Opens our free tool — no signup, no email required.
What this calculator does
Enter a building width and a pitch (in degrees or X-in-12). The calculator gives you the apex height, the rafter length, the number of sheets, and a per-sheet cut list for the rake on a gable wall. All measurements in millimetres. No login, no email, no catch — just numbers you can use today.
Roof pitch, in plain language
Pitch is the angle of your roof slope. It's measured either in degrees (8° is shallow, 22.5° is a standard gable, 30°+ is steep) or as a rise-over-run ratio like 3-in-12 or 6-in-12. In WA, most flat-roof patios sit at 4°–6°. Standard gable sheds run 10°–22.5°. Skillions are usually 5°–10°.
Why it matters: pitch controls how much water runs off the roof. Below about 3° you need standing-seam profiles like Klip-Lok or Spandek — Trimdek isn't rated below 2°. Pitch also drives ridge height, which feeds into council compliance (overall height limits) and the engineering of your trusses or portal frame. Get the pitch wrong and the rest of the quote follows it.
On a gable wall, pitch also drives the sheet cuts. The sheets at the rake have to be cut on an angle that matches the pitch, and each sheet is a different length because it sits further out from the apex. The calculator works out every cut so you can order pre-cut from your fabricator, or do the cuts on site without guessing.
Worked example
Inputs: 9,000 mm × 6,000 mm gable shed, 10° pitch, 4,000 mm eave height, Trimdek profile (762 mm cover).
Outputs: apex height 4,793 mm. 12 sheets of Trimdek across the gable wall. Longest sheet 4,838 mm (#6, straddles the apex). Total linear length 54.02 m for one gable end. Multiply by two for both ends. Order to the nearest standard length your supplier carries.
That's enough to phone the supplier or load a quote in five minutes flat.
FAQ
What's the difference between pitch in degrees and pitch in X-in-12?
Same thing, expressed differently. 10° works out to roughly 2.12-in-12 — meaning the roof rises 2.12 mm for every 12 mm of horizontal run. The calculator shows both. Builders in Australia mostly use degrees; the US convention is X-in-12. Use whichever your fabricator gave you.
What's the minimum pitch for a Colorbond roof?
Depends on the sheet profile. Lysaght rates Trimdek to 2° with end-lap sealing; most installers prefer 5°+ for peace of mind. Klip-Lok and Spandek can go down to 1°–2° because they're standing-seam profiles with sealed ribs. Below 1° you're into membrane roofing.
What's a normal pitch for a shed in WA?
10°–22.5° is the sweet spot for a gable shed. 10° looks modern, sheds water cleanly, and keeps the overall height under most council limits without needing a planning exception. Steeper (25°+) reads more traditional and gives you more headroom if you want a mezzanine. Skillions usually sit 5°–10°.
How do I check rafter length by hand?
Rafter length = √(half-span² + rise²). For a 9,000 mm wide gable at 10° pitch: half-span 4,500 mm, rise = 4,500 × tan(10°) ≈ 793 mm, so rafter ≈ √(4,500² + 793²) ≈ 4,569 mm. Add your eave overhang. The calculator does this and adds the top buffer and hang-below-slab adjustments automatically.
What pitch do I need for solar panels?
Solar panels work best at a pitch that matches your latitude. Perth's latitude is around 32°, so 30°–35° is optimal for year-round generation. Most flat-roof patios in WA (4°–6°) still work fine — output is about 90% of optimal at that angle. If you're committing to solar on a new shed, mention it before we quote — we can angle the roof to suit.
Does the calculator account for sheet overlap?
Yes — it uses cover width (the effective width after side laps). For Trimdek that's 762 mm. No manual lap maths needed.
Can I use it for a skillion roof?
Yes — pick the Skillion tab in the calculator. The maths is simpler (single slope, no apex), and you'll get the rafter length and sheet count for that single plane.
Need a real shed or patio?
Use this calculator for as many jobs as you like. If you'd rather we handle the whole thing — slab, frame, sheets, doors, the lot — we do custom sheds and patios across WA. 25-year warranty on every build.