Kumar IronUpdated
In this guide
The two common screw diameters
Most roofing sheet jobs use a self-drilling screw in 5.5 mm or 6.3 mm diameter, with a hex head and a bonded washer. The 5.5 mm screw is the everyday size for light steel purlins. The 6.3 mm screw is a stronger screw, often chosen for thicker steel and heavier fixings.
Diameter is only half the story. The drill point and the purlin thickness decide whether the screw drills cleanly, and the manufacturer's data sheet gives the actual range.
| Diameter | Lengths seen in listings | Points to note |
|---|---|---|
| 5.5 mm | about 22 mm to 85 mm | Common on light gauge purlins. Some listings cap drilling at about 5 mm steel, so check your purlin. |
| 6.3 mm | about 25 mm to 80 mm | Used where a stronger screw is needed. Capacity varies by product, so read the label. |
Point types and length
The tip of a self-drilling screw is called the drill point. Many packs print a point number, such as #2, #3 or #4, which is a rough guide to how thick a steel it can drill through. Generally, a higher number drills a thicker steel. A screw that is too short for the stack will not hold, and one with the wrong point will jam or snap.
Length is set by the stack you are fixing: the sheet, the purlin, and a few threads showing past the far side. As a rough rule, add the purlin thickness and the sheet thickness together, then allow several threads more. Retail lengths of 50 to 75 mm are common for single sheets on steel purlins, but the final length should be confirmed with the purlin size before buying.
- Check the point number or drill capacity on the box against your purlin thickness.
- Do not use a screw that is too short, because the threads will not grip the purlin.
- Wood purlins need a different wood-thread roofing screw, not a self-drilling metal screw.
J hook and U hook sizes
J hooks and U hooks are an older way of fixing corrugated and fibre cement sheets. A J hook has one leg through the sheet and a hook that wraps the purlin flange. A U hook, or U bolt, goes round the purlin with both legs through the sheet and a nut and washer on top.
Indian listings for galvanised J hooks and U bolts commonly show 8 mm diameter, with lengths such as 75 mm and 150 mm. These are catalogue sizes, not an engineering rule. The right hook length depends on the purlin flange thickness and the sheet profile, so check the length with the supplier against your actual purlin before buying.
| Fixing | Where it is used | Typical size seen in listings |
|---|---|---|
| Self-drilling screw, 5.5 mm | Light steel purlins, C or Z sections, colour coated and GI metal sheets | 22 to 85 mm |
| Self-drilling screw, 6.3 mm | Thicker steel purlins and heavier fixing | 25 to 80 mm |
| J hook, galvanised | Angle or channel purlins, corrugated sheets | 8 mm dia, check length against flange |
| U bolt or U hook, galvanised | Round or flat purlins where the hook must go around the purlin | 8 mm dia, check length against purlin |
Washers: EPDM or steel
A washer does two jobs. The steel part spreads the load so the sheet is not torn, and the rubber part seals the hole so water does not run down the screw. For exposed roofs the rubber is the critical part.
EPDM is a synthetic rubber that vendors and builders describe as more resistant to sun and ozone than neoprene, and it stays flexible longer. Neoprene is common on cheaper washers and can harden and crack in sunlight, which opens a gap. The life figures quoted by sellers are not independently tested, so treat them as claims.
- Bonded EPDM washer: rubber bonded to a steel carrier, one piece. This is the usual choice for roof screws.
- Plain steel washer with a separate rubber gasket: cheaper, but check that the gasket is EPDM and that the steel is coated.
- For coastal or very humid areas, ask for a stainless carrier. Stainless grade 316 is the one usually recommended for marine exposure, while 18-8 suits general use.
- Replace any washer that has cracked, hardened or been squeezed out of shape.
Coating and corrosion basics
Screws and hooks rust for the same reasons sheets do: a broken or thin coating, a cut edge, or two metals touching in damp conditions. Use screws and hooks with a corrosion coating that matches the sheet, and avoid plain black steel fixings on a coated roof. Mixing metals is what causes the rust streaks that show up in a season or two.
To compare coatings, ask the supplier for a salt spray test report. Salt spray hours depend on the test standard used, so only compare like with like. Do not accept a vague claim without a test report.
What to ask when buying
- Diameter, length and drill point number, and the purlin thickness they are meant for.
- Washer type: EPDM bonded or steel with gasket, and the carrier grade.
- Coating type and whether a test report is available.
- Hook size and length, with the purlin flange thickness you are fixing to.
For how many screws per sheet, sealing at the overlaps and why screws rust early, read the roofing sheet screws, washers and fixing guide. Tell us your sheet profile and purlin size, and we will suggest the screw and washer to ask for.
Frequently asked questions
- What size screw is used on roofing sheets?
- 5.5 mm or 6.3 mm self-drilling screw with an EPDM bonded washer is common. The length depends on the purlin and sheet.
- What is the difference between a J hook and a U hook?
- A J hook hooks under the purlin flange through one leg. A U bolt wraps the purlin with both legs through the sheet.
- Is EPDM better than neoprene for roof washers?
- Vendors and builders generally say EPDM lasts longer in sun. Ask for the rubber grade and check the coating.
- Which screw grade is best near the sea?
- Stainless 316 carriers are usually recommended for coastal exposure. Confirm with the supplier.