A steel carport is the second most common DIY structure after gates: four posts, a couple of trusses, polycarbonate or corrugated sheet on top. It looks simple, and in calm weather it genuinely holds. The question arrives in February, when wet snow settles on the roof — and that is when it becomes clear whether anyone sized the structure.
Start with snow, not with dimensions
A canopy carries snow, not its own weight. Across most of Ukraine the design snow load is roughly 130–150 kg per square metre of horizontal projection. On a 3×6 m carport that is nearly two and a half tonnes over the whole roof. This is why a structure welded by eye from thin tube stands for years — and then folds overnight after one wet snowfall.
The second factor is wind. A roof with no walls works like a wing: it generates lift, so the roofing fixings and the post anchors have to work upwards as well as downwards.
Sections for a typical car canopy
- Posts — square tube 80×80×3 for a canopy up to 6×3 m; 100×100×3–4 for longer spans or a single-slope roof with a cantilever.
- Trusses or beams — 60×40×2 chords with 30×20×2 webs; a solid beam instead of a truss only works up to about 4 m.
- Purlins — 40×20×2 at 60–80 cm centres under polycarbonate, up to 1 m under corrugated sheet.
- Bracing — 40×20 diagonals in at least one plane, or the frame sways in wind.
These are starting points for an ordinary domestic canopy. Spans over 6 m, canopies over a garage entrance and anything people or vehicles stand under need an engineer to size the sections: there the cost of an error is collapse, not deflection.
Pitch: why 5° is not enough
The minimum pitch for a single-slope canopy is 15–20°, not just enough for water to run off. A shallow roof does not shed snow: it accumulates, melts underneath and refreezes into ice that weighs three times what fresh snow does. A double-slope roof can go lower, 12–15°, because snow sheds both ways.
Pitch is set either by different post heights or by a truss of varying depth. The second costs more but keeps every post identical and looks tidier.
Foundations and posts
A post cannot simply be concreted into a half-metre hole — winter ground heave will lift it, concrete and all. The correct approach is a bore below the frost line (0.8–1.2 m in most regions), reinforcement, concrete, and a head with an embedded plate above ground level.
The other common option is a steel base plate with anchors, where the canopy sits on an existing concrete slab. Slab thickness decides here: anchors in a 10 cm path will not resist wind uplift.
Polycarbonate or corrugated sheet
Polycarbonate is lighter, lets light through and bends into an arch. It also moves a lot with temperature: fixing holes are drilled 2–3 mm larger than the screw, otherwise the sheet cracks in its first summer. Fix through thermal washers, run the flutes vertically and close the ends with profile, or condensation and dirt collect inside.
Corrugated sheet is cheaper, stiffer and better under snow, but casts full shade and is noisy in rain. Over a car that rarely matters; over a seating area it does.
Common mistakes
- 60×60×2 posts on a 6×3 canopy — good until the first wet snow;
- a 5° pitch — snow stays put and turns to ice;
- no diagonal bracing at all — the frame sways in wind;
- concrete above the frost line — posts get pushed out in the first winter;
- fixing holes in polycarbonate drilled to size — the sheet cracks as it expands;
- welds painted without primer — rust appears exactly along the seams.
What to build and what to order
Concrete work, roofing and painting are well within reach at home. The trusses are the part where accuracy decides everything: chords and webs cut with any error give a crooked truss, and it shows from twenty metres away. Tube cut to length and angle produces identical members that meet without fitting.
We fabricate steel structures — canopies, awnings, trusses and posts — sized for your span and snow load, with powder coating and installation on site. Send the dimensions of the area through the form and we will propose a structure.
