How to Build a Shed Floor: Joists and Plywood

To build a shed floor, you make a rectangular frame of two rim joists and a row of joists, set it on skids or beams, square it, and cover it with tongue-and-groove plywood. On the sheds our designer draws, the frame is pressure-treated 2x6 and the deck is 3/4 inch plywood, glued and screwed down.

This guide covers the frame, the joist size and spacing, the span check, the plywood, and how to keep moisture out. The counts come from the shed designer, so they match the cut lists on our free plans. The floor sits on a foundation, so start with the shed foundation guide if you have not picked one, and see the how to build a shed guide for the whole order of work.

What a shed floor frame is made of

The floor frame is a simple platform. Joists span across the width, two rim joists close the ends, and the plywood ties it all together into one stiff deck.

Rim joists and joists

The designer’s floor has two rim joists that run the depth of the shed, and joists that run across the width between them. Each joist is the shed width minus 3 inches, because two 1-1/2 inch rim joists take up the ends. On a 10x12 that is 10 joists of 117 inches between two rim joists of 144 inches. A 2x6 is 1-1/2 by 5-1/2 inches in real size (Southern Pine Inspection Bureau, citing PS 20-20), which is why 5-1/2 inches shows up in every height in this guide.

The rim joists do a second job. The 2021 IRC, R502.7, asks for the ends of joists to be held from rotating, either by solid blocking or by attaching them to a full-depth rim or band joist. The rim joist is that piece. R502.6 also asks for at least 1-1/2 inches of bearing on wood. The code speaks about house floors, and the locally adopted edition and amendments decide, but the frame works the same way.

Why the frame is pressure treated

The designer lists every piece of the floor frame as pressure-treated lumber rated for ground contact, and the 4x6 skids under it as well. The Preserved Wood use-category sheet defines UC4A as “Ground Contact, General Use”. It also says sawn components installed less than 6 inches above ground are to be treated to UC4A or higher. The 2021 IRC, R317.1, asks for treated or naturally durable wood where floor joists or the bottom of a wood structural floor sit closer than 18 inches to exposed ground. The joists on the designer’s skid floor sit 5-1/2 inches above the pad. Treated is the right call.

Two details matter when you work with treated lumber. Field-cut ends, notches and drilled holes need treatment on site (2021 IRC, R317.1.1), and fasteners must be hot-dipped galvanized, stainless steel, silicon bronze or copper (R317.3.1). Use exterior-rated hardware for the whole floor.

Skids or beams underneath

The frame needs something under it. On skids, the designer lays 4x6 pressure-treated skids on edge, front to back, in 3 to 5 lines depending on width, spaced up to about 4 feet apart. On blocks, it uses 4x4 pressure-treated beams on concrete blocks. Our shed on skids guide covers how many skids, what size and how far apart. The concrete block shed foundation guide covers blocks and beams. Our gravel shed base guide covers the pad the skids sit on, which the designer sizes at 4 inches thick and 12 inches past the footprint on every side. If you pour a slab, the slab is the floor and this frame is not needed: see the shed base guide.

The joist layout is the same on either support. On the designer’s 10x12 with blocks and beams, there are still 10 joists of 117 inches with the same span check. Only the height changes: the foundation is 11-1/2 inches with blocks and 5-1/2 inches with skids.

Shed floor joist spacing and span

Joist spacing is the distance from the center of one joist to the center of the next. The designer offers 12, 16 or 24 inches, and 16 is the default. The span is the unsupported distance a joist crosses, and on a skid floor that is the gap between skid lines.

The span check

The designer checks each floor joist as a simple beam, for 2x6 SPF No. 2 at 50 psf live load and 10 psf dead load, with deflection limited to L/360. The 2021 IRC, Table R301.7, gives L/360 for floors. The allowable spans of a 2x6 follow. The last column is a control: the 2012 IRC table for 40 psf, read from a city handout (City of Republic, MO).

The designer’s numbers land within about 3/16 inch of the older code table at 40 psf. Its skid lines sit about 38 to 47 inches apart, so the span it actually checks is 3’ 2-13/16” on a 10 ft wide shed and 3’ 10-13/16” on a 12 ft wide one. That is well under half of what a 2x6 is allowed. These are planning checks with beam math, not engineering, and they ignore a heavy machine parked on one wheel.

12, 16 or 24 inches on center

Here is what each spacing changes on two designer sheds. Sheets, screws and adhesive do not change, because the plywood covers the same area.

All six layouts pass. The 12x16 at 24 inches has the least room, and the designer still reports 90 percent to spare on the span. At 12 inches you buy 3 or 4 more joists than at 16, and closer joists are commonly felt as a stiffer floor under point loads (general practice). At 24 inches you save lumber and nailing, and the plywood must carry more between joists (see the plywood section).

The cheapest way to frame it

The cheapest floor is the one with fewer members that still passes. On the 10x12, moving from 16 to 24 inches on center drops 3 joists and the check still passes. Keep the 2x6 size, the treated lumber and the 3/4 inch plywood: those are the parts that keep the floor dry and stiff. Do not cut supports to save money. The gap between skids is what the span check is built on.

2x4 or 2x6 joists

The designer never draws a 2x4 floor. The same span arithmetic, at 50 psf and 16 inches on center, gives a 2x4 about 66 inches, against about 104 inches for a 2x6. Both clear the 38 to 47 inch gap between skid lines on paper. A 2x6 is the size the designer draws, cuts and counts, it has a deeper section, and it is what the designer’s numbers describe. Buy 2x6.

Joists and sheets for every gable size

The table below is the designer’s floor for the 16 gable presets, with skids and joists at 16 inches on center. Joists are 2x6 pressure treated, and the rim joists are two boards. “Sheets” are 4x8 sheets of 3/4 inch tongue-and-groove plywood. The screws are 2 inch exterior screws, and the tubes are construction adhesive. The last column is the span the check uses.

Every row passes against an allowable 8’ 7-13/16” at 16 inches on center. The 10x12 gable plan and the 12x16 gable plan list the boards to buy, including the lengths.

Reading the table

Joists run across the width, so their length depends on the width, and the rim joists depend on the depth. Two sheds with the same width have the same joist length. The joist count follows the depth: at 16 inches on center it is the depth divided by 16, rounded up, plus one. A 10x12 is 144 divided by 16, which is 9, plus 1, so 10 joists. The sheet counts are the designer’s cut plan with the long edge across the joists and the end joints staggered, and your own layout can differ by a sheet.

Fasteners for the frame

For nails, the designer counts 16d common nails, 3 per joist end through the rim joist, following the 2021 IRC fastening schedule in Table R602.3(1). It counts 10d box nails, 3 per joist, toenailed into each skid. All of these must be galvanized for treated wood. For the deck it lists 2 inch exterior screws at about 50 per sheet and one tube of adhesive for every 2 sheets.

Build the floor frame step by step

The steps below are general practice around the designer’s numbers. The instructions with your lumber, hardware and adhesive override them.

Cut, mark and assemble

1. Cut the parts. Cut the joists to the shed width minus 3 inches. Cut the rim joists to the shed depth. Take the length from the designer’s cut list, not from memory.
2. Mark the layout. Lay the two rim joists side by side and mark the joist positions on both at once. The first joist sits at the end, and the rest follow at 12, 16 or 24 inches on center.
3. Nail the frame together. Stand the rims on edge, set each joist at its mark and drive the nails through the rim into the joist end. Use the hot-dipped galvanized or stainless nails the code asks for on treated wood.
4. Treat the cuts. Brush preservative on every cut end you made, as the model code asks (2021 IRC, R317.1.1).

Square it

5. Check the diagonals. Measure corner to corner both ways. If the two lengths match, the frame is square. If they do not, push the long diagonal’s corners together until they do.
6. Set it on the skids. Slide the frame onto the skids, check that it is level and flush, and toenail each joist to each skid. The designer counts 3 nails per joist at each skid.
7. Anchor it. The designer lists 4 ground anchors for a skid floor, one near each corner. See the how to anchor a shed guide for the how.

Squaring comes before the plywood, and the plywood then locks the shape in. A frame that is out of square makes the wall framing on top fight you. The frame shed walls guide is the next step.

Plywood shed floor thickness and the deck

The deck is the part you walk on and stack shelves against, so it needs to be stiff between joists.

How thick

The designer uses 3/4 inch tongue-and-groove plywood in 4x8 sheets. APA’s floor guide (Form E30W) lists a maximum span by panel thickness in its subfloor table: 15/32 and 1/2 inch to 19.2 inches, 19/32 and 5/8 inch to 24 inches, and 23/32 and 3/4 inch to 32 inches. So 1/2 inch plywood is on the table for joists at 16 inches, but the same guide adds that heavy traffic and concentrated loads can require construction in excess of the minimums shown. A shed floor holds mowers and workbenches. Stay with 3/4 inch, and go there for joists at 24 inches.

The panel span rating is printed on the panel, and it is a rating for the long side across the joists (APA). The 2021 IRC, R503.2, asks for a grade mark on structural panels and sends you to Table R503.2.1.1(1) or APA E30 for span. We did not read the row for a 3/4 inch panel in that code table, so read the stamp on the panel and ask your building department.

Pressure treated plywood for a shed floor

The frame is treated. Whether the plywood must be is a fair question. The 2021 IRC, R317.1, asks for treated wood where “the bottom of a wood structural floor” is closer than 18 inches to exposed ground in crawl spaces or unexcavated areas. On the designer’s skid floor, the underside of the deck is about 11 inches above the pad. The designer lists ordinary 3/4 inch plywood, not treated. Whether your local code counts a shed’s floor as one of those cases is for your building department to say. If it asks for treated plywood, or you want the extra protection, the Preserved Wood use-category sheet shows plywood is treated to different categories: UC2 for a damp sub-floor above ground, UC3B for exterior above-ground plywood and UC4A for ground contact. Ask for the category tag on the panel.

The panel also has a glue rating. APA says Exposure 1 or Exterior panels have suitable bond performance for moisture during or after construction, and that long-term weather exposure calls for Exterior panels. One OSB maker, Huber, says its Exposure 1 subfloor is not suitable for permanent exposure. So keep the deck covered soon after you lay it.

Glue, gaps and screws

APA’s glued floor system uses adhesive plus fasteners for a stiffer, quieter floor. Its steps, in short:

• Tongue-and-groove panels. APA says they remove the need for blocking along the long edges.
• Adhesive. Use one that meets ASTM D3498 or APA AFG-01. Run a continuous line about 1/4 inch across on the joists, and two lines where panel ends meet on one joist.
• Start. Lay the first panel with the tongue side to the wall.
• Gap. Leave a 1/8 inch space at every end joint and edge, including the tongue-and-groove joints. Huber says the same, and suggests a 10d nail as a spacer.
• Stagger. Offset the end joints from row to row, and lay the long edge across the joists.
• Fasten fast. Drive every fastener before the adhesive sets.

For screws, Huber’s manual asks for fasteners 3/8 inch from the panel edge and at least 1 inch into the frame, and lists a #8 screw for 23/32 inch panels. It spaces fasteners 6 inches on supported edges and 12 inches at the intermediate supports. A 2 inch screw through 3/4 inch plywood goes 1-1/4 inches into the joist, which meets that 1 inch minimum. The manual is for one OSB panel, so follow the maker’s sheet for yours.

Waterproof shed floor: moisture, vapor barrier and paint

No shed floor is waterproof in the sense of a boat hull. The goal is to keep water from reaching the wood and to let the wood dry when it does get wet.

Keep water from the bottom

The designer’s height helps: 5-1/2 inches of skid, 5-1/2 inches of joist and 3/4 inch of plywood put the top of the floor at 11-3/4 inches above the pad. A block floor lifts it to 17-3/4 inches. Treated framing, a gravel pad that drains and open air under the joists do the rest. The gravel shed base guide covers drainage. The height also sets how long a ramp is: see the shed ramp guide.

Vapor barrier under a shed floor

The designer lists no ground cover under the floor. Huber’s manual says crawlspace foundations shall have a durable ground cover vapor barrier. A shed on skids over gravel is not a crawlspace, and we did not read a source that says a shed needs one. On a concrete slab, the 2021 IRC, R506.2.3, exempts garages, utility buildings and other unheated accessory structures from its vapor retarder rule. Common practice varies: some builders lay plastic on the pad, and others avoid it because water can pool on top. If you use one, the pad must still drain, and your local code has the last word.

Shed floor paint and finish

Paint does not make the floor waterproof. Common practice is to paint or seal the top once the deck is dry, and APA says a floor that got wet during construction should dry before a finish floor goes on. Match the finish to the panel. Huber’s manual for its OSB lists oil-based paints, primers and stains and two-part epoxy garage floor coatings, and says not to use water-based products or water sealers on it. For a vinyl or rubber covering, APA says always use a plywood underlayment under a thin resilient finish floor. Read the label of the paint and the panel before you buy either.

Where the floor fits in the build

The floor is step one above the ground, and everything on top of it depends on it being flat and square. To try your own size, open the shed designer with joists at 24 inches, or see the 12x16 gable plan. Check your building department before you build: thresholds and the code edition are local, and the shed permits guide explains how to ask.