What Thickness Plywood for a Shed Floor?

Quick answer: For most shed floors, use 3/4″ (18–19mm) exterior-grade plywood over joists spaced 16″ on center. That’s the thickness that won’t sag under a riding mower, a workbench, or stacked boxes. You can drop to 5/8″ (15mm) if joists are 16″ or closer and the load is light, but 1/2″ (12mm) is the bare minimum – and only on tightly spaced joists. Go thinner than your joist span allows and the floor flexes, then the fasteners loosen, then it bounces. The fix costs more than just buying the right sheet the first time.

Here’s how to land on the right number for your build.

The short version: thickness by joist spacing

Plywood thickness for a floor isn’t really about the plywood – it’s about how far it has to bridge between joists. The wider the gap, the thicker the sheet needs to be.

If you’re not sure what you’ll keep in the shed five years from now, go thicker. Nobody regrets a stiff floor. Plenty of people regret a springy one.

What actually decides the thickness

Four things move the number up or down:

1. Joist spacing. Covered above. This is the big one.

2. The load. A garden shed holding pots is a different animal from a workshop with a cast-iron table saw or a shed you drive a mower into. Concentrated point loads – a jack stand, a heavy bench leg – need a stiffer deck than evenly spread boxes.

3. Moisture. Sheds sit close to the ground, and ground throws up damp. A standard interior sheet will swell and delaminate within a couple of seasons. You want exterior-grade, weather-resistant board here – a waterproof plywood or WBP (weather and boil proof) plywood that’s built to handle wet-dry cycling. There’s even a dedicated plywood for flooring grade made for exactly this kind of deck.

4. Span rating. Some structural sheets carry a span rating stamp. If the sheet is rated for your joist spacing, you’re covered. If it isn’t stamped, fall back to the table above.

Recommended thickness by shed type

For floors you walk on with wet boots or sawdust underfoot, an anti-slip plywood surface is worth a look – the textured face stops the slick spots a smooth sheet develops over time.

Why not just use the cheapest 1/2″ sheet?

It’s tempting. It’s also where most sagging shed floors come from. Half-inch plywood on 16″ joists deflects under a point load – stand a heavy toolbox in one spot and you’ll feel the give. Over time that flex works the screws loose and the sheet starts to creak and bounce.

The price gap between 1/2″ and 3/4″ across a typical 8×10 shed is small – a few sheets. The labour to tear out a failed floor and redo it is not small. Spend the difference up front.

The one case where thin works: joists at 12″ on center, light storage only, dry site. Even then, 5/8″ is the smarter floor.

Plywood type matters as much as thickness

A thick interior sheet in a damp shed is still a bad floor. Match both.

Softwood options like pine plywood and pine/eucalyptus are common, cost-effective choices for shed decks when they’re in a waterproof grade. For a floor that takes real abuse, a hardwood plywood or film-faced plywood gives you a tougher, wear-resistant surface.

Installation tips that protect your floor

• Run sheets perpendicular to the joists so each panel crosses several joists, not parallel to one.
• Leave a ~1/8″ gap between sheets for expansion. Butt them tight and they’ll buckle when they swell.
• Stagger the seams brick-style instead of lining them all up in one row.
• Glue and screw to the joists – construction adhesive plus deck screws kills squeaks before they start.
• Lift the shed off the dirt. Skids, blocks, or a gravel pad with a vapor barrier keeps ground damp away from the underside of the floor, which is where rot usually begins.

Seal the edges. Cut edges drink up water fastest; a coat of sealant on them buys years.