A 40 ft long building with trusses at 24 in on center needs 21 trusses: 40 ft ÷ 2 ft = 20 spaces, plus one, which counts the two gable-end trusses. At 16 in on center it is 31. For a 28 ft span at 6/12 with 16 in overhangs, each top chord runs 17 ft 1 11/16 in from tail to peak and the peak stands 7 ft 3 15/16 in above the bottom chord with a standard heel.
How many trusses do I need?
Trusses sit at a fixed spacing along the building, with one at each end. Divide the building length by the spacing, round up to whole spaces, and add one.
Trusses = building length ÷ spacing, rounded up, + 1
Worked example with the defaults: 40 ft ÷ 24 in = 20 spaces, so 21 trusses: 19 common trusses and 2 gable-end trusses, which carry studs for the sheathing and siding at the ends. If the length does not divide evenly, the last space is shorter than the rest; never wider.
Top chord length, heel and height
The top chord is the sloped member. Its length from the tail to the peak is the level run (half the span plus the overhang) times the pitch multiplier. The height of the truss is the heel height plus the rise, and the rise is half the span times the pitch.
Top chord = (span ÷ 2 + overhang) × √(1 + (pitch ÷ 12)²) · Height = heel + span ÷ 2 × pitch ÷ 12
For the defaults: (14 ft + 16 in) × 1.118 = 17 ft 1 11/16 in of top chord. The rise is 14 ft × 6 ÷ 12 = 7 ft. A standard heel is the plumb depth of a 2×4 top chord over the wall, 3½ in × 1.118 = 3 15/16 in, so the peak is 7 ft 3 15/16 in above the bottom chord.
Raised or energy heel
A raised heel lifts the top chord over the wall so attic insulation can run full depth to the outside of the wall. Pick "Raised heel" and enter the height the truss plant quotes; the peak height grows by the same amount, which matters for the siding and for any height limit on your lot.
King post, Fink and Howe webs
The drawing shows the members on their center lines, the way a truss is laid out on the plant's table. The lengths are for planning and checking a quote, not cut lengths.
| Type | Webs | Default 28 ft span, 6/12 |
|---|---|---|
| King post | one vertical post at the center | 7 ft 3 15/16 in |
| Fink (W) | four diagonals from the third points of the bottom chord to the peak and the quarter points of the top chords | two of 4 ft 5¾ in, two of 8 ft 8¼ in |
| Howe | verticals at the quarter points and center, diagonals from the center up to the quarter points | verticals of 3 ft 9 15/16 in and 7 ft 3 15/16 in, diagonals of 7 ft 11¾ in |
More webs mean shorter unsupported lengths of chord, which is why the web pattern changes as spans and loads grow. Which pattern your roof gets is the truss designer's call, based on span, loads and bearing; the count, top chord and height above do not depend on it.
Trusses are engineered
The IRC requires wood trusses to be designed to accepted engineering practice and ANSI/TPI 1, and the truss design drawings must be approved by the building official and delivered with the trusses (R802.10.1, R802.10.2). They must be braced as the drawings or the SBCA BCSI guide say (R802.10.3), and no truss member may be cut, notched, drilled or spliced without a design professional's approval (R802.10.4). Use this calculator to plan the building and to check that a quote counts the right number of trusses; order the trusses themselves from a truss manufacturer.
Trusses or rafters?
Framing with rafters leaves the attic open and needs no truss design; the rafter calculator gives lengths and birdsmouth cuts, and the IRC rafter span tables (R802.4.1) set the sizes. Either way, start from the pitch in the roof pitch calculator, size any beams with the beam span calculator, frame the walls with the framing calculator, and order the roofing with the roofing calculator.