Steel carport: frame calculator
A mono-pitch canopy in hollow section. Give length, projection and height, and the calculator works out the number of posts, rafters and battens, the mass of the frame, the covered area and the welding current.
A worked example: 6000 × 3500 × 2400
A canopy for one car: 6 m long, 3.5 m of projection, a 2.4 m post, posts of 60×60×3, battens of 40×20×2 and 0.7 mm profiled sheet.
| Item | Section | Length, mm | Qty |
|---|---|---|---|
| Post | 60x60x3 | 3300 | 6 |
| Longitudinal beam | 60x60x3 | 6000 | 2 |
| Rafter | 60x60x3 | 3500 | 7 |
| Batten | 40x20x2 | 6000 | 6 |
Mass is worked out without the corner radii of the section, so it comes out two to three per cent below the standard tables. No allowance is added for saw kerf or cutting errors: when buying, add one commercial length.
Pitch is the one number you cannot fudge
A mono-pitch canopy stands on a single figure: the pitch. Everything else can be approximated; the pitch cannot.
- Profiled sheet — from 8–10°. Below that, water climbs the corrugation by capillary action and gets under the lap, and in winter snow stays on a roof that was never meant to hold it.
- Polycarbonate — from 5–8°, and always with the flutes running down the slope: across the slope they become gutters full of condensation.
Over a 3.5 m projection, 10° is about 600 mm of height difference between the front and back rows of posts. It is easier to build that in with posts of different length than by cutting the rafters: a mitre on 60×60 tube needs a jig, while another 600 mm on four posts is simply a different row in the cutting list.
Pitch is not in the model: the rafter is taken from the projection with no correction for slope. At 10° that understates the length by one and a half per cent — less than the saw allowance. Snow load is not calculated at all: in snowy regions the rafter spacing comes down to 700–800 mm, and that decision belongs to the local code, not to a calculator.
Posts and embedment
- Spacing no more than 3 m; the calculator sets them in pairs every three metres and adds a pair at the end.
- Embedment of 0.9–1.2 m in concrete: here it is not only frost that can lift a post, but wind.
- The top tie is not optional — it turns separate posts into a portal frame.
Welding settings
A 3 mm wall welds more confidently than the 2 mm of a gate, but there are three times as many joints here and all of them overhead. The values are the same as on the welding square tube page; the calculator above shows the row for the wall thickness you chose.
| Wall, mm | Section | MIG/MAG | MMA |
|---|---|---|---|
| 1.2 | 20×20, 20×40 | 0.8 mm · 50–70 A · 16–17 V | 2.0 mm · 50–65 A, stitch welding |
| 1.5 | 40×20, 40×40 | 0.8 mm · 70–90 A · 17–18 V | 2.0 mm · 55–70 A, stitch welding |
| 2.0 | 40×40, 60×40 | 0.8 mm · 90–110 A · 17–19 V | 2.5 mm · 70–90 A |
| 3.0 | 60×40, 80×40 | 0.8–1.0 mm · 120–140 A · 19–21 V | 3.2 mm · 90–120 A |
| 4.0 | 80×40, 100×50 | 1.0 mm · 140–170 A · 19–21 V | 3.2 mm · 110–140 A |
What usually spoils the job
| What went wrong | Cause | How to avoid it |
|---|---|---|
| The frame twisted | Welds run one after another on the same side | Alternate corners, weld in short runs |
| The diagonals came out unequal | Nobody checked them before tacking | Measure both diagonals twice |
| Burnt through the wall | Too much current for 1.2–1.5 mm | Turn it down, stitch it with pauses |
| The corner welded on one side only | Saving time | Weld both sides wherever the joint takes load from two directions |
| Rust from the inside within a year | Open ends | Caps, or weld them shut |
Frequently asked questions
What pitch should a mono-pitch canopy have?
With profiled sheet, not less than 8–10°; with polycarbonate, 5–8°. Below that water sits in the corrugation and gets under the lap, and snow does not slide. Over a 3.5 m projection, 10° is about 600 mm of height difference between the front and back rows.
What section for posts and rafters?
Posts of 60×60×3 for projections up to 4 m and heights to 2.5 m; above that, 80×80×3. Rafters of 60×40×2 or 60×60×3, battens of 40×20×2. The common mistake is making the battens from the same tube as the posts: over a hundred kilograms of steel for nothing.
How much does such a canopy weigh?
A 6 × 3.5 m frame in 60×60×3 with 40×20×2 battens is around 370 kg, and 0.7 mm sheet adds another 130. Half a tonne has to be lifted somehow: this is not a one-person build.
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