Roofing 5 min read
Roofing sheet thickness: what TCT actually means
A 0.45 mm sheet quoted as 0.50 mm is the most common overcharge in roofing. It takes a screw gauge and ten seconds to catch.
Short answer
TCT means total coated thickness — the steel plus its zinc and paint coating, which is the figure worth comparing. For a house roof or veranda use 0.35 to 0.45 mm TCT; for sheds, workshops and godowns use 0.45 to 0.50 mm. On plain galvanised corrugated sheet, 26 gauge is about 0.50 mm, 24 is 0.63 mm, 22 is 0.75 mm and 20 is 0.90 mm.
TCT, TPT and why quotes differ
TCT is total coated thickness: the base steel plus zinc plus paint. TPT is the paint thickness alone. Some sellers quote the base metal thickness, some quote TCT, and the difference is enough to make two quotes look far apart when they are selling the same sheet — or look identical when they are not. Ask which figure is being quoted and compare like with like.
How to check it yourself
Carry a screw gauge, or a vernier. Measure the flat of the sheet, not the rib. If the sheet is meaningfully under what was quoted, you are paying for steel you did not get — and on a large roof that adds up quickly.
Gauge on galvanised corrugated sheet
Gauge runs backwards: the higher the number, the thinner the sheet. 26 gauge is roughly 0.50 mm and suits light agricultural cover. 24 gauge (0.63 mm) is the general farm shed choice, 22 gauge (0.75 mm) suits godowns and workshops, and 20 gauge (0.90 mm) is heavy-duty industrial roofing.
Cut to length beats overlapping
Cross laps are where roofs leak. Colour coated profile sheet is cut to the length you give us, so a single sheet runs from ridge to eaves with no horizontal joint at all. Measure the rafter length plus your eaves overhang before ordering — a sheet cut short cannot be fixed.
Slope matters more than thickness
This belt sees around 3000 mm of rain a season. Do not go below about 1 in 5 — roughly 11 degrees — on profiled steel sheet, and keep cement sheet steeper still. A flatter roof pushes water back under the side laps in driven rain no matter how thick the sheet is.
