What roof trusses are and why you build them instead of rafters
A roof truss is a triangular frame made of wood or steel that spans from one wall to the opposite wall and carries the weight of the roof. You build trusses on the ground, then lift them into place as complete units. This is different from traditional rafter framing, where you cut and nail individual pieces directly on top of the walls — a slower, more labour-intensive method that requires more skill and more time working at height.
Trusses are faster to install because they arrive prefabricated or you assemble them all at once in a staging area. They distribute weight more evenly across the walls, which means you need fewer support posts inside the building. Most residential roofs built in the last 40 years use trusses rather than rafters.
Building your own trusses on-site is possible if you have the space, tools, and experience, but most builders and homeowners order them from a truss manufacturer who designs them to your roof specifications and delivers them ready to install.
Key Takeaways
- Trusses are triangular frames built on the ground and lifted into place as complete units, which is faster and safer than cutting rafters at height.
- A basic truss has a top chord (the sloped pieces), a bottom chord (the tie beam), and internal webs that brace them together.
- You need the roof pitch, span distance, and local snow and wind loads before you can design or order trusses.
- Trusses are usually ordered from a manufacturer, but you can build them yourself if you have a flat work area, a pneumatic nailer, and experience reading truss plans.
- Installation requires temporary bracing, a crew to handle the weight, and nailing or bolting trusses to the top plate of the walls in a specific sequence.
The parts of a truss and how they work together
Every truss has three main components. The top chord is the two sloped pieces that form the peak and carry the weight of the roof sheathing, shingles, and snow. The bottom chord is the horizontal tie beam that connects the feet of the truss and prevents the top chords from pushing the walls outward. The web members are the internal braces — usually vertical and diagonal pieces — that stiffen the top and bottom chords and transfer loads down to the walls.
The shape and size of these members depend on the roof pitch (how steep the roof is), the distance the truss has to span, and the loads it must carry. A shallow pitch over a wide span needs deeper, heavier trusses. A steep pitch over a short span can use lighter material. Snow load, wind load, and the weight of the roof covering all factor into the calculation.
Common truss types include the king post (one vertical post in the middle), the queen post (two vertical posts), and the scissors truss (bottom chords that slope upward to create a vaulted ceiling). The type you need depends on your roof design and what the space below the roof will be used for.
Information you need before designing or ordering trusses
Before you can build or order trusses, you need exact measurements and load data. Measure the span — the distance from the outside of one wall to the outside of the opposite wall. Measure the rise — how many inches the roof goes up for every 12 inches it goes across (this is the pitch, written as "6:12" or "8:12"). Know the length of the building so you know how many trusses you need and how far apart they will be spaced (usually 16 or 24 inches on centre).
You also need the design loads for your area. Your local building department publishes the snow load (how much weight snow can pile up on the roof) and wind load (how much sideways force wind can exert). These vary by region and elevation. A truss manufacturer or structural engineer uses these numbers to size the wood or steel and determine the internal bracing pattern.
If you are ordering from a manufacturer, provide the span, pitch, spacing, and your address so they can look up the snow and wind loads. If you are building trusses yourself, you need a truss design — either from a structural engineer or from a standard plan that matches your roof dimensions and local loads.
Materials and tools for building trusses on-site
Trusses are usually built from 2×4 or 2×6 lumber, depending on the span and load. You will need enough boards to cut the top chords, bottom chords, and web members for all the trusses you are building. You also need gusset plates — plywood or metal plates that reinforce the joints where the chords and webs meet — and fasteners (nails or bolts, depending on the design).
To assemble trusses, you need a flat work surface large enough to lay out a full truss. A concrete floor or a level patch of ground works. You need a circular saw or miter saw to cut the pieces to length and angle. You need a pneumatic nailer or a hammer and nails to fasten the gussets. A speed square or framing square helps you mark the correct angles. A tape measure, pencil, and a truss plan or template are essential.
If you are using bolted connections instead of nails, you need a drill, wrenches, and bolts. If you are building many trusses, a jig — a wooden frame that holds the pieces in the correct position while you fasten them — saves time and ensures consistency.
Step-by-step assembly of a straightforward truss
Lay out the two top chord pieces on your work surface in a V shape, with the peak angle matching your roof pitch. Lay the bottom chord (tie beam) perpendicular to the peak, connecting the feet of the top chords. Check that the angles are correct using a speed square and that the overall dimensions match your plan.
Cut and position the web members — the internal braces — according to your truss plan. These are usually cut at angles on both ends so they fit snugly against the chords. Nail or bolt the gusset plates over each joint where a web member meets a chord. Start with the joints at the peak, then work down to the joints at the feet. Fasten both sides of each gusset plate.
Once all the gussets are fastened, flip the truss over and fasten gusset plates to the other side of each joint. Check that the truss is square by measuring diagonally from corner to corner — both diagonals should be the same length. If the truss is twisted or out of square, loosen the fasteners, adjust the pieces, and re-fasten.
Repeat this process for each truss. Stack finished trusses flat on top of each other, separated by spacers so air can circulate and the wood does not warp.
Lifting and installing trusses on the walls
Before you lift any trusses, the walls must be complete and the top plate (the horizontal board on top of the wall) must be nailed down. Install temporary bracing — diagonal boards nailed from the top of each truss down to the floor or ground — to hold the trusses upright and prevent them from tipping while you work.
Lift the first truss into place and nail it to the top plate with multiple nails (usually 3 to 4 per foot of bearing). Use a level to make sure the truss is plumb (vertical). Install the second truss next to it, spacing it 16 or 24 inches on centre from the first. Nail it down and brace it. Continue across the building, installing and bracing each truss before moving to the next.
Once all trusses are in place and braced, install permanent lateral bracing — horizontal boards that tie the trusses together and prevent them from racking (twisting sideways). These are usually 1×4 or 1×6 boards nailed to the top chords or the webs. The pattern depends on your truss design and local building code.
After the trusses are fully braced and nailed, you can remove the temporary bracing and begin installing roof sheathing.
When to order trusses from a manufacturer instead of building them
Ordering trusses from a manufacturer is the standard choice for most residential and commercial projects. Manufacturers have computer design software that calculates the exact size and bracing pattern for your roof based on your span, pitch, loads, and local code. They have the equipment to cut and assemble trusses quickly and consistently. They deliver trusses ready to install, which saves weeks of on-site work.
Building trusses yourself makes sense only if you have a large, flat work area, the right tools, experience reading construction plans, and time to spare. Even then, you still need a structural engineer or a truss design that accounts for your local snow and wind loads — you cannot guess at the internal bracing pattern.
The cost of ordering trusses is usually lower than the labour cost of building them on-site, especially for a large roof. Get quotes from two or three truss manufacturers in your area. Provide them with your roof dimensions, pitch, and address, and they will give you a price and a delivery date.
Frequently Asked Questions
What is the difference between a truss and a rafter?
A rafter is a single sloped board that runs from the wall to the peak; you cut and nail multiple rafters in place at height. A truss is a complete triangular frame with internal bracing, built on the ground as one unit and lifted into place. Trusses are faster to install and safer because the assembly work happens on the ground.
Can I use any lumber for trusses, or does it have to be a specific grade?
Truss design specifies the lumber grade and size needed for your roof load. Using a lower grade or smaller size than the design calls for is unsafe and violates building code. Your truss plan or manufacturer will tell you exactly what grade and size to use.
How far apart should trusses be spaced?
Trusses are usually spaced 16 or 24 inches on centre (measured from the centre of one truss to the centre of the next). The spacing depends on the thickness and strength of the roof sheathing and the truss design. Your truss plan will specify the correct spacing.
Do I need a building permit to build roof trusses?
Yes. Roof framing is part of the structural system and must be inspected. Your building department will want to see the truss design or manufacturer's drawings before you install them. Building without a permit can result in fines and problems when you sell the property.
What happens if a truss gets damaged during delivery or installation?
Do not install a damaged truss. Contact the manufacturer or the person who built it and request a replacement. A cracked chord or a bent web member weakens the truss and can cause the roof to fail. The cost of replacing one truss is far less than the cost of repairing a collapsed roof.