Roof Truss Calculator

Background.

A roof truss is a component you specify and buy, not one you cut. That single fact decides both what this page computes and, more usefully, what it refuses to compute.

What you need in order to order trusses is a short list: the span bearing to bearing, the pitch, the heel height, the overhang, the spacing, and how many. This page produces those, plus the overall height and the chord lengths so you can check clearances, size the gable-end wall, and put a rough number on the lumber. For a 30 foot gable truss at 6:12 with a 3.5 inch heel and a 24 inch eave, at 24 inches on centre over a 48 foot building: twenty-five trusses, 7 ft 9 1/2 in tall at the peak, with top chords 19 ft 0 1/16 in long including the tail, and a 30 foot bottom chord.

Two of those numbers are worth dwelling on. The overall height is measured from the top of the bottom chord to the top of the top chord at the peak, and it is heel height plus the run in feet times the rise — so a raised energy heel lifts the whole truss without lengthening a single chord. Going from a 3.5 inch conventional heel to a 12 inch energy heel takes the same truss from 7 ft 9 1/2 in to 8 ft 6 in, changes nothing about the top chord, and adds most of a course to the gable-end wall. The overhang is the other one: 24 inches of eave is a horizontal projection measured on the plan, and along a 6:12 slope it becomes 26.8 inches of top chord. Treat it as a slope length and you will short every chord on the roof.

What this page does not give you is web member lengths, and that is a correctness decision rather than a gap. A profile name like Fink, Howe, king post or queen post describes the web pattern, but the panel-point positions that fix the actual web lengths are generated by the truss plant's engineering software from the loads, the connector-plate capacities and the lumber it happens to stock. Two suppliers will hand you different web layouts for the same profile name at the same span and pitch. There is no published geometric standard to quote, so quoting one would mean inventing a specification. The page emits none and says so beside the answer. The same reasoning applies to chord sizes, plate sizes and any capacity check: the chord size input here exists only to turn lineal feet into board feet for a budget, and it changes no dimension on the page.

There is also no cut angle anywhere on this page, which is the sharpest difference between it and the rafter length calculator. IRC R802.10.2 requires metal-plate-connected wood trusses to be designed in accordance with accepted engineering practice and to comply with ANSI/TPI 1. R802.10.1 requires the truss design drawings to be provided to the building official and approved before installation, and to be delivered with the trusses. R802.10.3 requires bracing to prevent rotation and provide lateral stability, either per the construction documents or per accepted industry practice such as the SBCA BCSI guide. And IBC 2303.4.5 requires written concurrence and approval from the truss designer before a truss member is cut, notched, drilled, spliced or altered. A truss calculator that printed a saw angle would be inviting a code violation, so this one does not.

Use the numbers here to get a quote, check a delivery clearance and budget the lumber. Confirm the section numbering against the code edition your jurisdiction has actually adopted, because adopted editions and local amendments differ. The sealed truss design drawings govern over everything on this page, and a licensed structural engineer or architect must sign off on the framing before work proceeds.

What is roof truss calculator?

A roof truss is a prefabricated triangulated frame that replaces site-cut rafters and ceiling joists with one engineered component. Its outline is defined by four numbers: the span between bearings, the pitch of the top chord, the heel height at the bearing, and the overhang.

The geometry follows from those directly. The overall height at the peak is the heel height plus the horizontal run in feet times the rise per foot. Each top chord is the run in feet times the unit length √(144 + rise²), which is the amount of chord per 12 inches of horizontal run, plus the overhang converted the same way. The bottom chord is the span. On a common gable truss the run is half the span and there are two top chords; on a monoslope or shed truss the run is the whole span and there is one, which makes a monoslope twice as tall as a gable of the same span and pitch.

Inside that outline sit the web members, and they are not geometry — they are engineering. ANSI/TPI 1 governs the design and manufacture of metal-plate-connected wood trusses, and the truss designer sets the web configuration, the panel points, the chord and web sizes and the connector plates from the loads the truss has to carry. The truss design drawings record all of it and, under IRC R802.10.1, have to reach the building official and be approved before the trusses go up.

This calculator produces the outline dimensions, the truss count and a chord-lumber estimate. It does not design a truss, size a member, compute a web length, check capacity or print a cut angle.

How to use this calculator.

1. 01Choose the type. A common gable truss peaks in the middle; a monoslope slopes one way across the whole span and is twice as tall for the same pitch.
2. 02Enter the span bearing to bearing, outside face of plate to outside face of plate. This is also the bottom chord length.
3. 03Enter the pitch as the rise per 12 inches of run — the first number of an x:12 pitch.
4. 04Enter the heel height. If you do not know it, leave the default and ask your supplier: a conventional heel is shallow and an energy heel is commonly 12 inches or more, and the difference only affects the height, not the chords.
5. 05Enter the eave overhang as a horizontal projection on the plan, not as a length along the slope.
6. 06Pick the spacing — 24 inches on centre for most residential work — and enter the building length for the count.
7. 07Read the truss count, then the overall height for delivery clearance and gable-end wall height, and the chord lengths for the take-off.
8. 08Read the note beside the answer. It lists what is excluded from the count and why there are no web lengths or cut angles, and it names the code requirements that apply before the trusses can be installed.
9. 09Send the span, pitch, heel, overhang, spacing and quantity to a truss supplier and let them engineer it. Do not cut, notch or drill a delivered truss without written approval from the truss designer.

The formula.

The outline comes from one unit length and one run. The unit length is the amount of top chord per 12 inches of horizontal run, √(144 + rise²). The run is half the span for a common gable truss and the whole span for a monoslope. Overall height = heel height + (run ÷ 12) × rise. Top chord = (run ÷ 12) × unit length + (overhang ÷ 12) × unit length. Bottom chord = span. Truss count = ⌊building length × 12 ÷ spacing⌋ + 1. Working the example: a 30 ft gable truss has a run of 180 inches. At 6:12 the unit length is √(144 + 36) = √180 = 13.4164 inches, so the top chord body is (180 ÷ 12) × 13.4164 = 201.246 inches, the 24 inch overhang becomes (24 ÷ 12) × 13.4164 = 26.833 inches, and the top chord totals 228.079 inches — 19 ft 0 1/16 in. The overall height is 3.5 + (180 ÷ 12) × 6 = 3.5 + 90 = 93.5 inches, which is 7 ft 9 1/2 in. Over a 48 ft building at 24 inches on centre the count is 576 ÷ 24 = 24 bays, so 25 trusses. Chord lumber per truss is two top chords at 228.079 inches plus a 30 ft bottom chord: 68.013 lineal feet, or 1,700.3 lineal feet over 25 trusses, which at a nominal 2x4 is 2 × 4 × 1,700.3 ÷ 12 = 1,134 board feet of chord lumber. That figure excludes every web, so treat it as a floor. The same truss as a monoslope has a 360 inch run: 183.5 inches tall and a 429.3 inch top chord. Every intermediate value is carried at twenty significant digits and rounded only at the end, and dimensions are rounded to the nearest sixteenth of an inch.

A worked example.

Frequently asked questions.

References& sources.

1. [1]International Code Council, via a STRUCTURE magazine review of the residential truss provisions — IRC Section R802.10.2 requires wood trusses to be "designed in accordance with accepted engineering practice" and to "comply with ANSI/TPI 1", and lists the required content of the truss design drawings (slope or depth, span, spacing, joint locations, reaction forces, bearing widths, chord and web forces, connector plate specifications, lumber grades, adjustment factors, connection requirements, deflection and permanent bracing locations). The article discusses the 2018 edition; confirm the section numbering in the edition your jurisdiction has adopted.
2. [2]International Code Council — IRC Section R802.10.1: truss design drawings must be "provided to the Building Official and approved prior to installation", and must also be provided with the shipment of trusses delivered to the job site. This is the requirement that makes the truss design drawing, not a calculator, the governing document.
3. [3]International Code Council — IRC Section R802.10.3: trusses shall be braced "to prevent rotation and to provide lateral stability", either as specified in the construction documents or the individual truss design drawings, or in accordance with accepted industry practice such as the SBCA BCSI Guide to Good Practice for Handling, Installing, Restraining and Bracing of Metal Plate Connected Wood Trusses.
4. [4]International Code Council — IBC Section 2303.4.5: truss members and components shall not be "cut, notched, drilled, spliced or otherwise altered" without the written concurrence and approval of a registered design professional. This is why this page publishes no cut angles. The IRC carries an equivalent prohibition on cuts, notches and bored holes in trusses except where permitted by the manufacturer's recommendations or specifically considered in the design by a registered design professional.
5. [5]Truss Plate Institute — ANSI/TPI 1, National Design Standard for Metal Plate Connected Wood Truss Construction, the standard the IRC and IBC both incorporate by reference for truss design and manufacture. It is the authority for web configuration, panel points, member sizing and plate design, none of which this page computes. Access is by purchase from TPI.