Roof Truss Design Guide: Types, Spans, Spacing and Bracing

Truss anatomy: chords, webs and panel points

A roof truss is a rigid, triangulated structural frame built from relatively small, uniform-size lumber (commonly 2x4 or 2x6) connected at each joint by galvanized steel connector plates pressed into both faces of the wood. Because the truss is fully triangulated, loads travel through the members almost entirely as tension and compression rather than bending, which lets small members span distances that would require a massive solid beam if left unbraced.

• Top chords: the two sloped members that form the roof pitch and support the sheathing and roof covering; they work mainly in compression and bending under snow and wind load.
• Bottom chord: the horizontal member that forms the ceiling line and ties the base of the truss together; it works mainly in tension and also supports the ceiling drywall and any storage or insulation loads.
• Webs: the diagonal and vertical members inside the triangle that transfer load between the top and bottom chords; their layout defines the truss type (fink, howe, king post, etc.).
• Heel: the point where the top and bottom chords meet at the bearing wall; heel height controls how much insulation depth is available directly over the exterior wall.
• Panel points: the joints where webs meet chords, each reinforced with a metal connector plate sized by the truss engineering software for the specific forces at that joint.
• Camber: a slight built-in upward bow in long-span trusses to offset expected dead-load deflection so the truss reads flat once loaded.

Common truss types and where each one is used

The web layout inside the triangle is what distinguishes one truss type from another, and each layout suits a different span range and interior-space goal.

King post truss

The simplest truss: a single vertical web (the king post) runs from the peak to the center of the bottom chord. It is efficient and inexpensive but only economical for short spans, typically up to about 16-20 ft, because there is no diagonal bracing to control the long unsupported top-chord panel lengths beyond that.

Queen post truss

Two vertical posts plus a horizontal top member between them replace the single king post, shortening the unsupported top-chord length and extending the efficient span range to roughly 20-30 ft.

Fink (W) truss

The most common residential truss in North America. Its webs form a W shape between the chords, giving an efficient balance of material use and span for roughly 16-32 ft — which covers the overwhelming majority of single-family gable roofs. Its simplicity also makes it the cheapest truss per square foot of roof at typical residential spans.

Howe truss

Adds a more complex web arrangement with verticals and diagonals in compression/tension pairs, allowing efficient spans from about 30 to 50 ft. It is common on larger residential clear-spans, light commercial buildings and agricultural structures.

Double-W (double-fink) and multi-panel trusses

For spans beyond about 40-50 ft, the web pattern repeats or doubles to keep individual member lengths and connector-plate forces within practical limits, extending clear spans to 60 ft or more in engineered commercial and agricultural applications.

Scissor truss

The bottom chord slopes upward from each bearing point toward the center (though at a shallower pitch than the top chord), producing a vaulted or cathedral ceiling instead of a flat one. Scissor trusses cost more than an equivalent fink truss because the sloped bottom chord changes the force paths and generally requires deeper members, but they let a stick-framed-looking vaulted ceiling be built with prefabricated, engineered parts.

Attic truss (room-in-attic truss)

Designed with a raised, flat-bottomed void in the center — effectively a large rectangular opening built into the web layout — that becomes a usable room. The bottom chord is set to support a floor load rather than just a ceiling load, and the webs form an inverted-A or parallel-chord shape around the room instead of a full triangulated web field. Attic trusses need a wider building (commonly 28 ft or more clear span) and a steep enough pitch (typically 8:12 or steeper) to yield a usable room height and width inside the truss.

Gable-end (California) truss

Not a roof-supporting truss but a flat-faced vertical truss that closes off the gable end, built with vertical studs on 16 or 24-inch centers between the top and bottom chords to give a nailing surface for gable-end sheathing and siding.

Truss type and typical span ranges

These ranges assume standard residential design loads (roughly 20-40 psf combined snow/live and 10-15 psf dead load) and typical lumber grades (No. 2 SPF or SYP); the exact allowable span for any specific truss always comes from the manufacturer's engineered design, not from a generic table, because top-chord pitch, spacing, species and local wind/snow loads all shift the number.

Truss spacing: 16, 19.2 and 24 inches on center

Truss spacing (o.c. spacing) is the horizontal distance between truss centerlines along the length of the building, and it interacts directly with sheathing thickness, insulation strategy and total truss count.

• 16 in o.c.: the traditional residential default, compatible with 7/16 in OSB roof sheathing spans, and the safest default where ceiling loads, storage, or mechanical equipment concentrate weight unpredictably.
• 19.2 in o.c.: a common modern compromise (24 in divided by 1.25) that reduces truss count by about 17% versus 16 in o.c. while still working with standard sheathing thickness in most span/load tables.
• 24 in o.c.: reduces truss count by 33% versus 16 in o.c. and is common on simple gable roofs with asphalt shingles, but generally requires 15/32 in or thicker sheathing (or engineered rated sheathing) and closer attention to wind-uplift connector spacing.

Worked example: a 40-foot-long building needs roughly (40 x 12 / 16) + 1 = 31 trusses at 16 in o.c., versus (40 x 12 / 24) + 1 = 21 trusses at 24 in o.c. — 10 fewer trusses, but each bay of sheathing spans a third farther, so the sheathing spec and fastening schedule must be checked against the sheathing manufacturer's span rating for that spacing and snow load.

Heel height and the raised-heel (energy) truss

Heel height is the vertical dimension of the truss at the point where the top and bottom chords meet over the bearing wall. A standard truss has a very shallow heel — often just a couple of inches — because the top and bottom chords converge almost to a point at the wall line. That geometry is fine structurally but leaves almost no room for insulation directly above the top plate, which is exactly where uninsulated attics lose the most heat and where ice dams most often start.

A raised-heel truss (also called an energy truss or Arkansas heel) adds a vertical riser at the bearing point, lifting the point where the top chord starts sloping away from the wall. This creates enough clear depth above the top plate — commonly 9 to 16 inches or more, chosen to match the target attic insulation R-value — for full-depth loose-fill or batt insulation to run continuously over the wall instead of tapering to nothing at the eave.

Bracing and web layout

A truss is only as strong as its bracing while it stands alone; individually, a truss is a slender, unstable assembly out of plane until it is tied to its neighbors and the structure as a whole.

1. Temporary bracing goes up during installation, before sheathing, to hold each truss plumb and spaced correctly and to keep the whole row from toppling like dominoes in wind — this is specified on the truss placement diagram and is a jobsite safety requirement, not an option.
2. Permanent lateral bracing runs continuously across the top chords, bottom chords and webs at spacings called out on the engineered truss drawings (often every 8-10 ft along the bottom chord and at specific web locations), and stays in the building for its life.
3. Diagonal (X) bracing between trusses resists racking and out-of-plane buckling of the web members, especially on longer-span and higher-pitch trusses where individual webs are more slender relative to their length.
4. Roof sheathing, once nailed off per the fastening schedule, acts as the primary permanent bracing for the top chords across the whole roof plane; this is one reason sheathing pattern and fastener spacing on truss roofs is specified as carefully as on rafter roofs.
5. Girder trusses (typically 2, 3 or 4 truss plies fastened or bolted together) support the ends of other trusses at openings, valleys or load concentrations, and must be specified separately with their own engineering, including the hardware connecting the plies together.

The truss manufacturer's placement diagram is the controlling document on site: it specifies every truss's exact location, orientation, girder locations, permanent bracing layout and any web reinforcement required. Deviating from it — moving a truss, skipping a brace, or field-cutting a web — is one of the most common causes of truss-related roof failures and is generally a warranty and code violation.

When trusses beat stick framing (and when they don't)

In practice, trusses dominate new production and tract-home construction because the speed and labor savings outweigh their lead time and inflexibility, especially at spans over about 24 ft where an equivalent stick-framed roof would need ridge beams and structural ridge support. Stick framing still wins on small additions, complex custom rooflines with many intersecting planes, remodels tying into an existing irregular structure, and anywhere the attic itself needs to be a full-height usable space without ordering a specialized (and pricier) attic truss.

Ordering, engineering stamps and cost drivers

Trusses are not a stock lumberyard item; each job's truss package is custom-designed from the building's actual floor plan, roof pitch, span, snow/wind loads and any special conditions (skylights, chimneys, dormers, mechanical openings). The truss manufacturer runs each configuration through design software, produces individual engineered drawings for every truss type on the job, and has them reviewed and stamped by a licensed engineer before fabrication — the stamped drawings are typically required for the building permit and inspection.

• Span and pitch: longer spans and steeper pitches need larger lumber, more webs, and sometimes higher-grade material, all of which raise per-truss cost.
• Loads: higher snow loads, wind exposure categories, or added mechanical/PV loads increase member sizes and connector-plate sizes.
• Special trusses: girders, scissors, attic trusses, and hip-roof components (hip jacks, corner sets) cost meaningfully more per piece than a standard fink truss because of their added engineering and non-repeating fabrication.
• Lumber species and grade: high-grade Southern Yellow Pine or engineered lumber for high-load webs costs more than standard SPF.
• Delivery and crane/rigging: trusses over a certain length need a crane or boom truck to set, which is a real line-item cost separate from the truss price itself, and site access can materially change this cost.
• Lead time: rush fabrication fees apply if the schedule requires trusses faster than the manufacturer's standard production queue, which typically runs 1 to 4+ weeks depending on backlog.

Frequently asked questions