Your truss, drawn to scale
Fink web
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The basics
What is a Gable Truss?
The gable is the default for good reasons, and knowing them helps you tell when it is the wrong default.
Every gable truss is the same five parts. Two top chords carry the roof in compression, one bottom chord holds the walls together in tension, the webs move load between them, and the connector plates (opens in a new tab) hold it all at the joints.
The single most misunderstood part is the heel. It looks like a detail but it sets your energy performance. With a standard 3½ in heel there is barely room for two inches of insulation (opens in a new tab) over the wall plate, which is exactly where heat escapes. Raising the heel costs a few dollars per truss.
- Cheapest per foot of span The simplest joint count of any pitched truss, which is why it is the baseline every other shape is priced against.
- Attic volume you can use A gable keeps the roof space open, so ventilation, access hatches and gable vents are all straightforward.
- Weaker in high wind The flat vertical gable-end wall catches wind that a hip roof would deflect. That is the main reason to consider a hip instead.
- Gable ends are not common trusses The frame at each end has vertical studs instead of webs and bears continuously on the wall. Order and price them separately.
Never modify a truss
Cutting, notching or drilling a chord or web voids the engineering completely. If a duct or a flue has to pass through, the truss designer has to design around it before fabrication.
The maths
Gable Truss Formula
Gable truss geometry
Slope factor is √(1 + (Pitch ÷ 12)²). Multiply any horizontal distance by it to get the true length along the roof plane.
Method
How to Calculate a Gable Truss
A gable truss is a symmetrical triangle, so every dimension follows from two numbers: the span and the pitch.
- 1 Span and pitch first These two numbers drive everything else. The calculator picks an appropriate web pattern for the span automatically.
- 2 Set heel height A standard heel is 3½ to 4 in. An energy heel of 10 to 16 in lets full-depth insulation run all the way over the wall plate.
- 3 Add the building length This is what turns a single truss into a truss count, a lumber order and a delivered price.
- 4 Check the advisories The tool flags spans past the practical limit for your chord size and pitches too low for shingles.
The web pattern is not decoration. A top chord in compression fails by bowing sideways (opens in a new tab) long before it crushes, and how far it can bow depends on the unbraced length between web connections. More panel points means shorter panels, which means more span from the same lumber.
That is why this calculator changes the web layout as the span grows: a king post below 16 ft, a queen post to 20 ft, a fink W to 34 ft, and a double fink beyond that.
Worked example
A 24 ft span at 6:12 with a 4 in heel gives a 12 ft run, a 6 ft rise and a 13 ft 5 in top chord, with the peak sitting 6 ft 4 in above the wall plate. The heel is cut at 26.57 degrees and the peak at 63.43. Confirm the chord size for your load with a supplier before ordering.
The web layout this calculator selects automatically, and the reasoning behind it.
| Span range | Web pattern | Typical chord | Why |
|---|---|---|---|
| Up to 16 ft | King post | 2x4 | One vertical is enough, extra webs would only add joints |
| 16 to 20 ft | Queen post | 2x4 | Two posts plus a straining beam control top chord buckling |
| 20 to 34 ft | Fink (W) | 2x4 or 2x6 | The W splits the top chord into short, efficient panels |
| 34 to 48 ft | Double fink | 2x6 | More nodes keep unbraced chord lengths under control |
| 48 ft and up | Engineered | 2x8 or steel | Requires a sealed design, no rule of thumb applies |
Note: Chord sizes assume standard residential loading. Heavy snow or tile roofing pushes each row up a size.
The tool
Why Choose Our Gable Truss Calculator
The web pattern is the part most calculators leave out, and it is the part that decides how far your lumber will actually reach.
- Web pattern picked for you King post, queen post, fink or double fink, selected from the span rather than left as a guess.
- Heel height as a real input The dimension that decides your insulation depth at the eave, not an afterthought.
- Warnings before you order Spans past the practical limit for your chord size and pitches too low for shingles are both flagged.
- Imperial and metric One toggle converts every input and every result, including the drawing labels.
More tools
Explore Related Truss Calculators
Calculators that pair with the gable tool, each running the same geometry engine.
- Roof Truss Span Calculator Check any span against chord size, species, pitch and load in one pass. Open calculator
- Hip Roof Truss Calculator Flat-top width, step-down heights, girder position and the full set take-off. Open calculator
- Attic Roof Truss Calculator Room width, headroom and floor-load check for room-in-roof trusses. Open calculator
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Answers
Frequently Asked Questions
Common questions about gable roof truss calculator.
How do I calculate a gable roof truss?
Start with the run, which is half the span, and the pitch. Rise equals run times pitch divided by 12. Top chord length is the square root of run squared plus rise squared. Then add the overhang tail, which is the horizontal overhang times the slope factor.
What is the difference between a gable truss and a common truss?
A common truss is the load-carrying truss used throughout the roof. A gable truss, properly a gable-end frame, sits at each end of the building, has vertical studs instead of webs, and bears on the wall below along its full length. They look similar and behave completely differently.
How far apart should gable trusses be?
24 in on centre is standard in North America and matches sheathing modules exactly. 16 in is used for heavy snow, tile or slate roofs, and for spans at the top of a chord size's range.
What is a fink truss?
A fink truss is a gable truss whose webs form a W between the chords. It is the most common residential truss in the world because the W puts a node roughly every quarter of the top chord, which lets a 2x4 truss span 20 to 34 ft.
How high is the peak of a gable truss?
Peak height above the bottom chord equals the run times the pitch divided by 12, plus the heel height. A 24 ft span at 6:12 with a 4 in heel gives 6 ft plus 4 in, so 6 ft 4 in overall.
Can I store things on gable truss bottom chords?
Only if the truss was designed for it. Standard residential trusses are designed for a ceiling dead load and nothing more. Storage needs a designed bottom chord live load specified before fabrication.
References
Gable Truss References & Standards
Related calculators on this site, plus the standards and reference works the figures on this page are based on. External links open in a new tab.
Authoritative external references
- WIKI Gable roof, Wikipedia (opens in a new tab)
- WIKI King post, Wikipedia (opens in a new tab)
- STD ANSI/TPI 1, Truss Plate Institute (opens in a new tab)
- STD American Wood Council, NDS and span tables (opens in a new tab)
- CODE International Code Council, model building codes (opens in a new tab)
Codes and standards are revised on their own cycles. Confirm the current edition adopted in your jurisdiction with your local building department before relying on any figure.