If you live in an exceptionally wet area or have a major termite problem you might want to use pressure treated wood for the entire floor including the skids, the floor joists and the plywood sheeting.
This will add to the initial cost of your shed but it might also save you money in the long run because a floor built with pressure treated lumber will last longer in these areas.
Prepare your materials… See Table 1 and Table 2: Get the number of floor joists you will need from Table 3, and the cut length for the band boards and floor joists and skid spacing from Table 2.
See Figure 2: Cut your band boards and floor joists and lay out the band boards for 16 inches on center and drill two pilot holes for each 2x4joist, or three holes for each 2×6 joist.
If you are building an 8 wide and using 2″x4″x92 5/8″ precuts then you can use them here without any cutting. They will be 3/8″ short but this will be split in half to 3/16″ on either side when you cover the floor with plywood. You will save a few dollars over 2″x4″x8′ers and no one will be the wiser.
10 and 12 wide sheds use 2″x6″ joists so you will have to cut them to length.
Assemble the frame: Lay the joists out on top of the pressure treated skids, spacing them about 16 inches apart.
Attach the band boards to the joists with two 3 inch screws or 16d nails at each joist.
Attach the skids: Position each skid as per measurement “D” from Table 2. Then drill pilot holes and toe nail both sides of the floor joists to the skids with 3 inch screws or 16d nails.
Except the front and rear joist.
Leave these two free so you can manipulate them to match the outside edge of the plywood sheeting as you nail it.
Square: Check the square by comparing diagonal measurements across the floor. Slid a skid back or forth until the measurements are equal.
Now the floor is square.
Level: Level the floor in all directions with a builders level. Add or remove a little dirt or gravel under the skids until the floor is level.
If necessary place a concrete block under each skid every 48 inches or less. Add blocks or wedges between the blocks and skids until the floor is level in all directions.
Recheck the square by comparing the diagonal measurements again.
Any extra effort you spent now to carefully level the floor will pay off when you start sheeting the roof. If the floor is not level, the roof sheeting will not fit properly and you might have to re-level at that time.
Tie downs: If you want to attach tie downs to the floor of your shed, now is the time. Attach them to the floor joists rather than the skids for an extra measure of security.
Nail the sheeting: Lay your best sheet of plywood along the front of the floor frame.
Square it up to the front corners and rim joist and nail along the front edge. Check the side edges for square along the frame and nail every 8 inches with 8d nails.
Repeat with the second and third sheet, putting the worst sheet at the back of the floor.
Extra blocking on 10 & 12 wide floors: If you’re using regular CDX plywood as opposed to tongue and groove you should provide some extra support where the full pieces of plywood meet the cut pieces.
To make these blocks cut 12 inches off the ends of some of the 2×4′s that you will be using for studs. You will need one less than the number of floor joists you have. These blocks will fit loosely, but that’s ok.
Attach them to the underside of the plywood joint with three or four 2 inch screws. Leave about half of the block exposed.
After you nail the smaller pieces of plywood in place then put screws on the other side of the joint to match.
Nail the center of your sheeting along the chalk lines every 8 inches.