Any screw that penetrates a roof panel, siding, or an outdoor enclosure needs a rubber seal under its head — otherwise every fastener is a future leak. This guide explains the two ways to seal a screw — bonded sealing washers and plain rubber washers — which material survives outdoors, and how to size them.
Bonded sealing washers vs plain rubber washers
A bonded sealing washer is a steel washer with an EPDM rubber layer vulcanized to its underside. The metal spreads the clamping load and stops over-compression; the rubber seals against the panel. They're the standard for metal roofing screws and are usually pre-assembled on the fastener.
A plain rubber washer under a screw head (or paired with a metal flat washer on top) does the same sealing job and gives you full control over dimensions and material — useful when you're sealing an odd bolt size, restoring old hardware, or the bonded size you need doesn't exist. Stack order: screw head → metal washer → rubber washer → panel. The metal washer prevents the screw head from tearing the rubber as it turns.
| Bonded washer | Plain rubber + metal washer | |
|---|---|---|
| Best for | Production roofing/siding work | Odd sizes, repairs, restorations, non-standard bolts |
| Size range | Limited to stocked fastener sizes | Any OD/ID/thickness combination |
| Over-compression protection | Built in | Torque carefully or add metal washer |
Why EPDM is the only material to use on a roof
Roof fasteners bake in UV and swing from freezing to 150°F+ panel temperatures. EPDM is the material bonded roofing washers use for a reason: best-in-class UV and ozone resistance and a −40°F to 257°F service range. Neoprene is acceptable for shaded or vertical applications; nitrile and natural rubber crack within a couple of seasons in direct sun. Full material breakdown in our comparison guide.
Sizing a rubber washer for a screw
Match the ID to the screw shank so the washer centers itself — #10 screws take roughly 3/16" ID, #12 take about 7/32"–1/4", and 1/4" lag screws take 1/4" ID. Choose an OD at least double the screw head diameter so rubber extends past the head, and 1/8" thickness for most panel work (3/32" for light-gauge, 3/16" where the surface is ribbed or uneven). Measure precisely with our measuring guide and find the size on the size chart — Rubber Washer Warehouse stocks every combination in EPDM and six other materials. Outfitting equipment or furniture too? Our sister store Rubber Feet Warehouse carries rubber feet, bumpers and pads to match.
FAQ
Do roofing screws need rubber washers?
Yes — any exposed fastener through a roof panel must seal its own penetration. Use screws with bonded EPDM washers, or add an EPDM rubber washer under a metal washer on plain screws.
How tight should screws with rubber washers be?
Drive until the rubber compresses slightly and squeezes just to the washer's edge, then stop. Visible bulging past the metal means over-driven — the rubber will split and leak.
Can I replace just the rubber on a bonded washer?
No, the rubber is vulcanized to the steel. Replace the whole washer, or switch to the plain metal washer + rubber washer stack, which lets you renew the rubber alone in the future.
What thickness rubber washer for metal roofing?
1/8" is the standard for corrugated and standing-seam panel fastening; go 3/16" on ribbed profiles where the washer bridges an uneven surface.