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The overall span of the scissor truss.
Bearing to bearing, as the truss will sit. A scissor truss is more sensitive to this than a common truss because both chords are pitched: the span sets the top chord, the bottom chord AND the vaulted height between them, so one wrong figure moves the ceiling as well as the roof. Where the trusses bear on an inner leaf, measure to those centres and not to the outside of the wall.
The slope of the top (rafter) chord, expressed as rise per 12 units of run.
The OUTSIDE slope — the one the roof covering follows. The bottom chord is pitched too, and shallower, and the gap between the two is the vaulted ceiling you get. A common rule of thumb keeps the bottom chord at roughly half the top; running them closer flattens the vault, and running the bottom chord too steep leaves too little depth at mid-span for the truss to work as one.
The slope of the bottom (ceiling) chord, expressed as rise per 12 units of run.
A scissor truss's bottom chord slopes more gently than its top chord, creating the vaulted ceiling profile.
Net vaulted ceiling height gain
4.333 ft
- Top chord length
- 15.62 ft
- Bottom chord length
- 13.7 ft
- Truss depth between chords at the peak
- 4.33 ft
They open the calculator with your figures already in it
Scissor Truss Chord Geometry Calculator: 4.33 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Standard rafter-length trigonometry applied independently to each chord's own pitch: chord length = half-span × √(1+(pitch/12)²); vaulted ceiling height gain = the bottom chord's own rise, half-span × (bottom pitch/12); truss depth at the peak = top chord rise minus bottom chord rise
Inputs used
- Truss Span
- 26 ft
- Top Chord Pitch (rise per 12)
- 8
- Bottom Chord Pitch (rise per 12)
- 4
Intermediate steps
- Top chord length
- 15.62 ft
- Bottom chord length
- 13.7 ft
- Truss depth between chords at the peak
- 4.33 ft
What this calculation does not cover
- The net height is only the top chord's rise minus the bottom chord's rise at the centreline, so any positive difference is reported at high confidence: an 8:12 top against a 7:12 bottom returns a workable-looking number even though it leaves almost no depth at the apex for the webs and the peak connection. The calculation only flags a problem once the bottom chord pitch reaches or exceeds the top, because that is the point at which the two chords cross. Fabricators commonly hold the bottom chord to around half the top pitch, and that check is not applied here.
- The figure is the gap between two chord reference lines that both start from the same bearing point, which is not the clear height of the finished room. A real truss has a heel depth at the wall, the bottom chord is a member of real depth, and battens and plasterboard hang below it, so the usable vault is the wall height plus this figure, less the bottom chord's own depth and the ceiling build-up.
- Both chord lengths are theoretical centreline lengths from the bearing to the apex, for one side of the truss only. Nothing is added for an eaves overhang or rafter tail and nothing is deducted for the apex or ridge plate, the plumb cut or the heel seat cut. A cutting list also needs the opposite half of each chord and the web members, none of which this page produces.
- Scissor trusses flatten and spread horizontally at their bearings as they deflect, which is why the truss designer normally details one end as a sliding or slip bearing and specifies the wall plate fixing to suit. This page gives the fabricated geometry only, so it produces no horizontal movement figure and no bearing detail; the movement that must be accommodated comes from the truss engineer's analysis, not from this geometry.
- Both pitches are entered as rise per 12 units of run, not as an angle, even when the span is given in metric. A reader who means an 8-degree top chord and a 4-degree bottom chord and types 8 and 4 gets 8:12 (33.7 degrees) and 4:12 (18.4 degrees) instead — a plausible-looking net height that is badly wrong, and nothing in the calculation can detect the substitution.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Standard rafter-length trigonometry applied independently to each chord's own pitch: chord length = half-span × √(1+(pitch/12)²); vaulted ceiling height gain = the bottom chord's own rise, half-span × (bottom pitch/12); truss depth at the peak = top chord rise minus bottom chord rise
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