What a roof truss is and why you build it on the ground
A roof truss is a triangular wooden frame that spans from one wall to the opposite wall and carries the weight of the roof. Instead of building it in place high above the ground, carpenters build trusses flat on the ground, then lift them onto the walls. This is faster, safer, and more accurate than assembling them overhead.
The triangle shape is what makes a truss work: the sloped top pieces (called top chords) meet at a peak, the bottom piece (called the bottom chord) ties them together, and internal bracing pieces (called webs) prevent the triangle from collapsing sideways. The angle and spacing of these pieces depend on your roof pitch, the span you need to cover, and local building codes.
Building trusses yourself is possible if you have carpentry experience, a flat work surface, and the right tools. Most residential projects, however, use factory-built trusses ordered from a truss manufacturer who designs them to your specifications and delivers them ready to install. This article covers the manual building process for those who need to understand how it works or build a small number of trusses on site.
Key Takeaways
- Trusses are built flat on the ground using a full-scale layout called a truss pattern or jig, which shows exactly where each piece goes and at what angle.
- The three main parts are the top chords (sloped sides), bottom chord (horizontal base), and webs (internal bracing), all cut to length and angle before assembly.
- Pieces are fastened with metal plates called gussets or gang-nail plates that are hammered or pressed onto both sides of each joint.
- A single truss for a typical residential roof takes two to four hours to build once all pieces are cut, and requires a work surface at least as long as the truss span.
- Building codes and snow load requirements determine the size and spacing of internal bracing, so your design must match your local requirements before you cut the first piece.
Designing the truss and getting the dimensions right
Before you cut wood, you need a design that shows the exact length and angle of every piece. This design comes from your roof pitch (the slope, usually written as rise-over-run, like 6:12 meaning 6 inches of rise for every 12 inches of horizontal run), the distance you need to span, and your local building code requirements for snow load and wind.
If you are building a small shed or outbuilding, you can sketch this yourself using geometry or online truss calculators. For a house or any structure that requires a building permit, your local building department will require a design stamped by a structural engineer or provided by a licensed truss manufacturer. Do not skip this step: an undersized truss can sag or fail, and an oversized one wastes material and money.
Once you have the design, draw it full-scale on a flat surface (plywood sheets laid on sawhorses work well) or use a printed pattern at full scale. This pattern, called a jig or truss pattern, shows the outline of every piece and where they meet. You will use this to cut pieces to length and angle, and to position them during assembly.
Cutting pieces to length and angle
Each truss piece must be cut to the exact length and angle shown on your pattern. The angles are usually not 90 degrees—the top chords meet at the peak at an angle that matches your roof pitch, and the webs are cut at angles that match where they meet the chords.
Measure each piece directly from the full-scale pattern using a tape measure, or transfer the pattern to a story pole (a straight board marked with all the lengths and angles you need). Mark the cut angles on each piece using a speed square or angle finder set to the angle shown on the pattern. Cut with a circular saw or miter saw, making sure the blade is sharp and the cut is straight.
Cut all pieces for all trusses before you start assembly. This is faster than cutting one truss at a time, and it lets you stack and organize pieces so you can grab them quickly during assembly. Label each piece with the truss number and its position (for example, "Truss 3, Left Top Chord") so you do not mix them up.
Building the jig and laying out the first truss
Lay out your full-scale pattern on a flat, level work surface—plywood sheets on sawhorses, a concrete floor, or a large flat area of ground. The surface must be at least as long as your truss span and wide enough to work on both sides. Nail or screw the pattern to the surface so it does not shift.
Position the bottom chord along the pattern line. Then position the two top chords so they meet at the peak angle shown on the pattern. Use temporary braces or clamps to hold them in place while you check that all angles and lengths match the pattern. The pieces should touch at every joint shown on the pattern with no gaps.
Once the main three pieces are positioned correctly, lay out the web pieces (internal bracing) in their positions. Do not fasten anything yet—this is just a dry layout to make sure every piece fits and every angle is correct. If something does not fit, stop and check your cuts before you proceed.
Fastening joints with gussets or gang-nail plates
Roof truss joints are fastened with metal plates called gussets or gang-nail plates. These are steel or aluminum plates with teeth or nails stamped into them. The plate is positioned over the joint (where two pieces meet) and hammered or pressed onto both pieces, with the teeth biting into the wood and locking the joint.
Position the gusset plate centered over the joint on one side of the truss. Use a hammer or a pneumatic nailer to drive the teeth into the wood, or use a hydraulic press if you have one. The plate should be flush against the wood with no gaps. Then flip the truss and install a matching gusset plate on the other side of the same joint. Both sides must be fastened—a joint with a plate on only one side will fail.
The size and number of gusset plates at each joint depends on the load and the angle of the pieces. Your design will specify this. A typical interior web joint might use one 4-by-8-inch plate on each side, while a peak joint might use two larger plates. Do not guess—follow the design exactly.
Checking the finished truss and preparing to lift
Once all gussets are fastened, check that the truss is square and true. Measure the diagonals (corner to corner) to make sure they are equal—if they are not, the truss is racked (twisted) and you need to adjust it before it dries. Check that the bottom chord is level and the peak is centered over the midpoint of the span.
Let the gusset adhesive (if used) or fasteners cure according to the manufacturer's instructions before you move the truss. This usually takes a few hours to overnight. Do not lift or stress the truss until it is fully cured.
When you are ready to move the truss, use a crane, boom truck, or multiple people with lifting poles to raise it carefully onto the walls. Never drag a truss or drop it—the impact can crack the wood or loosen the gusset plates. Once on the walls, brace it temporarily with diagonal bracing until the roof sheathing is installed and fastened.
When to order factory-built trusses instead
Building trusses on site makes sense for small projects, repairs, or situations where you cannot get factory trusses delivered. For most residential construction, ordering trusses from a manufacturer is faster and more reliable. A truss plant has the equipment to cut pieces to exact tolerances, press gusset plates with consistent force, and deliver trusses ready to install.
Factory trusses also come with a design stamp from a structural engineer, which satisfies building code requirements without additional engineering on your part. The cost is usually lower than building them yourself when you factor in labor, equipment rental, and the risk of mistakes. If you are building a house or any structure that requires a permit, ask your building department and general contractor whether site-built trusses are even permitted in your area.
Frequently Asked Questions
What is the difference between a truss and a rafter?
A rafter is a single sloped piece that runs from the wall to the peak, and the ceiling is hung below it. A truss is a complete triangular frame that includes the ceiling (the bottom chord), so the ceiling is built into the truss itself. Trusses are stronger for long spans and faster to install.
Can I use nails instead of gusset plates?
No. Nails alone cannot transfer the load at a truss joint reliably. Gusset plates (or bolts and metal brackets in some designs) are required by building code. The teeth in the gusset plate bite into the wood fibers and lock the joint in place in a way nails cannot.
How much does it cost to build a truss versus buying one?
Cost varies widely by region, truss size, and your labor rate. Factory trusses typically cost $50 to $150 per truss for residential sizes, plus delivery. Building one yourself costs the wood (usually $30 to $80) plus your labor and equipment. For a single truss, building it yourself may be cheaper; for ten or more, factory trusses are usually faster and less expensive overall.
What tools do I need to build a truss?
You need a circular saw or miter saw, a tape measure, a speed square or angle finder, a hammer or pneumatic nailer, clamps, and a flat work surface. A hydraulic gusset plate press is ideal but not required—a hammer works, though it is slower and requires more force.
How long does it take to build one truss?
Once all pieces are cut and the jig is set up, building and fastening a single truss takes two to four hours depending on size, complexity, and your experience. A small shed truss might take one to two hours; a large residential truss with many web pieces might take four to six hours.