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.

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.

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.

Materials and weight, part by part

For the 6000 × 3500 × 2400 example the calculator gives this bill of materials — which is also your shopping list:

• 60×60×3 tube — 56.3 m: six posts of 3300 mm (2400 above ground plus 900 in concrete), two top beams of 6000 mm and seven rafters of 3500 mm. One metre weighs 5.37 kg.
• 40×20×2 tube — 36 m: six battens of 6000 mm, 1.76 kg per metre.
• 0.7 mm profiled sheet — 21 m² of roofing, about 132 kg.

The frame comes to about 366 kg, and close to half a tonne with the roof. More useful than the total is the weight of each piece: a 6 m beam is about 32 kg, a rafter 19 kg, a post 18 kg, a batten 11 kg. Nobody lifts a beam to 2.4 m alone and holds it level while it is being tacked — plan a second person for that stage. The 32 joints and 6.2 m of weld take about 0.23 kg of wire or 0.34 kg of electrodes, so consumables are not a budget line here. Lay the cuts out on your supplier's stock lengths (often 6 m) in the cutting optimiser: a 3300 post and a 3500 rafter do not fit in one bar together, and it is better to see the offcuts before you buy.

Tools you need

• A welder that holds the current for a 3 mm wall: up to 140 A on MIG or up to 120 A with a 3.2 rod. There are few joints, but you weld them back to back — check the machine's duty cycle at that current in the duty cycle calculator.
• An angle grinder with cutting and flap discs: cutting, deburring and cleaning the weld area to bright metal.
• Setting out: tape, spirit level, string line, magnetic squares, 4–6 clamps.
• For the posts: an auger or spade, temporary braces from offcuts, concrete.
• Protection: helmet, gauntlets, covered clothing. Much of the welding is above your head, and spatter falls straight into your collar.

Setting out and tacking

A carport starts with a string line, not a welder. Stretch one line along the front posts, measure the projection and stretch a second. Check the rectangle by its diagonals: in a square layout they are equal. Dig the holes to the lines, plumb the posts in both directions and brace them before the concrete goes in.

Bring the post tops to one level — with a laser or a water level — and cut them down after the concrete has set: that is more accurate than trying to set the posts at one height in the holes. The pitch comes from the back posts being taller than the front ones (about 600 mm over a 3.5 m projection at 10°).

Tack each joint on two opposite faces, short and flat, so the final weld covers the tacks. Tack everything first: beams to posts, rafters to beams. Then measure — the frame diagonals and the plumb of the posts. The rule from the square-tube page: a difference in diagonals of more than 2 mm per metre means the frame is already out of square. A tack can still be cut at this point; a full weld can only be cut out.

Step-by-step order with settings

1. Posts in concrete. No welding. Do not load the frame onto fresh concrete: wait for it to gain strength — that takes days, and concrete reaches its full design strength at 28 days.
2. Beams onto posts. The 60×60×3 beam sits on the post top, welded all round. Settings for a 3 mm wall: 0.8–1.0 mm wire, 120–140 A, 19–21 V, or a 3.2 mm rod at 90–120 A. This joint is welded from below and from the side — overhead technique is covered in overhead welding.
3. Rafters onto beams every metre. The rafter sits on top, so you weld it from a ladder in the flat position, same settings as the beams.
4. Battens onto rafters every 800 mm. The batten wall is 2 mm, so the settings go down to suit the thinner wall: 0.8 mm wire, 90–110 A, 17–19 V, or a 2.5 mm rod at 70–90 A, with the arc aimed at the heavier rafter.
5. Clean, prime, paint — before the roof goes on: afterwards you cannot reach the top of the battens.
6. Roofing. Profiled sheet is fixed with roofing screws through the low corrugation into the battens.

How to keep the frame from pulling

Steel pulls towards the weld as it cools. On a carport you see it at once: a beam welded on the same side of every post in a row bows, and the posts lean out of plumb. Three things help:

• Skip welding. Run the full welds not in sequence but alternately and across the diagonal: front left, back right, front right, back left.
• Faces in turn. Weld a tube joint like a picture frame: one face, the opposite face, then the other two — not all the way round in one pass.
• No water. Do not quench a joint or force it to cool: fast shrinkage pulls harder.

Painting and rust protection

A carport lives in the rain, and it rusts at the welds first. The order: knock off scale and spatter, dress the welds with a flap disc, degrease with solvent, apply an anti-corrosion primer, then two coats of enamel. Brush the welds and edges separately before the full coat (a stripe coat) — paint is thinnest on edges and in corners. Cap or weld shut the open tube ends: water inside a post rusts it from within and freezes in winter. If the frame is going for hot-dip galvanising, the opposite applies: closed sections need vent and drain holes to EN ISO 14713-2, agreed with the galvaniser before welding.

How long it takes

The welding itself is a small part of the job: 32 joints and 6.2 m of weld. The time goes elsewhere:

• Setting out, holes, posts in concrete — day one, then a break while the concrete gains strength.
• Cutting and deburring 21 pieces — as long as the welding or longer.
• Beams, rafters, battens — a day for two people, if the pieces are cut to the list beforehand.
• Primer and two coats — with drying time as per the product sheet, then the roof.

If you would rather work from a proven design than from scratch, typical structures are reviewed on the steel canopies page.

What usually spoils the job

Beginner mistakes on a carport

• All posts the same length. Six posts cut to one size and the pitch left for "later". It ends with rafters being mitred on site. Put the taller back posts into the cutting list from the start.
• Battens in 60×60. Over a hundred kilograms of extra steel that also has to be lifted. The 40×20×2 batten is the second section in the calculator.
• Full welds before measuring. Tack everything — measure — correct — only then weld out.
• MIG in the wind. The gas blows away and the weld is porous. Outdoors you need a screen on the windward side, or stick electrodes.
• Paint over mill scale. Enamel on uncleaned steel peels within a season, and the welds rust first.

Author: Dmitry Frolov, welder and metal fabricator Updated: 28 September 2026