Things You'll Need
Post hole digger
4-by-4-by-8-inch concrete blocks
1-by-4-inch sheathing boards
Bent metal flashing
1-by-2-inch batten boards
2-by-6-inch door header
Attaching a shed to a garage can save space, construction time and money. It eliminates the need for one wall. Attached sheds can be built like standalone sheds and finished with siding to match the garage or they can be basic lean-tos that provide a place for wheelbarrows, lawnmowers, gasoline cans and other tools or materials that take up valuable garage space or need to be stored in a more open environment. Some garage sheds are left open on one or both ends to provide easier access.
Plan the shed. Decide how big to make it and which garage wall to put it on. Build a simple storage shed with post-and-beam construction, concrete block flooring, corrugated roofing and board-and-batten siding that can be painted to match the garage. Attach the shed to the existing siding of the garage, assuming that is wood.
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Lay out the shed dimensions along the garage wall using a tape measure. Mark a spot for posts at corners and at least every 8 feet between. Dig holes with a post hole digger and set 4-by-4-inch posts in concrete, checking with a level to make sure they are plumb in all directions and the same distance from the garage wall. Set the posts about a third of the finished height; use 12-foot posts set 3 feet deep to create a 9-foot roof with an 8-foot interior. Let the concrete dry.
Nail a 2-by-4-inch ledger or support board horizontally on the garage wall a few inches higher than the top of the posts; vary this with the width of the shed, a wider shed requiring more elevation. Nail it with a hammer and framing nails to the studs in the garage wall. Check the ledger with a level. Nail 2-by-4 boards on top of the posts and other 2-by-4s vertically to the garage wall to match the 4-by-4 posts, with the 4-inch side to the wall.
Make a floor of solid 4-by-4-by-8-inch concrete blocks. Dig out the floor area 8 inches deep, lay down 3 inches of compacted gravel and an inch of sand. Place landscape fabric over the dirt if weeds are likely to be a problem. Set blocks in the sand and level them. Use half blocks to adjust the flooring around the posts and to stagger the joints of the full blocks. Sweep dry cement into the gaps with a broom and dampen it with a garden hose to set the flooring solidly.
Add 2-by-4 braces to connect the posts and the wall boards, nailed to the sides 8 feet above the floor. Put boards on both sides of the posts and wall boards for extra strength. Make rafter braces, 2-by-4s nailed to the top of the ledger board and the 2-by-4 cap on the posts. Don't bother to try to notch these unless the roof slope is severe; just nail flush for a slope of an inch or two.
Put 1-by-4 sheathing strips on the walls and across the rafter boards. These are boards nailed horizontally at the tops and bottoms of the posts and in the center of the wall. Place them end to end across the roof.
Install corrugated metal for a durable and inexpensive roof, nailed to the strips with valleys running away from the garage. Attach roofing to the garage wall with bent metal flashing that goes up the wall and over the roof; seal the top with clear caulk. Cover the walls with board and batten, 4-by-8-foot sheets of plywood nailed vertically to the sheathing strips, with the seams covered by 1-by-4-inch batten boards. Cut plywood with a circular saw to fit the triangular opening at the top of the shed ends.
Make a door on one end. Build a rough frame with a 2-by-6-inch header board at the top of the door opening with a full 2-by-4-inch stud on the outside and shorter studs from the bottom of the header to floor level. Set that frame into the shed opening. Secure it to the top wall brace and with a horizontal base plate nailed to the concrete blocks on each side with concrete nails. Add studs and horizontal braces on either side of the door if necessary.