What Is a Washer and Why Does It Matter?
A washer is a small but essential fastener, a flat disc with a hole in the centre, placed under a bolt head or nut. Without the right washer, you risk damaging surfaces, causing joints to loosen over time, or even structural failure in load-bearing applications. Choosing the correct washer size and material is just as important as choosing the right bolt.
Types of Washers
Not all washers are the same. Here are the most common types you'll encounter:
- Flat Washers: The most common type. They distribute the load of a bolt or nut evenly across a surface, preventing damage and reducing the risk of loosening.
- Spring (Lock) Washers: Designed to prevent nuts and bolts from loosening due to vibration. The split ring bites into both the nut and the surface to resist rotation.
- Penny (Repair) Washers: Have a very large outer diameter relative to the hole size, ideal for spreading load over soft or thin materials like wood or plastic sheeting.
- EPDM Washers: EPDM washers create a durable, weatherproof seal around fixings, helping prevent water ingress in roofing, cladding and outdoor applications.
How to Match Washer Size to Your Bolt or Screw
The most important measurement is the inner diameter (ID) of the washer, this must match the diameter of your bolt or screw. For example, an M8 bolt requires an M8 washer. Here's a quick reference guide:
| Bolt Size | Washer Inner Diameter | Standard Outer Diameter |
|---|---|---|
| M4 | 4.3mm | 9mm |
| M5 | 5.3mm | 10mm |
| M6 | 6.4mm | 12mm |
| M8 | 8.4mm | 16mm |
| M10 | 10.5mm | 20mm |
| M12 | 13mm | 24mm |
Always check the specific DIN or ISO standard for the washer you need, as outer diameters can vary between standard (Form A) and larger (Form B) versions.
Washer Material Guide
Choosing the right material is critical, especially for outdoor or corrosive environments:
- Stainless Steel A2 (304): Excellent corrosion resistance, suitable for most outdoor applications including garden furniture, decking, and general construction. A great all-rounder.
- Stainless Steel A4 (316): Superior corrosion resistance, ideal for marine environments, coastal locations, or chemical exposure.
- BZP (Bright Zinc Plated): A cost-effective option for indoor use or dry environments. Not suitable for prolonged outdoor exposure.
- Galvanised: Hot-dip galvanised washers offer good corrosion resistance for outdoor structural applications.
- Nylon: Lightweight, non-conductive, and resistant to many chemicals. Ideal for electronics, plumbing, and applications where metal contact must be avoided.
Common Applications
Here's where different washers are typically used:
- Decking & Fencing: Stainless steel A2 flat washers with coach bolts or hex bolts
- Automotive & Engineering: Spring washers to resist vibration, mudguard washers for bodywork
- Plumbing: Nylon or rubber washers for watertight seals
- Construction & Structural: Large flat or penny washers to spread load in timber frame construction. Also, EPDM washers can be used on roofing and corrugation work.
- Marine & Coastal: A4 stainless steel washers for maximum corrosion resistance
Flat Washer vs Spring Washer - Which Do You Need?
This is one of the most common questions we get. The simple answer is: use both where vibration is a concern. A flat washer protects the surface and distributes load, while a spring washer prevents the fastener from working loose. In static applications with no vibration (like furniture assembly), a flat washer alone is usually sufficient.
Frequently Asked Questions
What size washer do I need for an M10 bolt?
For an M10 bolt, you need an M10 washer with an inner diameter of approximately 10.5mm. The standard outer diameter is 20mm, though large (Form B) M10 washers have a 30mm outer diameter for greater load distribution.
Can I use a washer that's slightly too big?
The inner diameter must be close to the bolt diameter, too large and the washer won't sit properly and may slip. The outer diameter can be larger than standard if you need to spread the load over a wider area.
Do I need a washer under every bolt?
Not always, but it's good practice. Washers are especially important when fastening into soft materials (wood, plastic, plasterboard), when using oversized holes, or in any application where vibration could cause loosening.
What's the difference between Form A and Form B washers?
Form A (DIN 125A) is the standard washer with a smaller outer diameter. Form B (DIN 125B) has a larger outer diameter, providing greater load distribution. Form B is often preferred for softer materials or where a larger bearing surface is needed.