A roof truss is the wooden or metal framework that carries the weight of your roof
A roof truss is a rigid triangular frame made of wooden or steel beams that sits on top of your walls and holds up the roof covering, insulation, and anything else above it. Instead of using a single beam to span the distance from one wall to the other — which would sag under the weight — trusses use a triangle shape to distribute that load outward and downward to the walls below. The triangle is the strongest shape in construction because the angles and cross-bracing prevent the frame from collapsing inward.
Most residential roofs use wooden trusses built in a factory to exact measurements, then delivered to the job site and installed as complete units. Steel trusses are less common in houses but more common in commercial buildings. Either way, the truss does the same job: it spans the distance between your exterior walls without needing support posts in the middle of your attic or living space.
Key Takeaways
- A roof truss is a triangular frame that transfers the weight of the roof down to the exterior walls, not to interior posts or beams.
- The triangle shape and internal bracing give trusses their strength and allow them to span long distances without sagging.
- Most residential trusses are made of wood and are built to match your specific roof design and span before delivery to the site.
- Trusses are different from rafters, which are individual beams that require collar ties and ceiling joists to work as a system.
- The spacing, size, and grade of truss lumber depend on the roof load, local snow and wind codes, and the distance they must span.
How the triangular shape makes trusses strong
The strength of a truss comes from its geometry. A triangle cannot change shape without breaking one of its sides, so when weight pushes down on the peak of the truss, the angled beams push outward against the walls and downward through the internal bracing. This is different from a rectangle or square, which can collapse inward if the corners are not held rigid.
Inside the triangle, smaller beams called web members or chords create a lattice pattern. These internal pieces break the span into smaller sections and prevent the main beams from buckling under load. The more web members a truss has, the more load it can carry over the same distance. Engineers design the pattern based on how much weight the roof will bear — which depends on the roof pitch, local snow load, wind speed, and the materials used.
Common truss types and what they look like
The most common residential truss is the Attic truss or scissor truss, which has a flat or nearly flat bottom chord that creates usable attic space. The peak is still triangular, but the bottom beam is higher than a standard truss, leaving room to store items or run ductwork underneath.
A standard or common truss has a flat bottom chord that sits directly on the walls and a peaked top. This design leaves no usable attic space because the bottom beam is at wall height. Raised heel trusses have a small vertical section at the wall end that lifts the bottom chord slightly, which improves ventilation in the attic and allows for better insulation coverage at the eaves.
Other types include mono trusses (single-slope, used for additions or modern designs), gambrel trusses (two slopes on each side, used for barn-style roofs), and scissor trusses (angled bottom chords that create a vaulted ceiling). The type you need depends on your roof design, the span you need to cover, and whether you want usable space underneath.
Trusses versus rafters: what the difference means
Older homes often use rafters instead of trusses. Rafters are individual beams that run from the peak of the roof down to the walls, and they require additional support pieces called collar ties and ceiling joists to work as a system. Rafters give you more flexibility in design and make it easier to add skylights or cut openings in the roof later, but they take longer to install and require more on-site carpentry.
Trusses are faster to install because they arrive as complete units and bolt or nail directly to the walls. They are also more predictable — the factory has already calculated the load capacity and bracing, so there is less room for error on the job site. The trade-off is that cutting into a truss or removing web members to create an opening can weaken the entire frame and is usually not recommended without an engineer's approval.
What affects the size and spacing of trusses
The lumber grade, beam size, and spacing of trusses depend on several factors. Roof load is the main one — this includes the weight of the roof covering (shingles, metal, tile), insulation, drywall or plaster on the underside, and any snow or ice that will accumulate. Areas with heavy snow loads need larger or more closely spaced trusses than areas with light snow.
Span is the distance the truss must cover from one wall to the other. A truss that spans 30 feet needs to be larger and stronger than one that spans 20 feet. Roof pitch (the steepness of the slope) also matters — a steep roof sheds snow and water faster, which reduces the load, while a shallow pitch holds more weight.
Local building codes set minimum requirements for snow load, wind speed, and seismic activity in your area. The truss manufacturer uses these codes plus your specific measurements to design the frame. If you live in a high-snow or high-wind area, your trusses will be noticeably heavier and more braced than those in a mild climate.
How trusses are installed and what can go wrong
Trusses are delivered on a truck and lifted into place with a crane or by hand, depending on the size and number. They are then fastened to the top of the walls with nails or bolts, and sometimes braced temporarily with diagonal supports until the roof sheathing is nailed on. The sheathing — usually plywood or oriented strand board (OSB) — ties all the trusses together and prevents them from racking (twisting out of square).
Common problems arise when trusses are cut or notched to fit around obstacles, when web members are removed to create attic access, or when the bottom chord is cut to fit ductwork or pipes. Each of these weakens the truss. Another issue is inadequate bracing during installation — if trusses are not properly supported while the sheathing is being nailed, they can shift or collapse. Moisture damage to the wood, rust on steel trusses, or damage from impact can also reduce their strength over time.
Frequently Asked Questions
Can I remove part of a truss to create attic access or run ductwork?
Not without professional guidance. Removing or cutting web members weakens the truss and can cause sagging or failure. If you need to modify a truss, contact a structural engineer or the truss manufacturer to see if reinforcement is possible. Many modern trusses include built-in access holes designed by the engineer.
How long do roof trusses last?
Wooden trusses typically last 50 to 80 years or longer if kept dry and protected from pests. Moisture is the main enemy — wet wood rots and loses strength. Steel trusses last longer but can rust if not painted or galvanized. Regular attic inspections for leaks, pest damage, and rot help catch problems early.
What is the difference between truss spacing and truss size?
Truss size refers to the lumber dimensions and internal bracing — a larger truss can carry more load. Spacing is how far apart the trusses are placed, usually 16 or 24 inches on center. Closer spacing means more trusses sharing the load, so each individual truss can be smaller. The engineer chooses both based on the roof load and span.
Do I need to ventilate the space between the trusses and the roof sheathing?
Yes, in most climates. Air flow between the insulation and the roof sheathing prevents moisture buildup and ice dams. This is called a ventilated attic or cold roof. Some designs use a sealed attic instead, where insulation is placed against the roof sheathing and the space is not ventilated — this requires careful air sealing and is less common in residential construction.