Kumar IronUpdated
In this guide
Screws or J hooks
J hooks (and U hooks) loop around the purlin and pass through the sheet, usually with a nut and washer on top. They suit angle or channel purlins and are common on corrugated GI and fibre cement sheets. Self-drilling screws drill their own hole into a steel purlin and are now the standard for colour coated and trapezoidal metal roofs.
- Screws: faster to fix, tighter grip, suit RHS, pipe and channel purlins. Need the right drill point for the purlin thickness.
- J hooks: need a hole drilled in the sheet, and the hook goes under the purlin. Fine for angle purlins but can loosen over time with thermal movement.
- Fibre cement sheets use J hooks or special bolts, not metal-sheet screws.
Choosing the screw
For metal sheets on steel purlins, the usual choice is a 5.5 mm self-drilling screw with a hex head and a bonded EPDM washer. Check the label for the drill capacity: thin light-gauge purlins need a shorter drill point, thicker purlins need a longer one, or the screw will jam and snap.
Length depends on what it passes through. It must go through the sheet crest, the purlin, and still leave several threads showing past the far side. For a single sheet on a steel purlin, lengths of 50 to 75 mm are common, but confirm against your purlin thickness. Sandwich panels need much longer screws.
| Purlin | Better fastener | Note |
|---|---|---|
| Light gauge C or Z purlin | Self-drilling screw, short drill point | Do not overtighten |
| RHS or pipe purlin | Self-drilling screw or J hook | Screw needs drill capacity to match wall thickness |
| Angle or channel | J hook or screw | J hook is traditional for corrugated sheets |
| Wood purlin | Wood-thread roofing screw | Not self-drilling metal type |
Crest or valley
On corrugated sheets, fix through the crest (the high part) wherever the profile allows. Water runs in the valley, so a hole there is a hole in the drain. Crest fixing keeps the washer above the water line and is the standard advice from most sheet makers.
Some trapezoidal profiles are designed for fixing in the valley or flat pan because the crest is narrow, especially where the screw has to pull the sheet down tight against the purlin. Follow the profile maker's drawing. Using the right screw with a good EPDM washer makes either position workable.
How many fasteners per sheet
Rule of thumb only: fix at every purlin, with a fastener in about every second or third crest along the purlin line, and closer at the sheet ends, edges, ridge and eaves where wind uplift is highest. For a typical 3 ft wide corrugated sheet this often works out to 3 to 4 fasteners per purlin line, but wind exposure, height of the building and the sheet maker's table should decide.
Open sheds in the Delhi NCR and west UP belt face strong pre-monsoon winds. Add extra fasteners on the edge zones and around the ridge. Under-fixing is the most common reason sheets lift in a storm.
Sealing and washers
- Use EPDM washers on screws. They stay flexible longer in sun than cheap rubber.
- Tighten until the washer is just compressed, not squeezed out. Overtight washers crack, loose ones leak.
- Drive screws straight. A tilted screw leaves a gap on one side.
- Use a neutral cure sealant at end laps and around ridge caps if the maker recommends it.
- Replace any fastener whose washer has hardened or cracked.
Galvanic corrosion and thermal movement
Mismatched metals corrode where they touch in the presence of water. Plain steel screws on a coated sheet rust within a season or two and leave brown streaks. Use coated or galvanised screws that match the sheet, and do not mix copper, aluminium and plain steel contacts on the same roof without isolation.
Metal sheets expand in heat and contract at night. On long sheets this movement can work screws loose or elongate holes. Do not overtighten fasteners in a way that locks the sheet rigidly, and for very long runs ask the maker about maximum recommended length or expansion details.
Frequently asked questions
- What size screw is used for GI and colour coated roofing sheets?
- A 5.5 mm self-drilling screw with an EPDM washer is the usual choice. Length depends on purlin type and sheet profile, so tell us your purlin and we will suggest the right one.
- Should screws go in the crest or valley?
- In the crest for most corrugated sheets, so water does not sit on the washer. Some trapezoidal profiles are made for valley fixing, so follow the profile drawing.
- How many screws per sheet do I need?
- As a rule of thumb, fix at every purlin and in every second or third crest, with extra at the edges, ridge and eaves. Wind exposure and the sheet maker's table should decide the final number.
- Why do roofing screws rust early?
- Often because plain steel screws were used on a coated sheet, or the washer failed and water got in. Use matching coated screws with EPDM washers.