Washers

A washer is a thin disc with a center hole that sits between a fastener and the surface below. Sounds like throwaway hardware. It isn’t. A failed joint rarely fails because of the bolt. More often it fails because the wrong washer was specified, or no washer was specified at all. Washers control how load moves through a joint, how long preload holds, and whether a connection survives vibration, thermal cycling, and long-term service.

The right washer comes down to three calls: what type, what material, what size. Get those matched to the job and the joint stays tight, corrosion-free, and watertight for its service life. Skip the washer or grab the wrong one, and you invite the slow failures — surface damage, loosening, corrosion, or failure.

What a Washer Actually Does

Four core jobs. Without a washer, bolt clamping force concentrates on the small bolt head bearing face. A standard M10 flat washer increases bearing area from about 53 mm² to 236 mm² — a 4.5× increase that drops surface stress proportionally. This is the difference between a joint maintaining preload across its service life and one gradually loosening from surface deformation.

Then there’s surface protection. During tightening, the rotating nut or bolt head can score and damage the mating surface. A washer takes that friction instead.

The third job is holding preload over time, and the numbers here are why this matters. Embedding — the microscopic flattening that occurs when rough contact surfaces compress under clamping force — can cause up to 50% or more loss in preload depending on materials and joint geometry. That’s half your clamp force gone, and the bolt looks fine the whole time.

Does every joint need one? No. If the fastener and the material it’s being driven into are the same — for example, metal to metal — then a washer isn’t necessary. But the moment you’re fastening into something softer — wood, aluminum, plastic, thin sheet — the washer stops being optional. When the material you’re fastening to is weaker than the fastener material itself, without a washer the fastener can distort the material it’s being installed into. To prevent this, you need a large bearing surface, which the washer provides.

The Main Types and When to Use Each

Flat washers are the workhorse, and they come in two patterns. SAE pattern has a smaller outer diameter for tighter clearances. USS pattern has a larger outer diameter to spread load over a bigger area. Both follow ASME B18.22.1. Western Building Center carries both patterns across a wide size range — USS from 1/4” up through 1”, SAE from #6 machine-screw size through 1”, plus Grade 8 SAE washers from 1/4” to 3/4” for higher-strength work.

Fender washers are the fix for thin or worn material. They have a large outer diameter and a small center hole, built to spread load across a wide area on thin or soft materials. The wide bearing surface prevents the fastener from pulling through the material under load, which a standard flat washer often cannot do alone. WBC stocks these in combinations like 1/4×1”, 5/16×1-1/2”, 3/8×1-1/2”, and 1/2×2” — the oversized OD doing the work.

Split lock washers resist rotation, but know their ceiling. Split lock washers provide spring tension and resist rotation in lower-vibration applications. They work well in general assembly where moderate movement resistance is needed. In high-vibration conditions, micro-movements will overcome a split lock washer. A wedge-lock system or Belleville washer is the correct spec for those environments. A 1990 NASA manual put it bluntly — “a lockwasher of this type is useless for locking.” Fair warning. For light to moderate vibration they earn their keep.

Belleville (conical) washers hold clamp load when conditions swing. A wedge-lock system or Belleville washer is the correct spec for high-vibration environments. They maintain consistent axial force through thermal expansion, vibration, and cyclic loading. The retention numbers tell the story: Belleville-equipped joints maintain 85–95% of initial preload after 500 thermal cycles, versus 50–65% for flat-washer-only joints. That gap matters in a climate that runs cold winters into hot summers.

Sealing washers keep water out. Rubber-bonded sealing washers have a stainless steel base with a built-in rubber seal, giving you strength, reduced chance of loosening due to vibrations, and a leak-proof seal. They’re commonly used in plumbing, outdoor fixtures, metal roofing, siding, and truss systems to create a watertight seal while cushioning fragile materials. When the screw is tightened, the rubber seals against the screw shank and the hard surface to prevent air and water leaks. For metal roofing and siding facing Montana snow and rain, that seal is the whole point — don’t substitute a plain flat washer where one’s specified. The seal is a functional part of the assembly, not an add-on.

Hardened structural washers are not interchangeable with standard flats. ASTM F436 hardened washers are required with high-strength structural bolts like A325 and A490. A standard flat washer will embed under the load and bleed preload. If the drawing calls for F436, that’s a requirement, not a suggestion.

Washer Type	Primary Function	When to Specify
Flat washer (SAE)	Load distribution, surface protection	General assembly, tighter clearances
Flat washer (USS)	Spread load over larger area	Softer materials, oversized holes
Hardened structural (F436)	Prevent embedment, retain preload	Structural bolts A325/A490, high-load joints
Split lock	Resist rotation under light vibration	Low to moderate vibration, general use
Belleville (conical)	Maintain clamp load, absorb thermal movement	Thermal cycling, high vibration
Sealing	Load distribution plus watertight seal	Metal roofing, siding, plumbing, outdoor fixtures
Fender	Wide load spread on thin material	Sheet metal, oversized holes, soft substrates

Material Choice and Avoiding Galvanic Corrosion

This is where outdoor jobs get expensive if you cut corners. Washer material needs to match the fastener and the environment. Getting this wrong causes galvanic corrosion at the joint, which has nothing to do with the bolt quality and everything to do with the spec.

The rules are simple. Use stainless steel washers with stainless steel fasteners. Do not mix carbon steel and stainless in wet or corrosive environments. Why? Mixing dissimilar metals can create galvanic corrosion, even when the bolt itself looks fine. Moisture acts as the “bridge” that speeds up corrosion. In wet Montana springs and freeze-thaw cycling, that moisture bridge is always available. Pairing stainless fasteners with untreated carbon steel washers can cause accelerated corrosion at the inner and outer edges of the washers.

The material lineup, plain and simple:

• Carbon steel washers offer strength and affordability. Use zinc or hot-dip galvanized versions for coated carbon-steel systems outdoors.
• Stainless steel washers resist corrosion and handle harsh weather.
• Plastic and nylon washers don’t conduct electricity, won’t rust, and help protect softer surfaces. They’re good for insulation, spacing, and isolating dissimilar materials that could corrode when they come into contact.
• Rubber-bonded for sealing.

One more trap on high-strength hardware. Hydrogen embrittlement is a critical risk when zinc-plated washers are used with Grade 8 or ASTM A490 structural bolts. The industry fix is a post-plate bake — roughly 375°F for several hours — to drive the trapped hydrogen back out.

Installation: Order, Orientation, and Sizing

Stack order trips up more people than it should. The correct sequence for a standard bolted joint: Bolt → Flat Washer → Workpiece(s) → Flat Washer → Spring Washer → Nut.

The lock washer placement is the part folks get backward. The spring/lock washer sits between flat washer and nut — never directly against soft material, where it would dig in and reduce clamping force. Put the flat washer down first to give the lock washer a hard surface to work against.

Sizing runs off bolt diameter. A standard metric M10 flat washer has an inner diameter of 10.5 mm, an outer diameter of 20 mm, and a thickness of 2.0 mm — that half-millimeter of clearance on the ID is what lets the bolt pass while the OD does the load spreading. The imperial sizing WBC carries works the same way: the size on the box matches your bolt diameter.

And the one that costs you a callback: do not use a standard flat washer as a substitute where a sealing washer is specified. The seal is a functional part of the assembly, not an add-on.

Frequently Asked Questions

Does every bolt need a washer? Not every one. If the fastener and the material are the same — metal to metal — then a washer isn’t necessary. That said, certain types like a lock washer can still be advantageous to prevent the fastener and nut from loosening. The moment a fastener goes into something softer than the bolt — wood, aluminum, thin sheet — a washer becomes essential to keep the fastener from distorting or pulling through.

What’s the difference between SAE and USS flat washers? Outer diameter. SAE washers are smaller and tighter-fitting. USS washers are larger and spread the force over a wider area. Reach for SAE in tight clearances, USS when working softer material or covering an oversized hole.

Will a split lock washer stop a bolt from backing out under heavy vibration? Not reliably. In high-vibration conditions, micro-movements will overcome a split lock washer. A wedge-lock system or Belleville washer is the correct spec for those environments. Split locks are fine for light to moderate vibration and general assembly.

Which washer for metal roofing or siding? A bonded sealing washer. They’re commonly used in metal roofing, siding, and truss systems to create a watertight seal while cushioning fragile materials. When the screw is tightened, the rubber seals against the screw shank and the hard surface to prevent air and water leaks.

Can a stainless bolt pair with a galvanized or carbon steel washer outside? Don’t do it. Do not mix carbon steel and stainless in wet or corrosive environments. Mixing dissimilar metals creates galvanic corrosion even when the bolt itself looks fine, with moisture acting as the bridge that speeds it up. Match stainless with stainless and the problem never comes up.

Why do bolted joints loosen over winter and tighten back in summer? Thermal cycling bleeds preload from the joint. Belleville washers maintain consistent axial force through thermal expansion, vibration, and cyclic loading. They hold clamp load through wide temperature swings far better than a flat washer alone — 85–95% of initial preload after 500 thermal cycles versus 50–65% for flat-only joints.