EPDM sealing washers are used with roofing and cladding screws to seal the fixing point against water ingress. In practice, the material itself is only part of the answer. Compression, screw angle, washer support, sheet geometry and installation quality all affect whether the seal works as intended.
The washer should be compressed enough to form continuous contact around the fixing, but not crushed. The correct condition is controlled seating, not maximum tightening.
How the washer forms the seal
A typical roofing fastener uses a metallic backing washer with an EPDM element bonded beneath it. As the screw is tightened, the metal washer distributes the clamping force and the EPDM conforms to the sheet surface around the penetration.
A good seal depends on reasonably uniform contact around the fixing. Gaps, tilting, excessive extrusion or damage to the elastomer can interrupt that contact.
Compression is the main installation check
Too little compression can leave part of the sealing face open, especially on profiled or uneven sheet. Too much compression can deform the backing washer, damage the sheet coating or squeeze the EPDM out from under the washer.
On site, the target is a washer that is fully seated with visible, even compression and without severe bulging or distortion.
The backing washer is not just a carrier
The metal washer provides a stable bearing surface under the screw head and spreads the installation load. Its diameter, thickness and stiffness influence both sealing and local load transfer into the sheet.
If the washer dishes or bends excessively, the EPDM may be compressed unevenly even when the screw appears tight.
Screw angle matters
A screw installed at an angle normally compresses one side of the washer more than the other. This can leave poor contact on the opposite side and can also distort the local sheet surface.
On profiled roofing and cladding, this is particularly important because the fixing surface may already be curved or inclined. The washer has to seat on the actual profile, not only look acceptable from above.
Underdriven, correctly seated and overdriven
- Underdriven: the washer remains loose or only partly compressed.
- Correctly seated: the EPDM is compressed continuously around the perimeter and the backing washer remains stable.
- Overdriven: the EPDM extrudes excessively, the backing washer distorts or the surrounding sheet is indented.
Flat washers and profiled sheets
Flat sheet, trapezoidal profiles, corrugated sheets and sandwich-panel skins do not give the same seating condition. A standard flat washer may not sit properly on every profile location.
Where a saddle washer or other profile-matched support is specified, its role is to give the sealing washer a more stable surface and improve load distribution at the fixing point.
EPDM alone does not define watertightness
Calling the sealing element EPDM does not by itself establish the performance of the installed fixing. Washer dimensions, backing stiffness, screw geometry, sheet condition, compression and exposure all contribute.
For this reason, sealing performance should be judged using the actual fastener and washer assembly rather than assuming that all EPDM-backed washers behave the same way.
Exposure and durability
Roofing fixings see temperature cycling, ultraviolet exposure, moisture, wind-driven rain and movement of the surrounding sheet. The sealing element has to retain enough elasticity to maintain contact under those conditions.
The metallic parts also matter. Corrosion of the screw or backing washer can eventually affect the sealing interface even when the EPDM itself remains serviceable.
What to inspect after installation
- Check that the screw is in the intended location and is not visibly inclined.
- Confirm that the backing washer lies flat and has not buckled or dished.
- Look for continuous EPDM compression around the washer.
- Check for severe extrusion, tearing or displacement of the elastomer.
- Inspect the surrounding sheet for indentation or coating damage.
- Where a profile-matched washer is used, confirm that it sits correctly on the sheet profile.
Sealing and structural resistance are separate
A correctly seated washer does not prove that the connection has adequate pull-out or pull-over resistance. In the same way, a structurally adequate screw connection is not automatically watertight.
For project selection, use the technical data for the actual JOPAL screw and washer arrangement together with the roofing or cladding system requirements.
This resource explains selection principles. Final product suitability, drilling range and resistance must be verified against the applicable JOPAL technical data for the specific fastener and project condition.
© JOPAL Trading LLC SOC. This technical resource is published for reference and may not be reproduced or republished in whole or substantial part without written permission. Please cite the canonical JOPAL article when referencing this material.