are usually cut from 2- x 10-inch boards and other framing boards. Rafter size is influenced by such factors as the type of wood and the load of the roof, including potential snow loads.

Determine the number of rafters you need based on the length of the roof. For example, if you have a 20-foot length of roof and need a rafter frame every 2 feet, you’ll need 10 pairs of rafters, or 20 rafters in total.

After determining the size and quantity of rafters, purchase your framing boards. Choose rafter boards at least 1 foot longer than your target length to leave adequate wood for your cuts.

Use a tape measure or laser distance measurer and a pencil to mark the necessary cuts on the boards. A rafter square or framing square ensures more accurate angles.

Secure the boards on a flat surface with a table clamp and make the cuts with a circular saw.

Make the following cuts as needed:

• Plumb cuts are where the rafter meets the ridge board. Use the roof pitch to determine the cutting angle and cut with a circular saw.
• Bird’s mouth cuts are where the rafter meets the wall top plate. These consist of a horizontal seat cut and a vertical shoulder cut.
• Tail cuts are at the lower end of the rafter and establish the eaves. They can be cut plumb or square depending on the desired shape of the eaves.

Purchasing prefabricated wood trusses is a popular option when framing a roof. Pros with carpentry skills can build simple trusses to save money and make custom designs as needed. (See the roof framing diagram above for the parts of a truss.)

Before building roof trusses, check municipal building codes, as some areas have size requirements. Your local building authority may require plans to be submitted for approval. Do not begin construction until your plans have been approved.

Choose the truss design. These are some of the most popular options:

• The king post truss design features a single vertical joist for reinforcing smaller, more lightweight structures.
• The Fink truss design arranges the interior joists in a “W” pattern to ensure more even weight distribution, making them suitable for larger structures and heavier loads.
• The Howe truss resembles the other designs but arranges additional beams around the central joist. They can be the most challenging to build but support the largest, heaviest structures.

Consider these tips when building a simple wood truss.

• Compared to rafters, trusses are usually built from smaller boards, such as 2- x 4-inch framing boards.
• Size the bottom chord
  0.25 inches longer than the floor to ensure that it will fit.
• Use a tape measure or laser distance measurer and a pencil to mark the necessary cuts on the boards.
• Secure the boards on a flat surface with a table clamp and make the cuts with a jigsaw.
• Attach the chords and posts in the chosen design with construction adhesive.
• Secure the truss with gusset plates. For lightweight trusses, cut your own plywood gusset plates with a circular saw. For trusses that bear heavier loads, use galvanized steel mending plates.
• Allow the adhesive to set overnight. Begin using the trusses to build the roof frame the next day.

Pro Tip: Whether you’re using prefabricated trusses or building them on site, use our tool and large equipment rental services for quick access cranes and other equipment to make the job an easy lift.

These are the general steps for how to build a roof after the ceiling has been framed. (See our guide 101 Guide to House Framing.)

• Decide whether to use rafters or trusses.
• If cutting your own rafters, calculate the size and placement of the rafters and the roof pitch based on the roof design.
• Place the rafters diagonally so they meet at a ridge board at the peak of the roof.
• Nail horizontal 2- x 4-inch or 2- x 6-inch rafter ties to the rafters and top plates at both ends of the roof. This prevents the walls from spreading apart.
• When using trusses, begin by installing temporary braces to support the trusses during the build. First attach the end trusses at either end of the roof, then attach the standard trusses.
• Nail the sheathing to the roof with framing nails.
• Cap the roof rafters with 2- x 6-inch fascia boards.
• Attach the waterproofing underlayment.
• Attach the roof shingles. (See our guide How to Install Roof Shingles.)

Pro Tip: When building a roof, be sure to use a safety harness and other fall protection equipment as needed. Always use other construction site safety equipment such as hard hats, work gloves and safety glasses.

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