Scissor truss Calculator

Truss Count, Rafter Length and Ridge Height for a Scissor truss

What is a scissor truss?

A scissor truss slopes the bottom chord upward as well as the top, giving a vaulted ceiling without a structural ridge beam. Typical span: commonly 20 to 40 ft. Truss count comes from the building length rather than the span — divide the length by the spacing, add one, then add two gable-end trusses.

A scissor truss slopes the bottom chord upward as well as the top, giving a vaulted ceiling without a structural ridge beam. This page gives the truss count at 24 and 16 inch centres, the rafter length and the ridge height for the spans a scissor truss is normally used at, along with what distinguishes it from a standard fink truss, where it fits, and the constraint that decides whether it will work on your building. Typical span: commonly 20 to 40 ft.

Rafter length and ridge height by span

Scissor truss geometry by span, excluding the overhang
Span	4/12 rafter	6/12 rafter	8/12 rafter	6/12 ridge height
16 ft	8.4 ft	8.9 ft	9.6 ft	4 ft
20 ft	10.5 ft	11.2 ft	12 ft	5 ft
24 ft	12.6 ft	13.4 ft	14.4 ft	6 ft
28 ft	14.8 ft	15.7 ft	16.8 ft	7 ft
32 ft	16.9 ft	17.9 ft	19.2 ft	8 ft
40 ft	21.1 ft	22.4 ft	24 ft	10 ft

A scissor truss changes the webbing inside the outline, not the outline itself, so the rafter length and ridge height are the same as any other truss at that span and pitch.

How many trusses

Truss count comes from the building length, not the span. Divide the length by the spacing, add one for the final end, then add the two gable-end trusses.

Truss count by building length
Building length	At 24 in centres	At 16 in centres
24 ft	15	21
32 ft	19	27
40 ft	23	33
48 ft	27	39
60 ft	33	48

Counts include the two gable-end trusses, which are a separate component from the commons and are ordered as such.

What makes it a scissor truss

The bottom chord pitch is usually about half the top chord pitch — a 10/12 top over a 5/12 ceiling.
Deflection is larger than a standard truss, so the drywall needs slip connections at the walls to avoid cracking.
No structural ridge beam and no interior bearing wall, which is the whole point.
Insulation depth is limited by the chord separation, so the ceiling R-value needs checking early.

Where it fits

At a glance

Typical span	Commonly 20 to 40 ft
Best for	A vaulted or cathedral ceiling on a conventionally framed roof
Watch for	Scissor trusses deflect more than flat-bottom trusses, so the ceiling detail has to allow for movement

The rules that apply to every truss

Never cut, notch or drill a truss. It is an engineered assembly and any modification voids its design — even a chord drilled for a cable run needs sign-off from the truss designer.
Order after the wall plates are measured. Span is set at the plant and there is no adjustment on delivery.
Fit the permanent bracing shown on the truss drawings. Temporary erection bracing is not the same thing and is not a substitute.
The two gable-end trusses are a different component from the commons and are priced separately.
Lead time is weeks, not days. A truss order is one of the first things to place on a build programme.

Frequently asked questions

What is a scissor truss?

A scissor truss slopes the bottom chord upward as well as the top, giving a vaulted ceiling without a structural ridge beam.

What span can a scissor truss cover?

Commonly 20 to 40 ft. The exact limit depends on the timber species and grade, the spacing, and the snow and wind loads for your area, so the truss designer sets it rather than a table.

How many scissor trusses do I need?

Divide the building length in inches by the spacing and add one, then add two gable-end trusses. A 40 foot building at 24 inch centres needs 21 common trusses plus two gable ends.

Can I modify a scissor truss on site?

No. Cutting, notching or drilling any truss voids its engineered design. If it does not fit, the answer is a redesign from the truss plant, not a saw.