Choosing the right washer is as important as choosing the right bolt. The wrong washer — or no washer at all — leads to fastener loosening, surface damage, and joint failure. This guide to types of washers and when to use them covers every major category: from plain washers and lock washers to spring washers, sealing washers, and specialty types, with materials guidance for every environment.

What Are Washers and Why Do You Need Them?

Washers are essential hardware components used to distribute the load of a threaded fastener over a larger area. Without a washer, a bolt head or nut concentrates clamping force on a very small contact area — enough to crush, dent, or pull through softer materials. Washers distribute the load of a fastener over a larger area, protecting surfaces from damage during tightening, and improving the stability of fastener systems overall.

Beyond load distribution, washers serve three other critical functions: they prevent fasteners from loosening due to vibration, they can serve as spacers to ensure proper alignment of components, and they absorb vibrations, enhancing the stability of fastener systems across applications from automotive assemblies to furniture assembly.

Plain Washers (Flat Washers)

Plain washers are the most common type. A plain washer is a flat, ring-shaped fastening device with a central hole sized to fit a bolt or screw shank. Plain washers distribute load and protect surfaces from damage during tightening. Choose flat washers for general load distribution in virtually any application where the bolt head or nut might dig into or damage the mating surface.

Two standard specifications govern plain washer sizing:

• SAE washers have a smaller outer diameter relative to the central hole size. SAE washers are used in automotive and precision assemblies where minimal washer projection is desired.
• USS washers have a larger outer diameter relative to the inner hole. USS washers are better for load distribution on softer or less stable surfaces — the larger contact area reduces surface pressure. Use larger washers to improve load distribution on soft materials.

Standard flat washers are suitable for general use. Standard washers are available in carbon steel, stainless steel, galvanized steel, and plastic materials.

Fender Washers

Fender washers are a subtype of flat washers with an unusually large outer diameter relative to their central hole. Originally designed for use in automotive fenders (where thin sheet metal needs reinforcement), fender washers are now used anywhere a wider bearing surface is needed — thin plate, sheet goods, and soft materials where standard washers would pull through. Fender washers have a larger outer diameter for better load distribution and are the right choice for thin sheet metal and similar materials.

Lock Washers

Lock washers prevent fasteners from loosening due to vibration. They work by biting into both the bolt head (or nut) and the mating surface, creating mechanical resistance to rotation. Lock washers are the standard solution for fastening systems subject to vibration or dynamic loads.

The most common lock washer types:

• Split lock washer (helical lock washer): The standard lock washer. A single ring with a gap and slightly offset ends. The ends dig into the bolt head and surface when the joint is tightened, resisting loosening. This is the type most people picture when they say "lock washer."
• Tooth lock washers (internal and external): Have serrated teeth around the inner diameter (internal tooth) or outer diameter (external tooth) that bite into the mating surface. Tooth lock washers provide more contact points than split lock washers. External tooth lock washers work well under bolt heads; internal tooth types are better under small bolt heads where space is limited.
• Wedge lock washer: A pair of washers with angled cam surfaces that lock together under load. The wedge lock washer provides spring-like tension and positive locking even under high vibration — superior performance over standard split lock washers in demanding high vibration environments.
• K-lock nut: A nut with an integrated tooth lock washer. The K-lock nut combines the function of a nut and lock washer into a single fastening device.

Spring Washers

Spring washers maintain tension and resist loosening under vibration by providing a spring-like force that keeps constant pressure on the fastening system. Spring washers are used in applications where the joint may experience thermal expansion, shock loading, or cyclic vibration that would otherwise cause the fastener to work loose.

Spring washers are available in several designs:

• Belleville washers (conical washers): Disc-shaped spring washers with a conical cross-section. Belleville washers provide high load-bearing capacity in limited space — high load bearing capacity is the defining characteristic of the Belleville design. Multiple Belleville washers can be stacked in series (to increase deflection range) or in parallel (to increase load capacity). Used in compact assemblies where a conventional spring cannot fit.
• Wave spring washers (wave washers): Have a corrugated, wave-shaped profile. Wave spring washers provide moderate spring force over a large deflection range. Wave washers are used in automotive assemblies, bearings, and anywhere a light preload is needed. The wave spring washer produces consistent load across its travel range.
• Finger spring washer: Similar to a wave washer but with discrete spring fingers rather than a continuous wave. The finger spring washer is used in precision instruments and clutch mechanisms.
• Crescent spring washer: A curved, crescent-shaped design providing asymmetric spring force. The crescent spring washer is used in specialized bearing applications.

Spring steel is the most common material for spring washers — spring steel provides the elasticity and fatigue resistance needed to maintain consistent load through repeated deflection cycles.

Sealing Washers

Sealing washers create a watertight seal around a fastener to prevent fluid or gas leakage. Rubber or silicone washers are used to create watertight seals in plumbing. Sealing washers are commonly used in roofing, plumbing, HVAC, and any application where a fastener penetrates a membrane or passes through a panel exposed to liquid.

Rubber and elastomeric washers compress under bolt load to fill microscopic gaps between the fastener and the surface, creating a positive seal. Neoprene is the most common sealing washer material for outdoor roofing fasteners. EPDM elastomeric washers are used where broader chemical and weather resistance is needed — it handles UV exposure and temperature extremes without degrading.

Metal-backed sealing washers combine a metal outer ring with a bonded rubber center. The metal provides structural rigidity (preventing over-compression) while the rubber seals. These are standard in metal roofing fastener systems.

Specialty Washers

Shoulder Washers
Shoulder washers have an extended cylindrical neck (the "shoulder") that fits inside a bolt hole to provide electrical isolation between the fastener and the panel. Shoulder washers are used wherever a bolt must pass through an electrically conductive surface without making an electrical connection — printed circuit boards, electrical enclosures, and similar applications. Nylon washers are the most common material for shoulder washers, providing both electrical insulation and light load-bearing capacity.

Torque Washers
A torque washer is a square or toothed washer designed to embed into wood and prevent carriage bolt rotation. When a carriage bolt's square neck can't grip a pre-drilled round hole, a torque washer provides the anti-rotation function that the square neck would otherwise provide. The torque washer teeth embed into the wood surface as the nut is tightened, locking the assembly against rotation. Common in timber construction and wood-to-metal connections.

Slotted Washers
Slotted washers have a slot cut from the central hole to the outer edge rather than a complete ring shape. This design allows the washer to be installed or removed without fully removing the nut or bolt — useful in maintenance situations where complete disassembly isn't practical.

Hat Washers (Top Hat Washers)
Hat washers — also called top hat washers — are flange washers with a raised cylindrical center. The hat shape provides a standoff between the bolt head and the surface, used as a spacer in assemblies requiring precise axial positioning. Common in automotive assemblies and structural connections where the attachment point requires a specific clearance height.

Washer Materials

Different types of washers are available in a range of materials, and choosing the right material is as important as choosing the right washer type:

Select stainless steel washers for corrosion resistance in harsh environments. For treated lumber specifically, use hot-dip galvanized or stainless steel — the preservatives in ACQ and similar treatments accelerate galvanic corrosion in plain zinc-plated fasteners and washers.

How to Choose the Right Washer

Follow this decision sequence to find the appropriate washer for any fastening application:

1. Do you need to seal against liquid or gas? → Sealing washer (rubber, neoprene, or EPDM backed)
2. Will the joint be subject to vibration or dynamic loads? → Lock washer or spring washers. Spring washers maintain tension in high-vibration applications; lock washers prevent fasteners from loosening due to vibration.
3. Do you need to maintain consistent tension against thermal expansion or cyclic loading? → Belleville washers or wave spring washers
4. Is this a soft material like wood, plastic, or thin sheet metal? → Fender washer or USS flat washer for maximum surface protection and load distribution
5. Is electrical insulation needed? → Nylon or plastic washers, or shoulder washers for bolt hole isolation
6. General fastening on hard material? → Standard flat washers (SAE or USS) for load distribution

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