Engineered roof trusses replaced stick framing for one reason: they ship pre-built, drop in 30 minutes per truss with a crane, and the lumber is sized by an engineer instead of by you eyeballing span tables. The math for ordering is simple — count and spacing. The math for the truss itself is the supplier's problem. Here's what you need to know to order correctly.

Estimate only — engineered design required. Truss design (lumber sizes, plate connectors, web layout) is engineered by the truss manufacturer for your specific span, snow load, and roof loads. This guide and calculator handle count and spacing only, not truss design.

The truss count formula

Trusses are spaced along the building length, perpendicular to the ridge. The math is:

total_trusses = ceil(length_in ÷ spacing) + 1

The +1 is the truss at the starting end — every spacing interval adds one more truss after that, and you cap with one at the far end.

Example: a 30 ft long building at 24 in OC. (30 × 12) ÷ 24 = 15 intervals. 15 + 1 = 16 trusses total. If both end walls are gable trusses, that's 14 common trusses + 2 gable trusses. The roof truss calculator handles this and the gable end split.

16 vs 24 OC — which spacing

24 in on center is the residential standard for two reasons: it lets you sheath with standard 4×8 ft OSB or plywood with fewer pieces, and the truss count is 33% lower vs 16 in OC. The tradeoff is that 24 in OC needs heavier top-chord lumber and heavier sheathing.

24 in OC:

  • Standard residential framing — single-family, light commercial
  • Allows 7/16 in OSB sheathing on most roofs
  • Lighter overall framing weight, fewer pieces to lift
  • Lower lumber and labor cost

16 in OC:

  • Required for very heavy snow loads (above ~50 psf design)
  • Used on long spans (over 32 ft) to allow lighter top chords
  • Allows lighter sheathing options including some panel products
  • 33% more trusses to lift, set, and brace

Truss suppliers default to 24 in OC unless your engineer specifies otherwise. The cost difference for going to 16 in OC on a 30 ft building is roughly 6 extra trusses plus the labor to set them — typically $1,500-3,000 in 2026 pricing depending on region.

Gable trusses — a different beast

A gable end truss looks like a regular truss outline but the web members are vertical studs at 16 or 24 in OC, doubling as the gable wall framing. They cost about 50% more than a common truss because of the extra material and engineering, but they eliminate having to stick-frame the gable walls separately.

Skip gable trusses if your end walls are already conventionally framed up to the roof line. Most production builders use them; most owner-builders save the cost and stick-frame the gables.

Hurricane ties — never skip these

Every truss bears on the top plate of the wall. Code (IRC R802.11) requires uplift connection between truss and plate sized for the wind region. The standard is a Simpson Strong-Tie H1 or H2.5A hurricane clip, one per truss-to-plate connection.

In coastal regions (Florida, Gulf Coast, Outer Banks), the spec jumps to H10 or LTS clips and requires anchoring to the wall studs, not just the plate. Local code amendments override the base IRC — pull the spec from the building department before ordering connectors.

Lead time — order before you frame

Engineered trusses run 4-8 weeks lead time in most US markets in 2026. The supplier needs:

  • Foundation and floor plan (defines the span)
  • Roof pitch (set by the architect or calculated from the rafter design)
  • Snow load (psf) for your jurisdiction — pull from the snow load calculator
  • Wind exposure category (B/C/D)
  • Sheathing and roofing dead load (typically 8-15 psf)

Submit the order when foundation forms go in — by the time framing is dried in, trusses arrive. Showing up with framed walls and no truss order in the system means a 1-2 month framing pause while the trusses get engineered and shipped.

What pros do differently

Get the truss layout drawing approved before manufacture. Suppliers send a layout PDF with web member positions, bearing points, and uplift values. The site superintendent and truss installer should both sign off before the order ships. Field changes after delivery cost weeks.

Order extra bracing material with the trusses. Permanent diagonal bracing is required between trusses per the truss design drawing. Most suppliers will quote the bracing lumber as a line item if asked. Without it, the framers improvise with whatever 2×4 they have on the truck — often non-compliant.

Confirm the bearing surface is sized right. Trusses bear on the wall top plate. Long-span trusses (40+ ft) often need a doubled top plate or a beam at the bearing. The truss layout drawing specifies the required bearing length — verify your wall framing matches before the trusses arrive.

Quick FAQ

How long does a truss take to install? About 30-45 minutes per truss with a crane, faster as the crew gets into a rhythm. A 16-truss roof on a 30 ft house takes a single day with a 3-person crew and crane.

Can I cut a truss to fit? Never. Cutting any web member voids the engineering and the warranty, and weakens the load path. If a truss is wrong, the supplier modifies and reships.

Do trusses replace ridge boards?Yes — engineered trusses are self-supporting once braced. There's no ridge board, no rafter ties, no collar ties. The truss IS the framing system.

Engineered design — verify with supplier. The roof truss calculator handles count and spacing only. Truss design (lumber sizes, web layout, plate connectors) is engineered by the truss manufacturer for your span, snow load, and wind region.