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On an exposed-fastener metal roof the screws are the weakest link: a forty-year panel fitted with eight-year washers is an eight-year roof. The bonded EPDM washer is the actual seal, so cheap PVC washers that harden and crack become the leak source long before the panels fail. This guide explains what separates a good metal roofing screw from a bad one, why coating choice prevents rust streaking and galvanic corrosion, which length suits wood purlins, plywood decking, shingle recovers and steel framing, how many screws a roof needs per hundred square feet, correct drive depth, and the common mistakes that cause leaks.
On an exposed-fastener metal roof the screws are the weak point of the whole system — every one is a hole through the panel sealed only by a rubber washer. Which screw you use, and how it is driven, decides how long the roof stays watertight.
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What to look for
EPDM washers rather than cheaper rubber. EPDM holds up to UV for decades; ordinary rubber perishes in a few years.
A bonded washer, moulded to the head, so it cannot be fitted the wrong way round or lost.
Long-life coating rather than plain zinc. The coating is what stops the head rusting and streaking down the panel.
Stainless-capped heads in coastal air, where anything else corrodes.
Correct length: long enough to pass the panel and bite the purlin or deck fully, no longer.
A colour match to the panel, which is cosmetic but is what makes a roof look finished.
What is not worth paying for
Wood screws or deck screws pressed into service. They have no washer and no coating for the job.
Screws with an unspecified washer material. If the listing does not say EPDM, assume it is not.
Buying exactly the count you calculated. Screws get dropped and stripped; buy 10% over.
Common mistakes
Overtightening. The washer should compress slightly, not squash out — a crushed washer is the most common leak on this kind of roof.
Undertightening, which leaves a gap the washer never seals.
Driving at an angle, so the washer only contacts on one side.
Screwing through the rib instead of the flat next to it, where the panel is supported.
Mixing metals — plain steel fasteners in aluminium panels corrode the panel around every hole.
Frequently asked questions
What screws are used for metal roofing?
Self-drilling screws with a bonded EPDM washer and a long-life coating, sized to pass the panel and bite the purlin or deck fully. In coastal air, stainless-capped heads.
How tight should metal roofing screws be?
The washer should compress slightly and no more. Squashed-out washers are the most common leak on an exposed-fastener roof, and undertightened ones never seal at all.
How many screws does a metal roof need?
Roughly 80 per square for exposed-fastener panels. Buy about 10% over — screws get dropped and stripped.
Do metal roofing screws need replacing?
On an exposed-fastener roof they are a maintenance item. The washers should be checked every few years, and most such roofs need refastening once in their life.
Not sure how many panels you're fixing down? The metal roof calculator counts panels, screws and trim.