A carport prevents frost on a windscreen by putting a solid surface between the glass and the open sky. The mechanism is worth understanding, because it is also what decides which roof materials work and which only help a little.
Why this works, briefly
On a clear, still Highveld night the sky is effectively very cold, and any surface with a clear view of it loses heat by radiation. A windscreen, angled upward and thin, loses heat faster than the air and drops below dew point - so moisture condenses and freezes on it, even when the air temperature has not reached zero. Put a roof between the glass and the sky and that radiative path is blocked. The car still gets cold; the windscreen no longer gets colder than the air.
This is the same mechanism, running in the opposite direction, that makes a carport worth having in summer - see sun, heat and UV protection.
Roof types against frost
| Roof | Effect on windscreen frost |
|---|---|
| IBR or corrugated steel sheet | Blocks the radiative path - the effective option |
| Aluminium sheet | Same as steel; the sheet is what matters, not the frame |
| Polycarbonate | Reduces it; transmits some long-wave loss |
| Shade net / shade cloth | Partial - the knit is porous, so some sky is still in view |
| Open sky | Full frost on clear, still nights |
Depth matters as much as the material. A 3 m-deep roof leaves a windscreen partly exposed to the sky at an angle; the standard 6 m depth does not.
What it saves you
Not scraping a windscreen is the obvious part. The less obvious part is what repeated frost and hard morning sun do to wiper rubber, dashboard plastic and roof clearcoat over a few Pretoria winters - the same UV and thermal cycling that makes covered parking worth costing properly. Bands for the standard builds are on carport prices in Pretoria, and if the roof is going to be solid steel, steel and IBR carports covers the sheet options.
Choose the roof that does it
A solid sheet is what blocks the radiative path - see steel and IBR carports.
Before you build
The rest are on carport questions answered for Pretoria - including closing a side against low winter sun: How to Cover Carport Sides.
Related detail
Does it help with morning dew as well?
Yes, and by the same mechanism. Dew forms when the surface drops below dew point through radiative loss to the sky; blocking that path keeps the glass closer to air temperature, so it stays clear on more mornings.
Does a carport stop the inside of the car freezing?
No, it reduces it rather than stopping it. The cabin still cools to roughly ambient overnight. What changes is the windscreen, which no longer drops below ambient and ices over.
A closed flank keeps frost off the side glass as well as the windscreen — the options are under how to cover carport sides.
KB Carports builds carports in Pretoria across Tshwane - free on-site measure, and a written quote that states the steel, the footing method and the clearance height.