Which washer do I need — flat, penny, spring or serrated?

Washers are the cheapest component in any joint and the one most often left out or picked at random. Each type solves a specific problem, and picking by problem rather than by what is in the tin removes most of the ways a bolted joint goes wrong in soft material.

Step by step

1. Use a flat washer to give the nut a face to turn against.It spreads the pressure, protects the surface finish, and lets the nut turn without dragging on the material, which is what stops you mistaking friction for tension when you tighten.
2. Use a penny or repair washer in soft material.Much larger outside diameter, same small bore. This is the answer to a bolt head or a nut sinking into timber, crushing plastic, or pulling through thin sheet. It is also what to reach for over a slotted or oversize hole.
3. Use a penny washer to stop a joint working loose in timber.Most timber joints loosen because the material under the washer is being crushed and settling, not because the nut is unscrewing. A wider bearing area removes the cause instead of fighting the symptom.
4. Treat spring washers realistically.A split spring washer flattens as the bolt is properly tightened, and once flat it adds very little. It is a habit inherited from an era of lower-tensile fasteners. Where the joint keeps loosening, the answer is usually correct tightening, a bigger washer, or a nyloc nut.
5. Use a serrated or shakeproof washer where you actually need rotation resistance.The teeth bite into both the nut face and the material, which does resist turning — at the cost of permanently marking the surface. Fine on machinery and structural steel, wrong on a visible finish.
6. Use a dome or cup washer with a raised countersunk screw.It gives the conical head somewhere to seat on a flat surface, spreads the load, and stops the head crushing into the finish. This is standard on visible ironmongery.
7. Use a bonded or sealing washer where water is involved.A steel washer with a vulcanised EPDM face, used on roofing and cladding screws to seal the hole. It is compressed to the point where the rubber just begins to bulge, and no further.
8. Match the washer material to the fastener.A steel washer under a stainless nut in a wet joint sets up exactly the galvanic pairing you were trying to avoid. Keep the metals consistent, or use a nylon washer to isolate them.

Tips

• Two flat washers stacked is not a penny washer. The bore is what makes a penny washer work — a big outside diameter with a small hole.
• Nylon washers are excellent for isolating dissimilar metals and for protecting a painted or polished surface from a turning nut.
• If a bolt has to be re-tightened every few months, look at what is under the washer before you look at the nut. Something is almost always crushing.

Common mistakes

• Putting a washer under the domed head of a coach bolt — The head is designed to bear directly on the timber so the square collar can pull in and stop the bolt turning. A washer holds it off and the bolt spins.
• Relying on a spring washer to stop a joint loosening — It flattens under proper tension and then does almost nothing. The real causes are under-tightening and material settling, and neither is fixed by a split ring.