What a roof truss is and why you build it on the ground
A roof truss is a rigid frame made of wood or steel that holds up your roof. It sits on top of your walls and carries the weight of shingles, snow, wind, and the structure itself down to the foundation. Most residential trusses are built flat on the ground, then lifted into place as a complete unit — not assembled piece by piece in the air.
Building on the ground is faster, safer, and more accurate than assembling overhead. You work on a flat surface, use straightforward tools, and can measure twice before cutting. Once the truss is built and braced, a crew lifts it onto the walls with a crane or by hand, depending on size and weight.
Key Takeaways
- Roof trusses are built flat on the ground from lumber cut to specific angles and lengths, then lifted into place as complete units.
- You need a design plan or truss drawing that shows the exact angles, member lengths, and connection points before you cut any wood.
- The three main members in a straightforward truss are the top chord (angled), bottom chord (horizontal), and web members (internal bracing), all joined with metal plates or bolts.
- Cuts must be precise because even small angle errors compound across the truss and prevent it from sitting flat on the walls.
- Most residential trusses today are ordered pre-made from a truss manufacturer rather than built on-site, because the equipment and precision required are substantial.
Getting a truss design and understanding the drawing
Before you cut any wood, you need a truss design — a detailed drawing that shows every angle, every member length, and how pieces connect. This drawing comes from a structural engineer or a truss manufacturer. It specifies the roof pitch (how steep), the span (distance the truss covers), the load it must carry, and the type of connections.
The drawing shows three types of members. The top chord is the angled piece that forms the peak and slope of the roof. The bottom chord is the horizontal piece that ties the two sides together and prevents the truss from spreading apart. The web members are the internal diagonal and vertical pieces that brace the top and bottom chords and transfer loads.
Each member is labeled with its length and the angles at both ends. Angles are given in degrees or as a rise-over-run ratio (for example, 6:12 means the roof rises 6 inches for every 12 inches of horizontal distance). Do not guess at angles or lengths — even a quarter-inch error in length or a degree off in angle will prevent the truss from fitting flat on the walls.
Cutting the lumber to length and angle
Lumber for trusses is usually 2×4 or 2×6 dimensional lumber, depending on the span and load. You cut each member to the exact length shown on the drawing, then cut the angles at both ends so the pieces fit together tightly.
A miter saw or circular saw with an angle guide is the standard tool for these cuts. Set the blade angle to match the drawing — for example, if the top chord meets the web member at 35 degrees, set your saw to 35 degrees and cut both pieces so they fit together without gaps. A speed square or angle finder helps you mark the cut line accurately before you saw.
Cut all pieces for one truss before you start assembly. Lay them out on a flat surface and check that each piece is the right length and that the angles at both ends match the drawing. A small error in one member will throw off the entire truss.
Assembling the truss on a flat work surface
Lay out all the cut pieces on a flat, level surface — a concrete floor, a large table, or a sheet of plywood on sawhorses works well. Arrange the pieces in the shape shown on the drawing: top chord angled on both sides, bottom chord horizontal underneath, and web members filling the space between.
Start by connecting the top chords to the web members. Use the connection method shown on the drawing — this is usually a metal gusset plate (a flat steel plate with nail or bolt holes) bolted or nailed across the joint, or a metal connector (a shaped bracket) bolted to the sides of the pieces. Align the pieces so they sit flat against the gusset plate with no gaps, then drive bolts or nails through the holes.
Once the top and web members are connected, attach the bottom chord. The bottom chord is under tension (being pulled apart), so the connection must be strong. Use the same gusset plates or connectors specified on the drawing. Check that the truss is square by measuring diagonally from corner to corner — both diagonal measurements should be equal.
Checking the truss for square and proper fit
Before you lift the truss, verify that it is square and that all angles are correct. Measure the distance from the peak to the center of the bottom chord — this is the height. Measure the distance from one end of the bottom chord to the other — this is the span. Both measurements should match the drawing exactly.
Measure both diagonals of the truss (corner to corner). If the diagonals are equal, the truss is square. If one diagonal is longer than the other, the truss is racked (twisted), and you need to adjust the connections or re-cut pieces before lifting.
Check that the truss sits flat on a level surface with no rocking or gaps. If it rocks, one of the angles is wrong or one member is the wrong length. Do not lift a truss that does not sit flat — it will not fit on the walls and may fail under load.
Lifting and placing the truss on the walls
Once the truss is built and checked, it is ready to move to the walls. Small trusses (under 100 pounds) can be lifted by hand with four or more people. Larger trusses need a crane or a boom lift. Attach lifting straps or chains to the truss at points that will not damage the wood or connections.
Position the truss so the bottom chord sits evenly on both walls. The walls must be level and the same height, or the truss will sit at an angle. Brace the truss with temporary diagonal bracing from the peak to the ground on both sides to keep it from tipping while you position it and before you nail it down.
Once the truss is in place and braced, nail the bottom chord to the top plate of the wall using galvanized nails or bolts specified on the drawing. Do not remove the temporary bracing until all trusses are in place and the roof sheathing is installed.
Why most residential trusses are ordered pre-made
Building trusses on-site requires precision equipment, a flat work area, and skilled labor. Most residential construction today uses trusses ordered from a truss manufacturer. The manufacturer has a computer program that designs the truss, a saw that cuts all pieces to exact angles and lengths, and a press that applies gusset plates under controlled pressure.
Pre-made trusses arrive at the job site ready to lift and install. They cost less than building on-site because the manufacturer spreads the equipment cost across many trusses. If you are building a single house or a small addition, ordering trusses is almost always the practical choice. Building on-site makes sense only if you have the equipment, the space, and the experience to do it accurately.
Frequently Asked Questions
What tools do I need to build a truss?
A miter saw or circular saw with an angle guide, a speed square, a tape measure, a level, a hammer or impact driver, and a wrench for bolts. You also need a flat work surface large enough to lay out the entire truss. A pneumatic nailer speeds up gusset plate installation, but hand nailing works if you have time.
How do I know what angle to cut?
The truss drawing shows the angle in degrees or as a rise-over-run ratio. Set your saw to that angle and cut a test piece first. Check the fit against the drawing before you cut the actual members. A speed square or angle finder helps you mark the line accurately.
Can I use different lumber sizes than the drawing shows?
No. The drawing specifies lumber size because it affects how much load the truss can carry and how the connections work. Using smaller lumber weakens the truss; using larger lumber wastes money and may not fit the connections. Follow the drawing exactly.
What if the truss rocks or does not sit flat?
Do not lift it. A rocking truss means one or more angles are wrong or a member is the wrong length. Check all measurements against the drawing, re-cut the problem piece, and rebuild that connection. A truss that does not sit flat will not fit on the walls and may fail under load.
How much does it cost to order a pre-made truss instead of building one?
Costs vary by region, truss size, and complexity. A straightforward residential truss typically costs between $50 and $150 per truss, depending on span and pitch. Get quotes from local truss manufacturers. For most residential projects, the cost is lower than the labor and equipment needed to build on-site.