Withdrawal resistance, which comes from the leg geometry rather than from the crown.

A staple is two nails joined by a crown, and like a nail it holds well across its legs and poorly along them.

Divergent points make the legs spread as they drive, resisting withdrawal by geometry.

Barbed and ring-shanked legs resist it the way a ring shank nail does.

Plain-leg staples rely on friction alone and back out under movement and uplift.

On sheathing exposed to wind uplift, the leg type is what the fixing schedule depends on.

How much bearing the staple gives and how much of the fixing shows.

A wider crown spreads load so the staple does not cut into soft sheathing or tear a membrane.

That matters most on building papers, breather membranes and soft boards.

A narrower crown concentrates load and penetrates harder material more easily.

It also shows less, which matters where the fixing is visible.

Crown width is part of the tool system, so it is not freely chosen once a tool is bought.

Long enough that most of the length is in the member being fixed into, as with nails.

The holding is developed in the timber behind, so penetration there is what matters.

Too short and barely any of the leg is in the timber.

Too long and it comes through a thin batten, which is both a hazard and a sign of the wrong size.

Sheathing specifications state leg length alongside spacing.

Leg length is also constrained by what the tool will drive in the material.

No - crown width, gauge, leg length and collation are all specific to a tool system.

A staple that physically fits the magazine may still jam or drive badly.

Collation adhesive and strip form differ between manufacturers.

Driving depth and power are matched to a particular staple in a particular material.

The tool manufacturer publishes a compatible list, and that is the one to buy from.

Buying tool and consumables as a system avoids most of the problems.

For their gauge yes, and two legs per fixing has advantages - but specifications name one or the other.

Two legs resist rotation of the sheet and spread the fixing over two points.

Staples are thinner than an equivalent nail, so they split timber less.

Individual withdrawal capacity per leg is lower than a nail of equivalent diameter.

Where a structural fixing schedule names nails, staples are not an automatic substitution.

Where staples are permitted, the schedule states gauge, crown, leg length and spacing.

Matched to exposure and to any timber treatment - and they are often never seen again.

Sheathing staples are typically covered by a membrane and then by cladding.

That means a corroding fixing is invisible until something fails.

Galvanised is the general external answer, with stainless where exposure is severe or the timber is treated aggressively.

Where the timber is treated, the preservative used decides which fastener coatings will survive next to it.

Because inspection is impossible, specifying properly matters more here than in visible work.

By the sheathing or membrane manufacturer, and it is usually closer at edges and perimeters.

Uplift and racking loads are highest at panel edges and at building corners.

Spacing schedules reflect that with a closer pattern at those positions.

Exposure and building height change the loads and therefore the schedule.

Edge distance matters too, since a staple too close to a board edge breaks it out.

Following the published schedule is normally also a warranty condition.