What a roof truss is and why you build it before installation

A roof truss is a triangular wooden frame that carries the weight of your roof and transfers it down to your walls. You build it on the ground or in a shop, then lift it into place as a complete unit. This is faster and safer than building the roof structure piece by piece on top of the house.

The truss sits on top of your top wall plate (the horizontal beam at the top of your walls) and spans from one side of the building to the other. The triangle shape — with a horizontal bottom chord, two angled top chords, and internal bracing — is what makes it strong enough to hold shingles, underlayment, insulation, and drywall without sagging.

Most residential roofs use factory-built trusses ordered from a truss manufacturer, who designs them to your roof pitch and building width and delivers them ready to install. If you are building trusses yourself, you are usually doing so for a small outbuilding, a retrofit, or a project where factory trusses are not practical. The process requires accurate cuts, proper fastening, and understanding how loads move through the frame.

Key Takeaways

  • A truss is a triangular frame made of 2x4 or 2x6 lumber, joined with gusset plates or metal connectors, that spans from wall to wall and carries the entire roof load.
  • The three main parts are the top chords (angled sides), bottom chord (horizontal base), and internal webs (diagonal bracing that prevents the frame from collapsing).
  • Cuts must be precise — the angle depends on your roof pitch, and even small errors compound when you stack multiple trusses.
  • Gusset plates (plywood pieces bolted or nailed to both sides of the joint) or metal connector plates are what actually hold the pieces together; the wood itself is just the skeleton.
  • A single truss is built flat, tested for square and strength, then lifted into place; multiple trusses are spaced 16 or 24 inches apart along the length of the building.

Calculating angles and dimensions from your roof pitch

Your roof pitch — the slope expressed as rise over run, like 6:12 (6 inches up for every 12 inches across) — determines every angle in the truss. This is the first number you need, and it comes from your building design or local building code requirements for snow load and water drainage.

Once you have the pitch, you can calculate the length of the top chords using the Pythagorean theorem or a rafter table (found in most carpentry handbooks or online). For a 6:12 pitch on a 24-foot-wide building, for example, each top chord runs roughly 13.4 feet from the peak to the wall plate. The bottom chord is straightforward half the building width — 12 feet in this case.

The internal web members (the diagonal and vertical pieces inside the triangle) are sized and angled based on the truss design. For a straightforward truss, you may have two diagonal webs running from the peak down to the ends of the bottom chord, with a vertical member in the middle. More complex designs add more webs to handle larger spans or heavier loads. A structural engineer or truss design software will give you exact dimensions and angles for each piece.

Materials and tools you need

Lumber for a residential truss is typically 2x4 or 2x6 dimensional lumber, graded for strength (usually #2 or better). The size depends on the span and load; a 24-foot span usually requires 2x6 top chords and 2x4 webs, while a 16-foot span can use 2x4 throughout. Buy lumber that is straight and free of large knots or splits at the joints.

Fastening is done with either gusset plates or metal connector plates. Gusset plates are pieces of 1/2-inch plywood cut to fit over the joint on both sides of the truss, then bolted or nailed through. Metal plates are stamped steel or aluminum with teeth that bite into the wood when pressed on; they are faster but require a plate press or hammer to set. Nails (16d or 20d common nails for gusset plates) or bolts (1/2-inch diameter with washers and nuts) complete the connection.

Tools include a miter saw or circular saw for cuts, a drill for bolt holes, a tape measure, a speed square or framing square for marking angles, a level, and clamps to hold pieces while fastening. If you are using metal connector plates, you will need a plate press or a heavy hammer and a setting tool. A workbench or flat ground surface large enough to lay out the full truss is essential — trusses are built flat, not standing up.

Cutting the lumber to length and angle

Start by cutting the bottom chord to length — this is the simplest cut because both ends are square (90 degrees). Measure twice, mark with a pencil, and cut on a miter saw or with a circular saw and a guide.

The top chords are angled at both ends. One end sits on the wall plate at a shallow angle (determined by your pitch), and the other end meets its partner at the peak at a steeper angle. Use a speed square or framing square to mark these angles on the lumber, then cut carefully. The cuts must be tight — gaps at the joints weaken the truss and make fastening difficult. Test-fit each piece before you commit to the final assembly.

Web members are cut at both ends, with angles that vary depending on their position in the truss. Mark each piece clearly so you do not mix them up. A common mistake is cutting all the webs the same angle when they should vary; double-check your design drawing before you cut.

After cutting, lay all the pieces on a flat surface and arrange them in the truss shape. This is called a dry layout. Check that the bottom chord is level, that the top chords meet at the peak, and that all pieces fit without gaps. Make small adjustments with a plane or sandpaper if needed.

Assembling the truss with gusset plates or metal connectors

Once the dry layout is correct, you are ready to fasten. If you are using gusset plates, cut them from 1/2-inch plywood to cover the joint on both sides. The plate should extend at least 6 inches on each piece of lumber meeting at that joint. Lay the first gusset plate on one side of the joint, align it, and drill holes for bolts or mark nail locations.

For bolted connections, drill through the gusset plate and both pieces of lumber, then insert bolts with washers and nuts. Tighten the nuts snugly but do not over-tighten, which can crush the wood. For nailed connections, use 16d or 20d common nails spaced about 4 inches apart around the perimeter of the plate. Flip the truss and attach the second gusset plate on the other side, making sure it is aligned with the first.

If you are using metal connector plates, position the plate over the joint and press it on firmly. The teeth will bite into the wood. If the plate does not seat fully, use a plate press (a specialized tool) or a hammer and setting tool to drive it home. Metal plates are faster than gusset plates but require more force and precision to install correctly.

Fasten all joints in the same way — the peak, the two bottom corners, and any internal web connections. After fastening, the truss should be rigid. Pick it up by the peak and shake it gently; there should be no movement or creaking at the joints.

Testing the truss for square and strength before lifting

Before you lift the truss into place, verify that it is square (all angles are correct) and that it is strong enough to handle its own weight plus the roof load. Measure diagonally from corner to corner of the truss; both diagonals should be the same length. If they differ by more than 1/4 inch, the truss is racked (twisted) and needs adjustment.

Check that the bottom chord is level and that the peak is centered over the midpoint of the bottom chord. Use a level and a tape measure. Small adjustments can be made by loosening one fastener, shifting the piece slightly, and re-fastening.

To test strength, explore a downward load to the center of the truss — this can be done by standing on it (if it is strong enough) or by hanging a weight from a rope. Watch for deflection (sagging) or movement at the joints. A properly built truss should deflect slightly under load but return to its original shape when the load is removed. If a joint creaks or moves excessively, re-tighten the fasteners or add additional gusset plates.

Lifting and spacing trusses on the building

Once the truss is complete and tested, it is ready to be lifted onto the wall plate. For a small truss (under 100 pounds), two people can lift it by hand. For larger trusses, use a crane, a gin pole (a temporary mast with a pulley), or a come-along winch. Never drop a truss or allow it to fall; impact can crack the wood or loosen fasteners.

Position the truss so the bottom chord sits fully on the wall plate, centered over the building width. The wall plate should be level; if it is not, shim under the truss with shingles or steel shims until it is stable. Nail or bolt the truss to the wall plate with at least two fasteners per side — this keeps the truss from sliding or tipping during installation of subsequent trusses.

Space multiple trusses 16 or 24 inches apart along the length of the building (16 inches for heavier loads or longer spans, 24 inches for lighter loads or shorter spans). Snap a chalk line on the wall plate to mark truss locations before you start lifting. Install temporary bracing between trusses — 2x4 boards nailed diagonally — to keep them plumb (vertical) and prevent racking until the roof sheathing is installed.

Frequently Asked Questions

Can I use 2x3 lumber instead of 2x4 to save money?

Not for the top chords or bottom chord of a truss spanning more than about 12 feet. Undersized lumber will sag under load and may fail. 2x3 can work for internal web members in some designs, but your structural engineer or truss designer must approve it. Using the wrong size is a safety hazard and will likely violate building code.

What is the difference between a gusset plate and a metal connector plate?

Gusset plates are plywood bolted or nailed to both sides of a joint; they are slower to install but are straightforward to inspect and repair. Metal connector plates are stamped steel with teeth that bite into the wood; they are faster but require a press to install correctly and are harder to adjust if something goes wrong. Both work; the choice depends on your tools and experience.

Do I need a building permit to build roof trusses?

If the trusses are for a structure that requires a permit (most residential buildings do), then yes — the truss design must be reviewed and approved by the building department before you build. You may need a structural engineer to stamp the design. Check with your local building department before you start.

How much does it cost to build trusses versus buying factory-built ones?

Factory-built trusses usually cost less per truss than materials and labor for building them yourself, especially for standard pitches and spans. Building your own makes sense for small outbuildings, unusual pitches, or when factory trusses cannot be delivered. Get quotes from both before you decide.

What happens if a truss is slightly out of square when I install it?

Small errors (under 1/4 inch) will be corrected by the roof sheathing and bracing. Larger errors compound — if each truss is off by 1/2 inch, ten trusses in a row will be 5 inches off by the end of the building. Install temporary bracing and check alignment frequently as you go.