EPDM sealing washers are used with roofing and cladding screws to create a controlled weather seal around the fixing point. Their performance depends not only on the washer material, but also on compression, screw seating, substrate geometry, installation angle and the condition of the surrounding sheet.
The purpose of the sealing washer is to create reliable contact around the fixing without being under-compressed or crushed. Correct installation is a controlled compression condition, not simply maximum tightening.
How the sealing system works
A typical roofing fastener uses a metallic backing washer with an EPDM sealing element beneath it. As the screw is tightened, the washer spreads the clamping force while the EPDM deforms against the sheet surface and around the screw penetration.
The resulting seal depends on continuous contact around the fixing point. Gaps, uneven seating, excessive deformation or damage to the elastomer can interrupt that contact and reduce weather resistance.
Why compression must be controlled
EPDM requires sufficient compression to conform to the local surface and close the sealing interface. Too little compression can leave incomplete contact, particularly on profiled or slightly uneven sheet surfaces.
Excessive compression is also undesirable. Overdriving can force the elastomer beyond its intended deformation, distort the metallic washer, damage the sheet coating or squeeze the sealing material unevenly from beneath the washer.
The objective is therefore a stable seated condition in which the washer is compressed uniformly without visible crushing or severe extrusion.
The metallic backing washer matters
The metallic component is not merely a carrier for the EPDM. It distributes installation force over a wider area and helps maintain a stable bearing surface under the screw head.
Its diameter, thickness and stiffness influence both sealing behaviour and local load transfer into the connected sheet. A washer that deforms excessively may produce uneven compression, while an unsuitable bearing diameter can alter local sheet behaviour.
Installation angle affects sealing
The screw should be installed as close as practical to the intended perpendicular orientation of the fixing surface. An inclined screw can cause one side of the washer to compress more heavily than the other, producing non-uniform contact.
This becomes particularly important on profiled roofing and cladding, where the fixing location may already have local curvature. The washer must sit correctly on the actual profile geometry rather than only appearing tight from above.
Underdriving and overdriving produce different risks
- Underdriving: the washer may remain loose or only partially compressed, leaving an incomplete sealing interface.
- Overdriving: the washer or sheet may deform, the EPDM may extrude excessively, and the local coating or substrate may be damaged.
- Correct seating: the washer is visibly engaged around its perimeter while retaining a stable, uniform geometry.
Profile geometry must be considered
Flat sheet, trapezoidal profiles, corrugated profiles and sandwich-panel skins do not present the same local bearing surface. On strongly profiled sheet, a standard flat washer may not provide the most stable seating condition at every fixing location.
Where a saddle washer or profile-matched support is specified, its purpose is to provide a more stable interface between the fastener assembly and the sheet geometry. The sealing arrangement should therefore be considered as part of the complete fixing system.
Watertight performance is not defined by EPDM alone
Material identification such as EPDM does not by itself establish the watertight performance of an installed fixing. Final performance also depends on washer construction, dimensions, compression behaviour, fastener geometry, sheet condition, installation quality and exposure.
For this reason, sealing performance should be evaluated using the documented configuration of the actual screw and washer system rather than assuming that all EPDM washers behave identically.
Durability and environmental exposure
Roofing and cladding fasteners are exposed to temperature cycles, ultraviolet radiation, moisture, wind-driven rain and movement of the surrounding sheet. The sealing material must retain sufficient elasticity and contact over these service conditions.
Compatibility between the washer, fastener coating, substrate coating and environmental exposure should be considered together. Corrosion of the metallic washer or screw can eventually compromise the sealing interface even if the elastomer itself remains intact.
Inspection of an installed sealing washer
A practical installation review should consider the complete fixing rather than only torque or driver setting. Typical checks include:
- Confirm that the screw is seated at the intended location and angle.
- Check that the metallic washer lies flat and has not visibly dished or buckled.
- Confirm that the EPDM is compressed continuously around the washer perimeter.
- Check for excessive extrusion, tearing or displacement of the sealing material.
- Inspect the surrounding sheet for local indentation, coating damage or deformation.
- Where profile-matched washers are used, confirm that they follow the sheet geometry correctly.
Relationship to structural connection performance
The sealing washer contributes to bearing and load distribution, but watertightness and structural resistance remain separate design checks. Pull-over resistance, pull-out resistance, screw tension, shear and sealing performance can be governed by different parts of the assembly.
A connection should therefore not be accepted as structurally adequate solely because the washer is correctly compressed, nor considered weather-tight solely because the screw has sufficient structural resistance.
Product-specific verification
Washer dimensions, elastomer formulation, backing material and validated installation condition are product-specific. JOPAL selection should follow the technical documentation for the actual screw and washer assembly, together with the roofing or cladding system requirements for the project.
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.
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