Summary

Unlike a cut-roof rafter, which a competent carpenter can size from a published span table, a manufactured roof truss is never span-tabled on site — every truss is individually designed by the manufacturer's engineering software for the specific span, pitch, spacing, loading and timber grade of that job, as covered in full in roof trusses. This calculator does not replace that design process. What it does is answer the practical, non-structural questions that come up when pricing and ordering a truss roof: how many trusses does a given roof length need at standard spacing, how does spacing affect batten and sheathing quantities, and what should you flag to the manufacturer's designer before they quote.

Getting truss count and spacing right at the pricing stage matters because it drives delivery logistics (crane requirement, storage space, erection time) and the knock-on material quantities for battens, breather membrane, and fixings — all before the structural design itself is finalised. Treat every figure in this article as an estimating tool for quoting and ordering, confirmed against the manufacturer's actual truss design drawing before you commit to a fixed-price contract.

Key Facts

Quick Reference Table — Indicative Truss Count by Roof Length and Spacing

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Roof Length Trusses at 600mm Centres Trusses at 400mm Centres
6m 11 16
8m 14 21
10m 18 26
12m 21 31
15m 26 39
20m 34 51

Calculated as (length ÷ spacing) + 1, rounded up. Always confirm against the truss designer's actual layout drawing, which may adjust the count for hip ends, gable positions, or girder truss requirements not captured by this simple formula.

Detailed Guidance

Step 1 — Calculate an indicative truss count for pricing

Use the formula above to get a first-pass truss count for pricing a job before the manufacturer's design drawing is available. This is for quoting and ordering purposes only — the manufacturer's final layout may vary the count where hip ends, valleys, or girder trusses at openings change the standard spacing pattern locally.

Worked example — 9.6m roof length, standard 600mm centres, straightforward gable-to-gable fink roof:

Worked example — 14m roof length with a hip at one end, 600mm centres:

Step 2 — Understand when 400mm centres apply instead of 600mm

400mm centres are specified by the truss designer, not chosen freely by the builder, and typically apply where:

Pricing implication: 400mm centres increase truss count by 50% compared with 600mm centres for the same roof length (600 ÷ 400 = 1.5×), which has a direct and significant impact on both the truss supply cost and the erection labour time. Never assume 600mm centres in a quote without confirming with the truss designer — if the roof covering or span pushes the design to 400mm, an estimate built on 600mm will be badly wrong.

Step 3 — Batten and covering material take-off (independent of truss spacing)

Batten gauge — the spacing between the horizontal battens fixed across the trusses, to which roof tiles or slates are hung — is a completely separate calculation from truss spacing, and the two are frequently confused on site. Batten gauge is determined by:

Common concrete interlocking tiles typically use a gauge in the region of 300–345mm, while plain tiles and natural slate use a substantially tighter gauge — ****, since gauge varies by tile profile and cannot be assumed generically. Battens run perpendicular to the trusses (horizontally across the roof slope) and are fixed to every truss they cross, regardless of whether truss spacing is 400mm or 600mm — so the number of trusses affects the number of batten fixing points, but not the batten gauge itself.

Step 4 — Weight and lifting considerations by span

Span Typical Handling Lifting Note
Up to 6m Manual, 2 operatives Straightforward manhandling
6–8m Manual, 3+ operatives, or light mechanical assist Team size and site access dependent
Over 8m Mechanical lifting typical Manufacturer often arranges crane as part of delivery
Attic/room-in-roof (any span) Mechanical assist recommended regardless of span Significantly heavier timber sections than standard fink

Price crane hire or mechanical lifting into the job at enquiry stage for any span over 8m, and for attic trusses regardless of span — this is a commonly underestimated line item when a builder prices truss erection based on a standard fink roof's labour rate without accounting for the heavier attic truss sections.

Step 5 — What this calculator does not do

This calculator will not tell you:

Use this article to build a first-pass quote and material list, then confirm every structural detail against the manufacturer's design drawing before finalising a fixed-price contract or ordering materials.

Frequently Asked Questions

Can I specify truss spacing myself to save on truss count?

No — spacing is a structural design output based on span, pitch, loading and timber grade, calculated by the truss manufacturer's engineer. Requesting wider spacing than the design calls for to reduce truss count is not a specification choice available to the builder; it would require the manufacturer to redesign around a different (and non-standard) spacing, which they generally won't do outside of 400mm/600mm without a specific engineering justification.

Does the roof pitch affect truss spacing?

Pitch affects the truss design itself (member sizes, geometry) more directly than it affects spacing, but a steeper pitch combined with a heavier roof covering can push the design toward 400mm centres where a shallower pitch or lighter covering would have been fine at 600mm. This is exactly the kind of interaction that makes truss design a per-project calculation rather than a lookup table — get the roof covering weight and pitch to the truss designer early.

Why does my truss quote seem to assume more trusses than my simple length ÷ spacing calculation?

Hip ends, valleys, gable ladder framing, and girder trusses at large openings all add trusses beyond the simple straight-run calculation in this article's formula. A roof with a hip end, for example, needs a girder truss plus a series of jack trusses in addition to the standard trusses covering the straight run — these are additional structural elements, not covered by dividing roof length by spacing.

Is it worth ordering extra trusses as a contingency?

Not usually, because unlike a cut timber rafter, a truss cannot be trimmed or adapted on site if it's the wrong size — a spare truss ordered "just in case" is only useful if it's identical to one already in the design, which is rarely a meaningful contingency. Order to the confirmed design drawing quantity, and treat any damage in transit or handling as a manufacturer replacement issue (with appropriate lead time) rather than something a spare unit resolves.

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