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The basics
What is a Scissor Truss?
A scissor truss buys you a vault without the posts and footings a structural ridge beam demands. What it costs is movement.
The competition for a scissor truss is a structural ridge beam with rafters (opens in a new tab), or a raised-tie truss. The ridge beam gives a full cathedral ceiling with no obstruction, but it needs posts and a load path all the way to the footings, often through the middle of a room you wanted open.
The scissor truss needs none of that. It bears on the two exterior walls like any other truss. The trade-off is that you never get the full roof pitch as ceiling pitch, you get roughly half, and the truss moves more than a conventional one.
- No interior bearing Clear-spans the building like a normal truss, so the floor plan below stays completely open.
- About half the roof pitch Ceiling slope is limited to roughly half the top chord pitch, which is a design constraint rather than a preference.
- More deflection and spread One bearing is normally detailed to slide, and drywall at the peak should be floated rather than fastened tight.
- Insulation needs planning The cavity narrows toward the peak, so confirm the required insulation depth fits at mid-span with a ventilation gap above it.
Scissor trusses move
A scissor truss deflects and spreads more than a conventional truss. Float the drywall at the peak rather than fastening it tight, or you will see a crack along the ridge within a year.
The maths
Scissor Truss Formula
Scissor truss geometry
Depth at mid-span is what makes the truss stiff. Keep the bottom pitch at or below half the top pitch so that depth stays workable.
Method
How to Calculate a Scissor Truss
A scissor truss is two triangles sharing the same bearing points, so you calculate the top chord and the bottom chord separately and read the depth between them.
- 1 Set the top chord pitch This is your roof pitch, exactly as it would be on a gable truss.
- 2 Set the bottom chord pitch This is your ceiling slope. The classic rule is half the top chord pitch, so an 8:12 roof with a 4:12 ceiling.
- 3 Read the vault gain The calculator shows how much higher the centre of the ceiling sits compared with a flat-bottom truss on the same span.
- 4 Watch the difference As the two pitches converge the truss loses depth at the centre and gets soft. The tool flags this before you order.
The two-to-one convention is not a code rule, it is a rule about depth. The vertical distance between the two chords at mid-span is what gives the truss its stiffness, and that distance shrinks quickly as the bottom chord steepens.
Push past it and two things happen. Deflection (opens in a new tab) climbs, so you get visible sag and drywall cracking at the peak. And horizontal thrust at the bearing points rises sharply, because a scissor truss does push outward on the walls in a way a flat-bottom truss does not.
Worked example
A 28 ft span at 8:12 over 4:12 gives a 14 ft run, a 9 ft 4 in top rise and a 4 ft 8 in ceiling rise, so the ceiling centre sits 4 ft 8 in higher than a flat one and the truss has 4 ft 8 in of depth at mid-span. Have the horizontal reaction checked before finalising the wall design.
Height gained at the centre of the ceiling versus a flat bottom chord, for common pitch combinations.
| Span | 8:12 over 4:12 | 10:12 over 5:12 | 12:12 over 6:12 |
|---|---|---|---|
| 20 ft | 3' 4" | 4' 2" | 5' 0" |
| 24 ft | 4' 0" | 5' 0" | 6' 0" |
| 28 ft | 4' 8" | 5' 10" | 7' 0" |
| 32 ft | 5' 4" | 6' 8" | 8' 0" |
| 36 ft | 6' 0" | 7' 6" | 9' 0" |
Note: Gain is measured at mid-span from the bearing level. Add heel height for finished ceiling clearance at the walls.
The tool
Why Choose Our Scissor Truss Calculator
A scissor truss lives or dies on the depth left at mid-span, and this calculator shows you that depth instead of hiding it.
- Two pitches, one drawing Top chord and ceiling slope as separate inputs, drawn together so the vault is visible.
- Vault gain in feet and inches Exactly how much higher the ceiling centre sits than a flat one, which is the reason you are here.
- Deflection warning built in When the two pitches converge past the usual limit, the tool says so before you order.
- Clearance at the walls Heel height is included, so the ceiling height at the wall is not a surprise.
More tools
Explore Related Truss Calculators
Calculators that pair with the scissor tool, each running the same geometry engine.
- Attic Roof Truss Calculator Room width, headroom and floor-load check for room-in-roof trusses. Open calculator
- Gable Roof Truss Calculator Live geometry, automatic web pattern selection and a complete material take-off. Open calculator
- Roof Truss Load Calculator Snow, dead and live load combined into per-truss forces and bearing reactions. Open calculator
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Answers
Frequently Asked Questions
Common questions about scissor roof truss calculator.
How do you calculate a scissor truss?
Work out the top chord geometry exactly as for a gable truss, then work out the bottom chord as a second, shallower gable sharing the same bearing points. The vertical distance between the two apexes is the truss depth at mid-span, and the ceiling gain is the bottom chord rise.
What is the maximum pitch for a scissor truss bottom chord?
Half the top chord pitch is the standard limit. Some fabricators will go slightly beyond it on shorter spans, but every step past that increases deflection and horizontal thrust disproportionately.
How far can a scissor truss span?
Residential scissor trusses commonly span 24 to 40 ft. Beyond that the reduced centre depth makes deflection control expensive, and a raised-tie or parallel-chord alternative is often cheaper.
Do scissor trusses push the walls out?
Yes, more than a conventional truss. The sloping bottom chord cannot resist thrust as efficiently as a flat one, so the design must account for horizontal reaction at the bearings and one end is usually detailed to slide.
How much ceiling height does a scissor truss add?
Roughly half the span multiplied by the bottom chord pitch over 12. A 28 ft span with a 4:12 bottom chord gains 4 ft 8 in at the centre compared with a flat ceiling, which is a substantial difference in a room.
Can you insulate a scissor truss roof properly?
It takes planning. The cavity between the chords narrows toward the peak, so you need to confirm you can fit the required insulation depth at mid-span with a ventilation gap above it. Raised heels at the walls and a good baffle detail at the peak are what make it work.
References
Scissor Truss Sources & 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 Vault (architecture), Wikipedia (opens in a new tab)
- WIKI Truss, 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.