WASHERS

The ULAMEX stainless steel washer with EPDM sealing is a dual-function fastening component designed specifically for outdoor photovoltaic mounting systems. Combining a precision-stamped A2 stainless steel (1.4301 / AISI 304) disc with a factory-bonded EPDM rubber backing layer, this washer simultaneously distributes bolt clamping loads across a wider bearing surface and creates a watertight seal at every penetration point in the mounting structure. In solar installations — where hundreds or thousands of bolt connections are exposed to decades of rain, snow, UV radiation and thermal cycling — the bonded EPDM washer eliminates the single most common cause of long-term corrosion: water ingress along threaded fasteners into the supporting substructure.

Installation Guidelines

Place the washer with the EPDM rubber face towards the contact surface (roof hook flange, mounting plate, or profile slot edge) and the stainless steel disc facing the bolt head or nut. This orientation ensures the rubber compresses against the mating surface to form a seal, while the hard steel disc prevents the bolt head from cutting into the softer EPDM under torque. Tighten to the fastener manufacturer’s recommended torque — typically 12–14 Nm for M8 and 20–25 Nm for M10 connections in PV mounting systems. Over-torquing will extrude the EPDM beyond the washer perimeter and compromise the seal; under-torquing will leave gaps for water penetration.

When assembling connections on roof hooks, always use the DIN 9021 large-series washer to spread the load across the full width of the hook mounting slot. For standard T-bolt connections in PV profiles, the DIN 125 standard washer is sufficient. In high-vibration environments — such as installations on standing-seam metal roofs or exposed coastal sites — consider using a Nord-Lock or serrated flange washer in combination with the EPDM washer for additional anti-loosening security.

EPDM washers must not be reused after disassembly. The rubber takes a compression set after initial tightening and will not re-seal reliably if the bolt is removed and re-torqued. Always use a fresh washer when re-assembling a connection.

Applications in Photovoltaic Mounting

• Roof hook to rafter connections — DIN 9021 M10 washer seals the lag screw penetration through the roof tile and into the timber rafter, preventing rainwater from following the screw thread into the roof structure
• Profile-to-roof-hook bolting — DIN 125 M8 washer distributes the clamping load of the T-bolt securing the PV rail to the roof hook
• Ballast tray fastening — washers under the M8 allen bolts connecting ballast holders to PV profiles on flat roof systems
• PV triangle base connections — M10 washers at the triangle bracket anchor points distributing loads across the base plate
• Ground-mount post connections — large-series DIN 9021 washers on foundation bolts where maximum bearing area is required
• General anti-vibration duty — EPDM layer dampens micro-vibrations from wind-induced panel flutter, preventing bolt loosening over time

Material Properties

The A2 stainless steel (1.4301) disc is the most widely used austenitic stainless grade in European fastener applications. Its chromium content of 17.5–19.5% and nickel content of 8.0–10.5% create a self-healing passive oxide layer that resists corrosion in atmospheric conditions without additional surface treatment. A2 stainless is non-magnetic in the annealed condition and maintains its mechanical properties across the full operating temperature range of solar installations (-40 °C to +80 °C ambient).

The EPDM rubber sealing layer is specifically chosen for outdoor PV applications because of its exceptional resistance to UV radiation, ozone, and weathering — properties that silicone and neoprene alternatives cannot match at the same cost point. EPDM maintains its elasticity from -40 °C to +120 °C, does not harden or crack under prolonged sun exposure, and is chemically inert to rainwater, mild acids, and alkaline cleaning agents. The factory bond between the steel disc and EPDM layer is permanent and will not delaminate under normal service conditions.

For installations in coastal, marine, or chemically aggressive environments (chloride-rich atmospheres, swimming pool buildings, agricultural settings), upgrade to A4 stainless steel (1.4401 / AISI 316) washers with molybdenum content for enhanced pitting resistance. Contact our sales team for A4 grade availability.

Compatibility

ULAMEX EPDM washers are designed to work within the complete ULAMEX photovoltaic mounting ecosystem:

• M8 thread: Compatible with all M8 T-bolts, hammer-head bolts and hex bolts used in clamp assemblies and profile connections
• M10 thread: Compatible with M10 double-threaded screws (Stockschrauben), coach screws and hex bolts used in roof hook and triangle bracket connections
• DIN 125 (standard): For use with standard hex bolts, T-bolts and socket-head cap screws in profile T-slot channels
• DIN 9021 (large / fender): For use where maximum load distribution is required — roof hook base plates, mounting plates, ground-mount anchor points

Pair with ULAMEX T-bolts, hex bolts, and threaded rods for a complete, material-matched fastening system.

Frequently Asked Questions

What is the difference between DIN 125 and DIN 9021 washers?

DIN 125 is the standard-size washer with an outer diameter approximately twice the bolt diameter. DIN 9021 is a large-series (fender) washer with an outer diameter approximately three times the bolt diameter, providing a significantly larger bearing surface for load distribution on soft or thin materials.

Can EPDM washers be used with stainless steel bolts on aluminum profiles?

Yes. The EPDM layer acts as an electrical insulator between the stainless steel bolt and the aluminum profile, preventing galvanic corrosion between dissimilar metals. This is one of the key advantages of bonded EPDM washers in mixed-metal PV mounting systems.

How long do EPDM washers last outdoors?

EPDM rubber is rated for 25+ years of continuous outdoor exposure in European climatic conditions. It does not degrade under UV radiation and maintains its sealing properties across temperatures from -40 °C to +120 °C — well within the operating envelope of any solar installation.

Do I need washers on every bolt in a PV system?

Washers are recommended at all structural connections where load distribution or waterproofing is required. For T-bolt and clamp connections inside profile channels, a washer is optional but recommended. For roof penetrations (lag screws, coach screws), an EPDM sealing washer is essential to prevent water ingress.

Ready to order EPDM stainless steel washers for your solar project? Request a quote — ULAMEX supplies bulk quantities at B2B wholesale conditions with delivery across Europe. Combine with our full range of photovoltaic mounting components for single-source procurement.