It is a common and frustrating sight: a roof covering still in sound condition, dotted with rust stains running down from every fixing. The sheet has outlasted the screws holding it on. There are a few reasons this happens, and most are avoidable at specification.

The fixing is the weak point by design

A roofing sheet is protected across its whole face by its coating. A fastener is a small component that has been formed, threaded and driven, and it is doing a harder job in a more exposed position. Every fixing also creates a penetration through the coating you have just paid for.

Mismatched materials

Putting dissimilar metals in direct contact in the presence of moisture invites galvanic corrosion, where one metal corrodes preferentially. Fixings should be chosen to suit the sheet material and the environment, not simply because they were to hand.

Damaged coatings on the fastener

Driving a screw with a worn or wrong-sized bit chews the head and strips its protective finish. So does over-driving. Once bare steel is exposed at the head, corrosion starts there and stains the sheet below. Using the correct driver is a small detail with a long tail; we supply magnetic bit Tek drivers complimentary on request with all orders for this reason.

Washers

The sealing washer does two jobs: it keeps water out and it shields the head. Overtightening splays it, undertightening leaves a gap, and either way the fixing is exposed. Tighten until the washer is just compressed and no further.

Environment

Coastal sites, agricultural buildings housing livestock and anywhere with chemical exposure are considerably more aggressive than a suburban garage. The specification that is perfectly adequate inland may be marginal near the sea.

Specifying fixings that last

• Match the fastener to the sheet material and the exposure of the site.
• Choose the correct length. Our range runs from 25mm to 210mm so the fixing suits the build-up rather than the other way round.
• Fit square, and stop at the point the washer seals.
• Use the right driver, and replace worn bits.

If you are unsure what suits your building, ask us. We supply fixings alongside sheets so the two are specified together.