Roof Truss Calculator

How to count roof trusses

A roof truss is a prebuilt triangle of lumber that holds up the roof and the ceiling at the same time. Trusses are set in a row along the length of the building, all at the same spacing, with one at each end. The count is the number of spaces plus one:

Trusses = ⌈ length (in) ÷ spacing (in) ⌉ + 1

Truss spacing: 16 or 24 inches on center?

“On center” (O.C.) means the distance from the middle of one truss to the middle of the next. 24 in O.C. is the standard for most new homes and garages: fewer trusses, fewer lifts and less labor. 16 in O.C. appears where loads are heavy (deep snow, clay tile), where the ceiling drywall is ½ in, or in older or custom designs. 19.2 in spacing is used with some engineered layouts, since it divides an 8 ft sheet into five.

The spacing isn’t your call alone: the truss designer picks it with the loads, and your sheathing thickness must match. Ask for the spacing on the truss drawings and enter it here.

Truss height and chord lengths

The bottom chord is the level bottom member and equals the span. The two top chords form the roof slope. For a common truss, the height to the peak is half the span times pitch ÷ 12. Each top chord, including the tail, is (half span + overhang) × the slope factor.

Need the angle or slope for your roof? Use the roof pitch calculator. Framing with loose rafters instead? The rafter length calculator gives the cut list.

Truss count chart

How many trusses you need for common building lengths, including both gable ends. Lengths that don’t divide evenly by the spacing round up to one more truss.

Worked example: sheathing, shingles and total cost

Once the trusses are up, the roof needs sheathing and roofing. The calculator works these out from the same numbers. For a 40 ft long building with a 24 ft span, 6/12 pitch and 12 in overhangs at the eaves and gables:

1. Slope length of one side = (12 + 1) × 1.118 = 14.53 ft. Roof length with both gable overhangs = 40 + 2 = 42 ft.
2. Roof area = 2 × 14.53 × 42 = 1,221 sq ft.
3. Sheathing: 1,221 × 1.1 ÷ 32 sq ft per sheet = 41.97, so 42 sheets of 4 × 8 plywood or OSB.
4. Roofing: 1,221 × 1.1 ÷ 100 = 13.43, so plan on about 13⅔ squares, which is 41 bundles of three-tab or architectural shingles.
5. Cost: 21 trusses at $150 = $3,150, plus install labor at $40 a truss ($840) and a $400 delivery and crane charge, comes to $4,390. These prices are only an illustration; enter your own quotes.

Common truss types

Most homes use one of a handful of shapes, and you can pick six of them in the calculator to see their webs and cut list. Each changes how the inside of the roof can be used:

• King post: a single vertical web. Cheap and simple; for sheds, porches and spans up to about 20 ft.
• Fink (W): the most common house truss, with W-shaped webs. Efficient for spans of roughly 20 to 40 ft, but leaves little attic storage.
• Howe and Pratt: vertical and diagonal webs; used for longer spans and heavier loads.
• Scissor: a sloped bottom chord that gives you a vaulted ceiling inside.
• Attic (room-in-attic): an open rectangle in the middle for usable floor space; deeper chords and a higher price.
• Gable end: vertical studs at 16 or 24 in for sheathing; one at each end of a gable roof.

Trusses vs. rafters

Rafters are cut and assembled on site, one board at a time; trusses arrive ready-made from a factory. Both can build the same roof, but they suit different jobs:

For a standard garage or house, trusses are usually cheaper overall. Rafters still win for small sheds, additions that tie into an existing roof, and attics you plan to turn into a room.

Worked example: a 28 × 44 ft house

A single-story house is 44 ft long and 28 ft wide, with a 6/12 roof, 16 in overhangs and trusses at 24 in on center.

1. 44 × 12 = 528 in. 528 ÷ 24 = 22 spaces, + 1 = 23 trusses: 21 common and 2 gable ends.
2. Height = 14 × 6 ÷ 12 = 7 ft from the bottom chord to the peak.
3. Top chord = (14 + 1.33) × 1.118 = 17.14 ft per side, so each truss is about 31 ft wide tip to tip.
4. At a quoted $180 per truss, the trusses cost about $4,140 before delivery and crane time.

Trusses this wide are heavy and floppy; most crews use a crane or boom truck and set them with a spreader bar.

Delivery and installation

Have the walls framed, plumbed, braced and the top plates marked at your spacing before the delivery truck arrives. Check the truss drawings for which end goes where and for any girder or special trusses. Set the first gable truss and brace it well to the ground, then lift each common into place, nail or strap it to the plate with the specified connectors, and tie it back with temporary lateral bracing as you go. Sheathe as soon as the set is plumb and braced. Never cut a chord or web to fit ductwork or a ladder; a truss that has been altered needs a repair detail from the engineer.

What to tell the truss supplier

Truss plants design every order, so a phone call goes faster if you have these ready:

1. Building length and span (outside of wall to outside of wall).
2. Pitch, overhang and the tail cut you want (plumb, square or none).
3. Spacing, roof covering (shingles, metal, tile) and ceiling finish.
4. Your location, so they can apply local snow and wind loads.
5. Any vaulted areas, attic storage or HVAC in the attic.
6. Number of gable ends, and whether you need a girder truss or hip set.

Roof truss cost

Truss prices move with the lumber market, span, pitch, type and region, so get a current quote. As a rough guide, simple common trusses for garages and small homes often cost from under $100 to a few hundred dollars each; gable ends, scissor and attic trusses cost more, and delivery or crane time is usually extra. Enter your quoted price per truss in the calculator to see the total.

Truss cut list and lumber

The calculator lists every member of one truss with its centerline length: the distance between the joints (panel points) where the members meet. The truss plant’s software turns these into exact cut lengths and angles once it knows the lumber sizes, heel height and plate positions, so use these numbers for planning and lumber estimates, not for cutting.

Building trusses yourself? Most building departments only accept trusses designed by an engineer, either from a truss plant or from approved plans. For a small shed, check with your building department before you cut anything.

Roof loads on a truss

Trusses are designed for the loads they carry, measured in pounds per square foot (psf). The calculator multiplies these by the area each truss supports (its spacing × the span plus overhangs) for a rough idea of the weight on each truss and each bearing wall. It is a sense check for talking to your truss supplier, not a structural design.

How far can a truss span?

Trusses can span much further than rafters because the webs turn bending into tension and compression. The actual limit depends on the design, lumber, loads and pitch, and is always set by the truss engineer. As a planning guide:

Wider buildings use trusses with more panels (double Fink, double Howe) or attic and girder trusses. Ask your supplier what they recommend for your span.

Attic space and headroom

Under a flat bottom chord, the height at any point is the distance from the wall × pitch ÷ 12. The calculator shows how wide the floor is where you have your chosen headroom, for example 5 ft. On a 24 ft span at 6/12, that width is only 4 ft, and the webs of a Fink truss block most of it anyway. If you want a usable attic room, ask for attic (room-in-attic) trusses, which leave a clear rectangle in the middle, or choose a steeper pitch.

The math behind the calculator

Roof truss glossary

Top chord

The sloped members that form the roof line and carry the sheathing.

Bottom chord

The bottom member that ties the truss together and usually carries the ceiling.

Web

The inside members between the chords, in tension or compression.

Panel point

A joint where webs meet a chord, held together with a metal plate.

Gusset (truss) plate

The toothed steel plate pressed into the joints at the factory.

Heel

Where the top and bottom chord meet over the wall. A raised (energy) heel leaves room for full insulation at the eaves.

Tail / overhang

The part of the top chord that extends past the wall to form the eave.

Bearing

The point where the truss sits on a wall or beam.

Clear span

The distance between bearings with no support in between.

Girder truss

A multi-ply truss that carries other trusses, used at hips and openings.

Gable-end truss

The end truss with vertical studs for sheathing and siding.

Piggyback truss

A small cap truss on top of a flat-topped truss, used when one truss would be too tall to ship.

Permanent bracing

Lateral and diagonal bracing specified on the truss drawings that keeps webs and chords from buckling.

Notes for builders and estimators

Tips

• Order one or two spares only if the supplier agrees; trusses are custom and seldom returnable.
• Store them flat on blocking and off the ground, or upright and braced, never leaning.
• Check the walls are square and the plates level before the trusses arrive.
• Send the count to your supplier using WhatsApp or Email under the result.

Frequently asked questions