Because timber joints fail by the wood crushing under the washer, and area is what prevents it.

A bolt in steel is limited by the bolt; a bolt in timber is usually limited by the bearing strength of the wood beneath the washer.

A square washer presents more area than a round one of the same nominal dimension, so it raises that limit directly.

Timber design codes set washer sizes for exactly this reason, and the sizes are larger than general fixing practice suggests.

Thickness matters equally, because a thin plate dishes and concentrates load near the bolt again.

Timber also creeps under sustained load, so joints are commonly checked and retightened after a period in service.

Noticeably more than a round washer of the same nominal size, because the corners are additional bearing.

A square of a given width across the flats encloses more area than a circle of that diameter, and the bolt hole is the same in both.

In timber, where bearing area sets the capacity, that difference is worth having and is why designers specify square.

The corners only contribute if the washer is stiff enough not to lift there, which is the argument for thickness.

On a soft substrate the corners can also concentrate stress at their tips, which is why plate washers have generous corner radii.

Sizes are quoted across the flats, so comparing directly with a round washer's outside diameter is the correct comparison.

Against a rough or textured surface, largely yes - the corners resist rotation as the nut is tightened.

A round washer turning with the nut scuffs the bearing face and can polish a ring into a finished surface.

Square corners dig slightly into sawn timber or a rough casting and hold position.

On a smooth machined face the effect is much smaller, since there is nothing for the corners to engage.

In channel-type assemblies the flat sides bear against the channel walls and locate the fastener positively.

Where rotation must be prevented reliably, a tab washer or a locating feature is the proper answer rather than relying on corners.

On finished surfaces, in recesses and counterbores, and against curved faces.

Corners mark a finished surface first and most visibly, and the mark is square-shaped.

A counterbore or recess sized for a round washer will not accept a square one at all.

Against a curved surface a square washer bears at two points rather than spreading load, which is the opposite of what it is there for.

In tight assemblies the corners can foul adjacent components or reduce spanner access.

Round is the sensible default, with square as a deliberate choice for area, rotation resistance or location.

Thick enough to stay flat under full load - which in timber design is specified rather than estimated.

A thin plate under a bolt behaves like a diaphragm and lifts at the corners and rim, concentrating load back near the hole.

Structural plate washers are consequently several times the thickness of a general stamped washer.

The visible symptom of an inadequate plate is a shiny ring around the bolt hole and corners that are not bearing at all.

Timber connection design specifies plate dimensions including thickness, because they determine the joint capacity.

Substituting a thinner plate of the same face dimensions reduces the connection capacity without looking any different.

Usually hot-dip galvanised or stainless, and the choice interacts with the timber treatment.

Exterior timber is wet for long periods, so a plated finish is inadequate and stains the timber as it fails.

External structural timber connections are normally hot-dip galvanised throughout.

Some preservative treatments are corrosive to zinc and to plain steel, and the treatment supplier states which fastener finishes are compatible.

Stainless is specified where the environment is aggressive or where staining would be unacceptable.

Bolt, nut and washer should share a compatible finish, since a mixed assembly corrodes at the junction.

Yes - on tapered flanges when beveled, in channel assemblies, and generally wherever area or location matters more than appearance.

Beveled washers for rolled sections are conventionally square, because the straight edges align with the flange and make the taper orientation visible.

Strut and channel systems use square washers that bear against the channel walls to locate the fixing.

Agricultural, marine and heavy plant assemblies use them for the same bearing-area reasons as timber.

Fabrication and repair work uses them where a large plate washer is effectively a small distribution plate.

In machinery and finished assemblies round remains the norm, for clearance and appearance.