Photo: Niki Nagy

How thick should the plywood be for a shed floor?

The recommended shed floor plywood thickness is 3/4 inch pressure-treated exterior-grade CDX plywood. This type of plywood won't sag when used on a 12 inches joist structure. It is rot resistant with knots replaced with football shaped plugs to give it a smooth moisture-resistant surface.

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Building a shed involves selecting the correct materials for different components; the foundation, the walls, and the roof. The most commonly ignored, but equally important is the floor. It has to support whatever you put on it, withstand decades of abuse as boxes, crates, machinery, and feet scuff and scrape it. What is the best shed floor plywood thickness? The recommended shed floor plywood thickness is 3/4 inch pressure-treated exterior-grade CDX plywood. This type of plywood won’t sag when used on a 12 inches joist structure. It is rot resistant with knots replaced with football shaped plugs to give it a smooth moisture-resistant surface.

When Do I Need to Build a Floor?

If you’re installing a pre-built plastic shed, you may or may not need to make a base. This is especially true of small sheds. The best way to find out is to check the manufacturer’s recommendations.

You might be fine with simply laying it on level ground.

Or you might want to lay some crushed stone-down first so the shed is more level. If you’re building the shed from scratch, whether the structure is metal or wood, you’ll want a proper floor. You can use plywood to cover the floor joists. And support the joists with deck blocks to keep them up off the ground and away from moisture.

Size and Type of Plywood for Your Shed Floor

Let’s take a look at what 3/4 inch pressure-treated CDX plywood is and why you want it. The 3/4 inch thick plywood is sturdy. It can support the weight of a person and heavier equipment without fatiguing over time. With pressure-treated joists spaced 12 inches apart, this floor can last for 20 years or more. Pressure treatment protects the wood from moisture. It’s best used for anything close to the ground and prevents rot.

CDX refers to the grade of plywood.

The C and D refer to the quality of wood on either side of the board. It’s a lower classification than the A and B, which is a more expensive plywood. You’ll see the football-shaped patches on both sides, and there will be knots visible. Really, you’re building a shed floor. It’ll be taking some abuse, so there’s no point in shelling out extra cash for prettier plywood. It will get beaten down within the first year anyway. The X signifies exposure (eXposure?). This means it’s more resistant to moisture than the regular CD plywood. It’s not the same as marine plywood ($$$), but it can withstand intermittent exposure to moisture. Being pressure treated on top of this will make the floor last longer. Basically, for the X designation, the glue can’t soften and allow the board to delaminate. You should still keep it as dry as possible, though. Even though 3/4 inch CDX plywood is the most common, there might be reasons to not use this material for your shed floor.

Alternative Material Options for a Shed Floor

There is one single most important factor to when choosing shed floor material.

Moisture resistance.

I’ve seen my fair share of poorly thought out sheds. Let’s run through a few material types to see what would work and what you should avoid.

Decking Boards

These are fine for open-air decks, but for the floor of a small building, you’re going to run into headaches. The reason for this is that decking boards have a wood grain that is only in one direction.

How does this translate to shed flooring?

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The boards will expand and contract with humidity. This can make your flooring buckle and warp as the seasons change and moisture is introduced. All said and done, this will be an expensive and under-performing option.

OSB

OSB (Oriented Strand Board) is really common in roof and wall sheathing, as well as subflooring. It doesn’t do so well for shed floors.

The reason?

Shed floors need to be able to handle exposure to moisture. OSB doesn’t do well with moisture. You’ll almost immediately see signs of swelling around the edges of the boards. The boards will puff up and break loose, which means that you’ll need to redo the job sooner that you’d want to. To be fair, there is “waterproofed” OSB on the market, but you’ll still need to treat the cut edges. Otherwise, the boards will come apart at the seams. If the choices are plywood or OSB for your shed floor, the plywood wins every time.

Reinforced Concrete Slab

With a proper moisture barrier, this can be the best floor you could build! Usually anything under 10’x12’ or so can be done by a single person. You might need a helper if you want to go bigger than this. It’s pretty straightforward too, especially if you’ve laid a foundation before. A properly done 4 inches thick reinforced concrete slab is going to last you longer than any other option.

So why wouldn’t this be the #1 recommendation?

Because it’s expensive and a lot more work. And usually it’s overkill.

It also makes the shed a very permanent structure. If you ever want to tear it down or move it, you’d have several tons of material to get rid of. For a high-end solution that lasts (nearly) forever, go with the concrete option. You can check out this post about pouring concrete slabs to get an idea of the work that goes in to this project. It might be more than you’re up for.

For a more common, cheaper, and easier to build shed, use plywood.

Building a Sturdy Shed Floor from Plywood

Here are the best practices for building a wood shed floor from scratch:

Clear and level the area that you want to build your shed.

Adding gravel “pads” under the deck blocks can be a good idea if your soil is soft. This will help to spread the load and prevent the shed floor from becoming unlevel in the future. Use concrete or composite deck blocks to raise the structure off the ground and away from moisture. Make sure that everything is level using a laser level. If you don’t have one you can also use string and a line level. Start framing the floor! 2×6’s are a great, sturdy option that will take a lot of abuse. For a smaller shed (like 6’x6’) 2×4’s will usually work just fine. Make sure they’re pressure treated, and keep the joists 12” apart on center. Use a preservative, like Coppercoat, to protect the cut ends from moisture and insects. Use Simpson strong drive screws. They are coated to prevent them from being eaten by the copper in the pressure treated solution. Use 3/4 inch CDX pressure treated plywood for the floor. Keep the “D” side (the side with more imperfections) facing down.

How Much Should It Cost to Build a Shed Floor?

The cost is going to depend on the size of your shed and the local prices of materials.

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For the sake of reference, let’s run through a scenario.

Let’s plan a big 10’x12’ shed, using deck blocks and 2×6’s as outlined above. 10’x12’ is about the max size you’d want to build before changing the plan to a more permanent foundation that reaches below the frost line. We’ll build it with a grid of 12 deck blocks, so the weight is well distributed and it’s less likely to sink to one side. Where I’m from, the soil has a lot of clay. That means that putting the deck blocks directly on soil won’t be a problem. If you’re in an area with softer soil, or if it’s in a wetter area, you might need more deck blocks, in addition to preparing the area with gravel. Alternatively, using pilings that go below the frost line is a great solution. I don’t like to spend more than I need to, though. I can get concrete deck blocks for about $7 a pop from Lowe’s. $7 x 12 = $84. Next, the 2×6’s. For simplicity’s sake, I’ll get 12’ lengths and space them 12 inches apart. That means I’ll need 13 for joists and 2 more for the sides. 12’ long pressure treated 2×6’s are about $13 each in my area. $13 x 15 joists = $195. A small can of Coppercoat to protect the cut ends of the 2×6’s is about $14. For the plywood, I’d need four 4’x8’ sheets total, and I should have a 4’x2’ piece left over. 3/4 inch pressure treated CDX goes for about $40 each where I am. 4 x $40 = $160. For screws, a 1 lbs pack of 3 inches long #8 coated screws will cost around $10, I’ll use these for the framing. Three screws per connection. For screwing down the plywood, I’ll use a pound of 2.5 inches long #7 coated decking screws, which will be another $10 or so. $10 + $10 = $20.

For the total price of the shed floor:

$84 + $169 + $14 + $160 + $20 = $447 total.

That’s really not bad for such a large shed. 10’x12’ is a lot of room, and the average person could probably get by just fine with a shed half that size. Beyond just being a storage space for tools, this could easily include a little work area. I’d round that up to an even $500 to include a reasonable fudge factor. I always find that if I budget exact amounts, I’m guaranteed to go over every time. Prices are going to vary by area and season, so you might be able to score some good deals and get it for cheaper. This will get you in the ballpark though.

Conclusion

Building a shed floor is actually pretty straightforward. For many handymen, it’s a doable job in a day and a half, from picking up the materials to fastening the final screw. Using 3/4 inch thick pressure treated plywood for your shed floor is the most economical choice and will last you 20+ years if done well.

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