Storage sheds require strong floors to support the weight of the shed itself, plus any items you store and the people who enter the shed. Spacing your floor joists — the beams that support your flooring — on 12-inch centers ensures a strong floor that will meet or exceed building codes in nearly any area. Always check your local building codes before beginning a construction project to ensure your work meets safety and zoning regulations. Many areas require building permits for shed construction.
1. Determine your shed’s dimensions. Your floor joists will run in the shorter dimension if your shed is rectangular, referred to here as the width. For example, in a 6-by-8-foot shed, the floor joists will run the 6-foot width while the end joists — the end framing, also called band boards– will run the 8-foot length.
2. Measure and cut all of your floor joists to 3 inches less than your shed’s width. In a 6-by-8 shed, this means you will need 8 joists — one for each foot of length — cut to 7 feet 9 inches.
3. Cut your two end joists to the length of your shed — in this case, 8 feet.
4. Lay out three 4-by-4-inch pieces of lumber that are the same length as your shed, spacing them so two rest where the ends of your shed floor will be — in this case, with the outer edges 6 feet apart — and the third is in the middle. Line up the ends of the three pieces exactly, and then mark each foot along this lumber. Do this on the leveled area where you want the shed so you don’t have to move the unit later. If your shed is more than 8 feet wide, add another 4-by-4 for every 4 feet.
5. Set your band boards on the outside edge of your outer two 4-by-4s, on edge so the 4-inch face is vertical. Screw the band boards to the joists by driving deck screws in at a downward diagonal.
6. Lay out your joists between the band boards on your 1-inch marks.
7. Screw the frame together by driving deck screws through the band boards into the ends of the joists.
8. Lay out sheets of 3/4-inch plywood so the grain runs perpendicular to your joists. Cut the plywood to the size of your frame with a circular saw. For a 6-by-8 shed, you need two 4-by-8-foot plywood sheets, with one trimmed down to 2-by-8 feet.
9. Screw the plywood to your joists with a wood screw every foot along the edges and every 16 to 24 inches along the joists in the middle of the boards. The floor is complete once your plywood is attached.
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