Kumar IronUpdated
In this guide
What purlins do
Purlins are the horizontal members, such as rectangular hollow sections or channels, that run across the trusses and carry the sheets. Closer purlins mean a stiffer roof but more steel. Wider spacing saves steel but can make thin sheets sag, flutter and dent when someone walks on them.
Typical purlin spacing by sheet
Manufacturers publish maximum spans for each profile and thickness, and the numbers vary a lot between profiles. Deep trapezoidal profiles span further than shallow corrugated ones. The table is a rough range seen in practice, not a design value.
| Sheet type | Typical thickness | Common spacing range |
|---|---|---|
| Corrugated GI or GC | 0.35 to 0.50 mm | About 0.9 to 1.2 m |
| Colour coated corrugated | 0.40 to 0.50 mm | About 1.0 to 1.4 m |
| Trapezoidal or deep profile | 0.45 to 0.60 mm | About 1.2 to 1.8 m |
| Fibre cement sheet | Per maker | Per maker, usually closer than metal |
Thinner sheet means closer purlins. If you want wider purlin spacing to save steel, increase sheet thickness or choose a deeper profile instead of guessing. The saving in purlins can be cancelled by a heavier sheet bill, so compare both.
Minimum roof slope
Sheet laps are only watertight when water runs off quickly. As a common working figure, corrugated and trapezoidal sheets are laid at about 10 degrees (roughly 1 in 6) or steeper. Some long-run trapezoidal profiles with sealed laps are allowed at lower slopes, and some makers ask for more in heavy rain or dusty, flat areas.
A flat looking shed roof is a leak waiting to happen. If your design needs a lower slope, ask the sheet maker for their minimum in writing and use sealant at laps. For heat, a steeper roof with a ridge vent helps hot air escape.
Overlaps
- End lap (sheet over sheet along the slope): commonly about 150 mm, more for flatter slopes. Always lap over a purlin and fix there.
- Side lap (sheets side by side): one to two corrugations is typical, with the larger lap used on lower slopes or exposed sites. Many dealers quote one and a half corrugations as a safe average.
- Lay sheets so the side lap faces away from the prevailing wind direction.
- Use sealant tape at laps on flatter roofs when the maker recommends it.
Overlaps reduce the usable cover width of a sheet. Our roofing sheet calculator allows for this, so enter the correct cover width for the profile you buy.
Overhang
At the eaves, let the sheet project beyond the last purlin so water falls clear of the wall or gutter. Roughly 150 to 300 mm is common on houses and 300 mm or more is useful on cattle sheds to keep rain off animals and walls. Do not over-project thin sheets, because the end can flex in wind. At gable ends, a small overhang with a verge flashing keeps rain out.
Putting it together
- Choose sheet profile and thickness first.
- Read that sheet's maximum purlin spacing from the data sheet.
- Set slope at 10 degrees or more where possible.
- Lay out purlins so every end lap lands on one.
- Get the frame checked by an engineer for wind and span.
Share your span, length and slope and we will help you work out the sheet count and steel weights using our calculators and weight charts.
Frequently asked questions
- What is the right purlin spacing for 0.5 mm sheets?
- For typical corrugated 0.5 mm sheets, roughly 1.0 to 1.4 m is common, and deeper profiles can go further. Treat it as guidance and follow the maker's span table.
- What is the minimum slope for a tin roof?
- About 10 degrees (about 1 in 6) is a common working minimum for corrugated metal sheets. Some profiles with sealed laps allow lower. Confirm with the manufacturer.
- How much should sheets overlap?
- End laps are commonly about 150 mm and side laps one to two corrugations, increasing as the slope gets flatter.
- Can I fix sheets without a purlin under the lap?
- No. Both end laps and sheet ends should sit over a purlin so they can be fixed down properly.