Quick answer: number of trusses = building length in inches ÷ spacing, rounded up, + 1. A 40 ft building at 24 in on center needs 21 trusses: 19 common trusses and 2 gable ends.
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
A detached garage is 26 ft long with trusses at 24 in on center.
26 × 12 = 312 in. 312 ÷ 24 = 13 spaces. 13 + 1 = 14 trusses.
That is 12 common trusses and 2 gable-end trusses. If the length doesn’t divide evenly, the last space is simply shorter, but you still need the extra truss.
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.
24 ft span, 6/12 pitch, 12 in overhang:
Height = 12 × 6 ÷ 12 = 6 ft. Top chord = (12 + 1) × 1.118 = 14.53 ft each side.
Add the chord depth (usually a 2×4 or 2×6) and any heel height to get the real overall height for delivery and crane planning.
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.
| Building length | 24 in O.C. | 16 in O.C. | 19.2 in O.C. |
|---|---|---|---|
| 20 ft | 11 | 16 | 14 |
| 24 ft | 13 | 19 | 16 |
| 30 ft | 16 | 24 | 20 |
| 36 ft | 19 | 28 | 24 |
| 40 ft | 21 | 31 | 26 |
| 48 ft | 25 | 37 | 31 |
| 50 ft | 26 | 39 | 33 |
| 60 ft | 31 | 46 | 39 |
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:
- Slope length of one side = (12 + 1) × 1.118 = 14.53 ft. Roof length with both gable overhangs = 40 + 2 = 42 ft.
- Roof area = 2 × 14.53 × 42 = 1,221 sq ft.
- Sheathing: 1,221 × 1.1 ÷ 32 sq ft per sheet = 41.97, so 42 sheets of 4 × 8 plywood or OSB.
- Roofing: 1,221 × 1.1 ÷ 100 = 13.43, so plan on about 13⅔ squares, which is 41 bundles of three-tab or architectural shingles.
- 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. 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:
| Trusses | Rafters | |
|---|---|---|
| Speed on site | Very fast: a roof can be set in a day | Slower: every piece is measured and cut |
| Material cost | Lower, using 2×4s and metal plates | Higher, using deeper 2×8 to 2×12 lumber |
| Attic space | Webs fill the attic (except attic trusses) | Open attic, easy to finish later |
| Changes | Never cut or alter without the engineer | Easy to modify on site |
| Lead time | Usually one to several weeks | Lumber is in stock |
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.
- 44 × 12 = 528 in. 528 ÷ 24 = 22 spaces, + 1 = 23 trusses: 21 common and 2 gable ends.
- Height = 14 × 6 ÷ 12 = 7 ft from the bottom chord to the peak.
- Top chord = (14 + 1.33) × 1.118 = 17.14 ft per side, so each truss is about 31 ft wide tip to tip.
- 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:
- Building length and span (outside of wall to outside of wall).
- Pitch, overhang and the tail cut you want (plumb, square or none).
- Spacing, roof covering (shingles, metal, tile) and ceiling finish.
- Your location, so they can apply local snow and wind loads.
- Any vaulted areas, attic storage or HVAC in the attic.
- 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.
Notes for builders and estimators
Layout. Start layout from one end so the first common truss lands at the spacing measured to its center. Many crews mark the plate at 23¼ in (or 15¼ in) for the first truss so the sheathing edge falls on its center.
Gable overhang. If the rake overhangs more than a few inches, you may need a drop-top gable truss with outlookers, or ladder framing. Tell the supplier.
Bracing. Install temporary and permanent lateral bracing per the truss drawings and the BCSI guide. Most truss failures happen during erection, before the sheathing is on.
Hips and girders. This calculator covers gable roofs. Hip roofs use a girder truss, step-down hip trusses and jacks, which the truss designer lays out.
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
How many trusses do I need?
Divide the building length in inches by the truss spacing and add one. A 40 ft building with trusses 24 in on center needs 40 × 12 ÷ 24 + 1 = 21 trusses, including the two gable-end trusses.
How many trusses for a 30 ft building at 16 inches on center?
30 × 12 ÷ 16 = 22.5, which rounds up to 23 spaces, plus one = 24 trusses.
Should trusses be 16 or 24 inches on center?
Most modern house trusses are designed for 24 in on center, which saves material and labor. 16 in spacing is used for heavier loads, tile roofs, or when the ceiling or sheathing needs closer support. Follow the truss designer’s spec.
How much do roof trusses cost?
Prices change with lumber markets and region. A simple common truss for a garage or house usually costs from under $100 to a few hundred dollars each, with gable-end and special trusses costing more. Ask your truss plant for a quote and enter the price to get a total.
How many sheets of plywood do I need for a roof?
Work out the roof area (both slopes, including overhangs), add about 10% for cuts, and divide by 32, the area of one 4 × 8 sheet. A 1,221 sq ft roof needs about 42 sheets. The calculator does this for you.
How much does it cost to install roof trusses?
On top of the trusses, budget for labor and usually a crane or boom truck for larger roofs. Installers charge per truss, per hour or per job, and costs vary widely by region. Enter your quotes in the calculator to see the total.
What is a gable-end truss?
It is the truss at each end of a gable roof. It has vertical studs instead of diagonal webs so you can attach sheathing and siding to it, and it sits on top of the end wall.
Are trusses cheaper than rafters?
Usually, yes. Trusses use smaller lumber and go up much faster, so material and labor are both lower for a standard roof. Rafters can be the better choice for small sheds, additions or attics you want to finish.
Can I build my own trusses?
Building codes require trusses to be designed by an engineer, and most come from a truss plant with stamped drawings. For sheds, some areas allow site-built trusses from approved plans. Check with your building department first.
What is the truss height?
For a common truss without a raised heel, the height from the bottom chord to the peak is half the span times the pitch ÷ 12. A 24 ft span at 6/12 is 6 ft tall, plus the depth of the chords.