How to Build a 20 x 20 Shed? | From Foundation to Roof

A 20×20 shed is 400 square feet of new space, and the build follows six steps: plan, foundation, floor, walls, roof framing, and roofing.

For the full breakdown, see our best 20 x 20 Sheds guide.

A 20 x 20 shed adds 400 square feet of storage, workshop, or garage space — but at this size, local building codes start paying attention in ways they don’t for a small backyard unit. The sequence for how to build a 20 x 20 shed runs from site prep to the final roof covering, and the order matters as much as the materials. Two choices shape everything after them: the foundation type and the roof style.

Building a 20×20 Shed: The Step Order That Works

Every wood-framed 20×20 shed follows the same six stages in the same order: plan, foundation, floor frame, walls, roof frame, and roof covering. Work through them in this sequence and each stage gives the next one a square, level surface to build on.

1. Plan and permit first. Confirm local setbacks and code requirements before you choose the site — a structure this size can land in a stricter permit class than a typical backyard shed. If you’ll add electricity or plumbing, plan those systems now; they follow a separate code path.
2. Prep the site. Clear debris and vegetation, level the ground, and mark the foundation perimeter with stakes and string. Excavate for footings if your foundation type calls for them.
3. Build the floor frame. Many 20×20 plans call for a pressure-treated 2×8 perimeter with 2×6 or 2×8 joists at 16 or 24 inches on center. Square the frame by comparing corner-to-corner diagonals — when both measurements match, the corners are true — then set it on concrete deck blocks or 6×6 skids, check level, and add floor sheathing.
4. Frame the walls. Use 2×4 studs at 16 inches on center, and build window and door headers from 2×6 lumber with 1/2-inch plywood sandwiched between, fastened with 2 1/2-inch screws.
5. Frame the roof. A typical 20×20 gable plan uses a 6/12 pitch, a 20-foot span, and 2×6 rafters at 24 inches on center. Cut the rafters, collar ties, and ridge board to the plan, then cover them with plywood sheathing.
6. Finish with roofing and siding. Install shingles or metal over the sheathing, then add the exterior cladding — T1-11 is common, fastened with 6-8d nails every 8 inches and fitted with no gaps.

What a 20×20 Plan Actually Specifies

Published 20×20 plans differ on foundation and framing details, but the specifications below come from current plan documents and make a reliable baseline for ordering materials.