STAINLESS STEEL PENNY WASHERS

Penny Washers — There is no dimensional standard behind the name penny washer - it describes a large flat washer, historically sized against an old penny coin, and it earns its keep purely through its oversized bearing area. That size is the whole point: where a standard washer would concentrate a bolt or screw's clamping force onto a small ring of material, a penny washer spreads the same load over a much wider area, which is exactly what is needed when fixing into timber, roofing felt, insulation board, or any substrate soft enough that a normal washer would pull straight through it under load. That makes penny washers a fixture of roofing, cladding, fencing, and general construction and repair work, along with agricultural and automotive uses where they are sometimes called mudguard washers. Because the diameter is generous and the name is not tied to a formal spec, sizes vary by manufacturer and merchant; where a project genuinely needs a dimensionally standardised large washer rather than a commercial penny size, DIN 9021 (the 3xd flat washer, sometimes marketed under the same penny or mudguard name) is the closer formal reference, and it is worth confirming which one a specification actually requires before quoting against a drawing. Material choice matters more here than the name suggests: mild steel with bright zinc plating is fine for dry, indoor, short-term fixings, but roofing, cladding, and anything left exposed to weather calls for A2 (304-equivalent) stainless as the sensible default, since a rusting washer under a roofing bolt is a recurring maintenance headache. A4 (316-equivalent) is worth the jump specifically for coastal roofs, marine-adjacent cladding, or anywhere salt exposure is a genuine, ongoing factor - everywhere else, A2 covers the corrosion risk without the added cost. Sizing is simply a matter of matching the bore to the fastener shank and choosing the largest practical diameter the fixing detail allows, since with this washer type, bigger genuinely means better load spreading.

FAQs
What is the difference between a penny washer and a mudguard washer?
Usually none - both are trade names for the same large-diameter washer type, and both are commonly mapped to DIN 9021 where a formal dimensional standard is actually required.

Can penny washers be used under structural bolts?
No. They are a load-spreading washer for soft or thin materials rather than a rated structural component; structural steel connections should use a washer specified to the relevant structural standard, such as DIN 7989.

Do penny washers need a matching bonded seal for roofing use?
Not necessarily for load spreading alone, but where keeping water out at the fixing point matters as much as holding the panel down, a bonded sealing washer with an EPDM disc is the better choice.