Floor frames for sheds are simple versions of house floor frames. They have outside, or rim, joists that are set on edge and nailed to the ends of the common joists. Gazebos have floor frames similar to decks, with angled joists that are connected to support beams with joist hangers. On top of floor frames, a layer of tongue-and-groove plywood (or decking boards, for a gazebo) provides the floor surface and adds strength to the frame. To prevent rot, always use pressure-treated lumber and galvanized nails and hardware for floor frames.

TOOLS & MATERIALS
Circular saw	8d and 16d galv. common nails
square	3/4" tongue-and-groove exterior-grade plywood
pressure-treated 2 × lumber

HOW TO BUILD A SHED FLOOR FRAME

Step A: Cut the Joists & Mark the Layout

1. Cut the two rim joists and the common joists to length, making sure both ends are square. Note that rim joists run the full length of the floor, while common joists are 3" shorter than the floor width.

2. Check the rim joists for crowning—arching along the narrow edges. Pick up one end of the board and hold it flat. With one eye closed, sight down the narrow edges. If the board arches, even slightly, mark the edge on the top (convex) side of the arch. This is the crowned edge and should always be installed facing up. If the board is crowned in both directions, mark the edge with the most significant crowning.

3. Lay one rim joist flat on top of the other so the edges and ends are flush and the crowned edges are on the same side. Tack the joists together with a few 8d nails. Turn the joists on-edge and mark the common joist layout on the top edges: Mark 1 1/2"" and 15 1/4" from the end of one joist. Then, measuring from the 15 1/4" mark, make a mark every 16"—at 32", 48", 64" and so on, to the end of the board (if the plan calls for 24" spacing, make a mark at 1 1/2" and 23 1/4", then every 24" from there). Don't worry if the last space before the opposite end joist isn't as wide as the others. Make a mark 1 1/2" in from the remaining end. After each mark, draw a small × designating to which side of the line the joist goes—this is a handy framers' trick to prevent confusion. This layout ensures that the edges of a 4-ft. or 8-ft. board or sheet will fall, or break, on the center of a joist.

4. Using a square, draw lines through each of the layout marks, carrying them over to the other rim joist. Draw Xs on the other joist, as well. Separate the joists and remove the nails.

Step B: Assemble & Square the Frame

1. Check the two end joists for crowning, then nail them between the rim joists so their outside faces are flush with the rim joist ends and the top edges are flush. Drive two 16d galvanized common nails through the rim joists and into the ends of the end joists, positioning the nails about 3/4" from the top and bottom edges.

2. Install the remaining joists, making sure the crowned edges are facing up.

3. Check the frame for squareness by measuring diagonally from corner to corner: when the measurements are equal, the frame is square. To adjust the frame, apply inward pressure to the corners with the longer measurement.

4. If you're building the floor over skids, secure each joist to the outside skids with a metal anchor and toenail the joists to the internal skid(s) with 16d galvanized nails.

Step C: Install the Plywood Floor

1. Lay a full sheet of 3/4" tongue-and-groove exterior-plywood over the frame so the groove side is flush with a rim joist and one end is flush with an end joist. Fasten the plywood to the joists with 8d galvanized nails driven every 6" along the edges and every 8" in the field of the sheet. Do not nail along the tongue edge until the next row of plywood is in place.

2. Cut the second piece to fit next to the first, allowing for a 1/8" gap between the sheets. Install the second sheet with its outside edges flush with the frame.

3. Start the next row with a full sheet (ripped to width, if necessary). Install the sheet starting from the corner opposite the first sheet, so the joints between rows are offset. Make sure the tongue-and-groove joint is tight; if necessary, use a wood block and a sledgehammer to close the joint.

4. Cut and install the remaining piece of plywood.

Mark the joist layout onto the header or beam and use a square to draw parallel lines that represent the outer edges of the joist. Position the hanger with one side flush to a joist outline, and fasten it with joist hanger nails (1 1/2") or 10d or 16d galvanized common nails.

Using a scrap piece of joist as a spacer, close the hanger around the spacer. Check the position of the spacer's top edge, then nail the free side of the hanger in place.

To install the joist, set it completely into the hanger and fasten it through the hanger holes with joist nails.