How To Calculate Materials Needed For A Shed

Shed construction isn't rocket science, and doesn't have to be complicated. Designed to keep tools and goods out of the weather, a shed should be basic and affordable. Calculating materials is the first step for pricing, and to make choices.

Consult local building codes before you design your shed. Permitting — if required — should be at the top of the list.

Door options might include a double-door, sliding door, walk-through door or a piece of plywood. Doors are expensive. Building one out of plywood and 2-by-4 lumber is affordable and saves you money.

Common shed sizes of 8 by 12, 8 by 16, 12 by 16, or even 16 by 16, are not by accident. The sizes are calculated with affordability and waste in mind. Materials, such as plywood, sheathing, corrugated metal and plastic panels, are typically dimensioned in multiples of 48 inches. Framing is spaced with the same concept. Designing, calculating and building a shed with this in mind prevents you from buying more than you need, because everything fits without trimming.

Block Footings

The basic shed foundation may rely on the same principal as a deck foundation. Deck blocks with 4-by-4 holes will suffice to support the deck. Use two of them for support under horizontal beams of 6 feet or shorter. Add a center block for anything longer.

Alternatives to deck blocks include 4-by-4, 4-by-6, or 6-by-6 pressure-treated posts. This type of post can come into direct contact with the ground without rotting as fast as untreated woods. Depending on the load or size of shed, two of them parallel to each other is typically sufficient to serve as a base for the platform or floor.

For the sturdiest shed, plan on building the platform with treated 2-by-6 posts, 24 inches apart, or standard 2-by-4 studs for a lighter-duty shed. Calculate by area and add enough 3/4-inch, CDX plywood to cover the platform. This type of plywood has allowed defects, such as knots and patches, but it's strong and reliable. The C side — the face — is better than the D side. The X means that it has exterior glue. If you've followed the sizing recommendations — 48 inch increments — for shed dimensions, the 4-by-8 sheets should fit perfectly.

Calculate how many studs you'll need by dividing the number you'll need over the horizontal length of the wall, 16 inches apart. Add top and bottom horizontal studs that surround the perimeter on all four sides to form the walls.

Weather-resistant, 5/8-inch, CDX plywood is a good choice for affordable sheathing. Another option is T-111 siding. It consists of a 4-by-8, composite, textured sheet made to look like wood boards. The number of sheets needed are easy to calculate if you're using the sizing recommendations of 4-foot dimensions. If not, divide the width and length of the shed by 48 inches. Add another piece for each of the gabled, or angled ends.

Roof support joists are probably the most complicated part of your calculation. Skip the complicated math and draw or construct a full-sized mock-up using scrap lumber to get the pitch. Count each piece needed for a single joist, including some scrap plywood needed to join the mitered — angled — braces. And don't forget to add two metal brackets or hangers for each joist.

Sub-Roof

Exterior-grade, 5/8-inch , CDX plywood — the same type as the flooring, but thinner — is a good choice for sub-roof sheathing. A cheaper alternative might be oriented-strand-board. Both come in 4-by-8 sheets. Use the mock-up of the joist, plus the overall area calculation for the number of sheets.

Corrugated panels are typically 24 to 36 inches in width. Using the figure for the sub-roof, calculate how many panels you'll need for each side. Divide by the width needed for the total and add at least 12 inches to the length to allow for the pitch.

Trim is optional, depending on how much you want to spend. Use 3/4-by-4-inch cedar for high-dollar trim and 3/4-inch fir or pine for cheaper trim. Another option is composite trim, which is the most affordable. Add enough to go around the perimeter at the top, and one piece for each side, splitting it in the middle on corners.

Use screws for everything. It's not necessary to count them. Purchase a small box. If you run out, get another one; if you purchase too many, you can always use extra screws. Use 3-inch screws for framing and 1 5/8-inch for siding, OSB or plywood. Use 1-inch sheet metal screws with rubber grommets to seal out water on corrugated panels.

Do a rough, square foot calculation for area. Using this figure, refer to the labels on sealing or paint products when purchasing.