Roof Truss Calculator Metric

How the Metric Roof Truss Calculator Works

Everything on this page runs on a millimetre-first, metre-second, kilonewton-third workflow — the unit ladder used across UK, Australian and New Zealand structural documentation. You type the span as a metre value such as 4.5 (which is 4500 mm on the drawing), the pitch as a plain angle such as 25°, and the calculator resolves the right-triangle geometry that a gable roof forms over the building’s centreline. Because a symmetrical roof splits the span into two identical halves, every length the tool reports traces back to one small triangle with the run as its base.

The second difference from imperial tools is the pitch itself. American charts speak in “x/12” — how many inches the roof climbs per 12 inches of run — while metric drawings simply state the angle, because a protractor and a degree scale travel better than a rise-per-foot convention. A UK truss fabricator quotes 22.5°, not 5/12. The calculator therefore takes degrees as input and shows the ×/12 reading only as a courtesy output, so a plan imported from North America can still be checked without converting anything by hand.

• 1Run = span ÷ 2. For a 4.5 m span the run is 2.25 m — the horizontal base of the roof triangle from wall plate to ridge.
• 2Ridge rise = run × tan(θ). With a 25° pitch: 2.25 × 0.46631 = 1.049 m, which is 1049 mm of height above the wall plates.
• 3Top chord per side = run ÷ cos(θ). At 4.5 m and 25°: 2.25 ÷ 0.90631 = 2.483 m, so both chords together total 4.966 m.
• 4Rafter with overhang = (run + overhang) ÷ cos(θ). A 400 mm eaves projection extends the 25° rafter to (2.25 + 0.40) ÷ 0.90631 = 2.924 m per side.

Two derived readings ride along with the four lengths. The ×/12 equivalent is 12 × tan(θ), so 25° shows as 5.6/12 — useful whenever a drawing, a video, or a supplier quote slips into imperial language. The plumb cut angle is 90° − θ, which at 25° is 65.0°: that is the angle the ridge cut makes against the horizontal, and getting it right is the difference between top chords that meet cleanly at the apex and a ridge that needs re-cutting on site.

🏗️Millimetre-first roof framing

Millimetre-first roof framingPhoto: Unsplash

One habit separates a smooth metric takeoff from an expensive one: keep the units in a single lane per calculation. Convert the span to metres once, keep the overhang in millimetres until the last line, and only convert the final rise to millimetres for the drawing. Mixing them midway — adding 400 to 2.25 because both “look like lengths” — inflates the rafter length by a factor of a hundred and is the single most common arithmetic failure in metric roof work. The calculator’s inputs are labelled by unit precisely so the mixing never starts.

Truss Height Calculator Metric: Rise from Span and Angle

Ridge height is the number that decides whether a loft bedroom stands upright or stoops, whether a dormer clears the ridge, and whether a solar array keeps its winter sun. The formula never changes: rise = (span ÷ 2) × tan(θ). Work two derivations end to end and the pattern locks in. First, a 4.5 m span at 25°: the run is 4.5 ÷ 2 = 2.25 m; tan 25° = 0.46631; the rise is 2.25 × 0.46631 = 1.049 m, or 1049 mm. Second, a 6 m span at 22.5°: the run is 3.00 m; tan 22.5° = 0.41421; the rise is 3.00 × 0.41421 = 1.243 m, or 1243 mm. Notice that the wider building rises less — height scales with the angle much faster than with span.

The angle term dominates precisely because tan grows non-linearly. Move a 6 m span from 22.5° to 30° and the rise jumps from 1.243 m to 1.732 m — a 39% increase for a 7.5° change — while moving the same span from 4.5 m to 6.0 m at a fixed 25° lifts the rise only from 1.049 m to 1.399 m. That is why planning rules, height-to-boundary limits and ridge-line restrictions bite steep roofs first, and why the pitch cell of your set-out sheet deserves more scrutiny than the span cell.

One boundary case deserves a mention: at very low pitches the tangent flattens, so height becomes insensitive to span — a 10 m span at 15° rises only 1.340 m, less than a 4.5 m span at 25°. That is the whole appeal of shallow metric trusses under height-limited planning envelopes, and equally the reason shallow roofs demand careful drainage and load detailing, since the geometry that minimises height also slows water off the roof. Whatever the pitch, treat every figure here as geometry and have the structural side confirmed with a licensed engineer.

Regional Timber Grades: UK, Australia, NZ, South Africa & Canada

Geometry travels freely across borders; timber grades do not. A 6 m span at 22.5° produces the same 1.243 m rise in every country on this list, but the name on the timber, the grading machine’s stamp, and the design code behind it all change at the border. The notes below keep to what each system is called and how it is graded — the qualitative map you need before a conversation with a local supplier — without inventing allowable-stress numbers that belong to your engineer.

Roof Truss Calculator UK: C16 vs C24

Searches like roof trusses calculator uk or roof truss size calculator uk almost always end at the same fork: C16 or C24. Both are strength classes defined in BS EN 338 — the grading standard that sorts softwood into bands by stiffness and strength. C24 is the stiffer and stronger of the pair, which is why it is the common default in UK trussed-rafter specifications when the modest price premium is acceptable, while C16 remains widely used where loads allow. The practical consequence is simple: the same truss geometry can often use slightly smaller or more widely spaced members in C24 than in C16. UK structural timber design now runs through Eurocode 5 (BS EN 1995), which replaced the older BS 5268 permissible-stress method, so a fabricator’s quote today reflects limit-state design rather than your grandfather’s span tables. Use this page for lengths and heights, then let the fabricator’s Eurocode 5 software pick the section — and confirm the result with a licensed engineer.

Roof Truss Calculator Australia: MGP10 vs MGP12

A roof truss design calculator australia query lands in machine-graded pine territory. Australian framing softwood is stamped MGP — Machine Graded Pine — followed by a number that encodes stiffness: MGP10 is the volume-grade workhorse of residential roofs, and MGP12 is the higher-stiffness step up when a design wants a bit more performance from the same nominal size. The grading itself is done by machine, bending each piece and correlating its response to assigned properties, which is what makes the MGP label consistent across suppliers. Design context comes from AS 1720 (timber structures) with AS 1684 framing rules, and — uniquely for this list — Australian roof design carries a permanent cyclone and wind-region overlay that can dominate the member choices in the north. A roof truss calculator australia user should treat the geometry here as universal and the grade selection as strictly local: MGP10 and MGP12 change capacity, never length. Confirm the grade and sizing with a licensed engineer.

New Zealand and South Africa

New Zealand light-timber framing is organised by NZS 3604, the standard that underpins most house construction there, within a seismic design culture that shapes how connections and bracing are detailed — any truss design calculator nz workflow ends with that standard’s detailing rules, not just its span numbers. South Africa runs on SANS-graded structural pine: SA pine is sorted into strength grades under the SANS system, and local truss plants design to SANS 10160 loading actions with SANS 10163 timber rules, so a roof truss calculator south africa user should expect grades stamped directly on the timber and a fabricator who designs in-house. In both countries the pitch-in-degrees convention of this page matches local practice, and in both the final member sizes must be confirmed with a licensed engineer familiar with the local wind or seismic zone.

Canada

Canada is the imperial-speaking exception on a metric page — official units are metric, and drawings often carry millimetre dimensions, but the lumber trade still speaks nominal inches. Roof framing typically uses SPF dimension lumber (spruce-pine-fir, graded to the National Lumber Grades Authority rules), designed under the CSA O86 engineering standard with the National Building Code of Canada loading snow-load cases that dwarf wind in most regions. A truss calculator canada user can therefore enter spans here in metres, read lengths in millimetres for the set-out, and expect the fabricator to translate the member schedule into 2×4 and 2×6 nominal sizes. Snow depth, not pitch, usually drives Canadian member selection — another reason the geometry on this page is only half the answer, with the structural half belonging to a licensed engineer.

FAQ

🌤️Setting the top chords at 25 degrees

Setting the top chords at 25 degreesPhoto: Unsplash