Short answer. On a roof the screw goes into the bottom wave, the one pressed against the batten, and never into the top. Screw length depends on the wave height of the profile: 35 mm is enough for the bottom wave on any of our profiles, while the top wave needs from 45 mm on T10 to 90 mm on T57. Consumption is typically 6-8 pieces per m².
Screw length for each profile
The arithmetic is simple: the screw must enter the timber by at least 30 mm. Fixing into the bottom wave, the sheet rests on the batten, so the wave height is not added. Fixing into the top wave, it is added in full.
| Profile | Wave height | Bottom wave | Top wave |
|---|---|---|---|
| T10 | 10 mm | 35 mm | 45 mm |
| T20f | 20 mm | 35 mm | 55 mm |
| T20r | 20 mm | 35 mm | 55 mm |
| T35 | 35 mm | 35 mm | 70 mm |
| T45 | 45 mm | 35 mm | 80 mm |
| T57 | 57 mm | 35 mm | 90 mm |
This is why there is no universal screw. Buying 35 mm and fixing T57 through the top wave with it will not work: only 8 mm reaches the timber, and the first gale pulls the fixing out along with the sheet.
Why roofs take the bottom wave only
The top wave is the highest point of the profile and no water runs along it. Fixing there looks logical — the hole sits above the waterline. The real problem is not water but clamping.
In the bottom wave the sheet rests on the batten and the washer presses the steel into the timber, giving a rigid joint. In the top wave there is a void beneath: the screw pulls the steel down, the wave sags, the washer tilts and stops sealing. A year or two later a pinhole leak appears there. On a wall the effect does not arise — no water, no snow load — so either option is acceptable.
Consumption on an example
| Item | Value |
|---|---|
| Roof area | 60 m² |
| Typical consumption | 6-8 pieces per m² |
| Screws in total | 360-480 pieces |
| Length for T35, bottom wave | 4.8 × 35 mm |
| For side laps | 4.8 × 19 mm, into the top wave |
The lower figure of 6 per m² covers the field of the sheet. The upper figure of 8 allows for eaves, ridge and verges, where fixings sit closer together. The exact number depends on batten spacing: the denser the battens, the more rows of fixings.
Three mistakes you can see at a glance
| Mistake | Consequence |
|---|---|
| Overtightened screw | the washer is crushed and the rubber squeezed out — no seal from day one |
| Undertightened screw | the washer is not pressed home, water and air move under it; visible as a ring of daylight |
| Screw driven at an angle | the hole is oval and the washer sits on one side. Usually from drilling while unsupported |
A correctly tightened screw compresses the rubber washer by about a third and does not deform the steel around it. You can see it — an even circle with no ripple.
How to order
Step 1. Decide the profile and the fixing method
Roof — bottom wave. Wall or fence — your choice. That sets the screw length from the table above.
Step 2. Work out the area
Multiply by 6-8 and add 10%: some screws are always lost to misdriven fixings.
Step 3. Order them with the sheet
We stock roofing screws with EPDM washers, the head painted to match the sheet. Call (067) 077-56-77, Mon-Fri 08:00-17:00.
What we do not do
We do not install and we do not travel for on-site measuring. We supply the sheet, colour-matched screws and the profile geometry the fixing length is calculated from.
FAQ
Can rivets be used? On side laps and flashings, yes. Sheets are not riveted to battens: there is no clamping and no seal.
How many waves apart? At the sheet edges, into every bottom wave; in the field, every other one. Closer at the ridge and eaves.
Do I need to pre-drill? No, a roofing screw has a drill point. Pre-drilling is only needed on steel from 0.7 mm.
Why do screws rust before the sheet? Usually it is not the screw rusting but a grinder cut nearby, or the head coating damaged by overtightening.
How long is the wait? Production 1-2 working days, delivery a further 1-5 days.