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How to Build a Garden Shed Addition

Updated: Feb. 22, 2023

Plenty of storage for outdoor tools and toys, only a step away from your back door

How to Build a Garden Shed AdditionFamily Handyman
This spacious cedar garden shed has tons of storage space for lawn and garden tools plus shelves for supplies, a potting bench and windows. It's close at hand too, since we attached it to a garage wall. An outdoor overhang shelters the potting bench. Or use it as a parking place for the barbeque. The inexpensive barn sash windows let in plenty of light and ventilation, so you can easily find what you need or even work inside. Anyone with a green thumb, energetic kids or a big yard will love it.

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Click the links below to download the construction drawings and material list for the 2000 shed.

2000 Shed Construction Drawings

2000 Shed Material List

Project overview

Spacious interior storage

This shed measures 7-1/2-ft. x 13-ft., giving you about 100 sq.ft. of storage space.

Built-in shelving

Build in shelving for all those little items. The windows bring in a flood of light.

Sliding door access

The sliding door opens wide enough for wheelbarrows to roll through.

Even though we’ve engineered this shed to keep complicated figuring and cutting to a minimum, it’s still a big construction project that will take you at least five or six weekends to complete. If you’ve built a deck or similar structure, you’ll have no trouble putting in the foundation and building the walls. To help you through the trickier roof-framing stage, we’ll show you how to make a rafter pattern without using any complex math. You may have to modify our plans slightly to fit your house, but all the basic building techniques we show will be the same.

You won’t need any special tools for this project. You probably already own most of the basic carpentry tools you’ll need to build this shed. Besides hand tools like a hammer, tape measure, square, utility knife, chalk line, sharp chisel, handsaw and a few screwdrivers, you’ll need a 4-ft. level, a line level (Photo 2) and a power drill, plus the bits listed in the story. A circular saw will work for most of the cutting, but the windowsill and drip cap (Fig. C) require bevel cuts that would be easier to make with a table saw. If you don’t own a table saw, ask a woodworker or a full-service lumberyard to cut these pieces for you. A power miter box is another optional tool that would add speed and accuracy to your cutting, especially for finish work. You can rent a power miter box, but for a project this big, I’d recommend buying one.

Step 1: Pick a site and order materials

Shed added to garage

We extended our garage roof to cover the shed as well. But you can also butt the shed roof into a garage wall.

Scope out the perfect location.

With its simple roof style, this shed can go just about anywhere. Look for a spot on the back of your house where windows and doors aren’t in the way. Behind or to the side of the garage is a good location. We tied our shed roof into the garage roof, but you can also butt the shed roof against a wall, as long as you install metal flashing under the siding and over the shingles where the roof and wall intersect.

In addition, consider: Ground level. The ideal site is flat and sloping slightly away from the house. If the ground slopes steeply, either toward or away from the house, your job will be a lot harder. You’ll have to excavate and build retaining walls or bring in fill. In any case, regrade around the shed to ensure good every drainage and provide a ramp for your lawn mower and wheelbarrow. Roof pitch and headroom. This is the trickiest stage of planning. Our garage has 9-ft. tall walls and a shallow pitched roof. This combination allowed us to continue the roof in a straight line and still have enough headroom at the outside shed wall for standard-height windows and doors. But your house/garage might be different. To ensure adequate outer wall height, stretch a string along the roof and brace a 2×4 temporarily at the location of the outside wall to figure out how much headroom you’ll have (basically the same procedure shown in Photo 9 for making the rafter template). Measure from the string representing the roof down to what will be the top of the 6×6 foundation beam. If this distance isn’t 90-3/4 in., you’ll have to modify the wall height or change the roof slope of our shed plan.

Then call your local building inspections department to find out if your shed location is OK and what’s required to get a building permit. Most cities will accept a plan drawn to scale on 1/4-in. graph paper if it includes all the structural details.

Every region has a few unique building requirements. In cold Northern climates like ours, deep footings are required to prevent frost heaving. In areas with high winds or earthquakes, you’ll need special metal framing anchors to tie everything together. Ask your building inspector what’s required in your area.

Most of the materials for this shed are available at home centers and lumberyards. For a complete shopping list is in the Additional Information below. Here are a few pointers for your shopping trip:

  • Pick straight, dry cedar 4x4s. Bowed or twisted lumber will cause trouble when it comes time to install the windows and door. Check at both lumberyards and home centers.
  • Make sure the 6x6s are .60 treated, not the less durable .40 that’s sometimes used for landscaping timbers. You’ll probably have to order them.
  • Barn sashes are sold at some home centers and farm-supply retailers. If you’re willing to modify the plan dimensions, consider using old storm windows in place of the barn sashes. Contractors specializing in window replacements often throw these away.
  • Search for an interesting old entry door at salvage yards or architectural antiques dealers, or buy a new one like ours.

Figure A: Framing Details

Note: You can download Figure A and enlarge it from the Additional Information below.

Figure B: Foundation Plan

Note: You can download Figure B and enlarge it from the Additional Information below.

Step 2: Set the foundation

Photo 1: Stake out the corners of the shed

Establish the size of the shed by laying out and cutting the treated 2×4 bottom plates. To accurately space and mark the posts, use as patterns a 2×4 cut to the width of the window sash plus 1/8 in., and a small section of 4×4. Drive reference stakes along the house or garage to mark the corners and establish the floor height. Then lay the plates on the ground to establish the approximate perimeter

Photo 2: Mark the floor height

First pound wooden stakes into the ground about 18 in. outside the perimeter of the shed to support the 1×4 batter boards. Then establish the floor height by leveling across from the reference stake with a string and line level. Mark each stake.

Photo 3: Level and set batter boards

Align the top edge of the 1×4 batter boards with the marks and screw the boards to the stakes. Then stretch strings between the batter boards and reference stakes to indicate the locations of the 6×6 beams and two 4×4 support posts. Subtract 1 in. from the length of the outside wall plate and 1/2 in. from the end plates (the plates overlap the beams 1/2 in.) to determine the beam lengths and positions. Check that the strings form right angles by measuring diagonally from one corner to the other. Adjust the strings until the diagonal measurements are equal and all the sides are the correct length. Tie the strings to nails to hold them taut.

Photo 4: Dig and pour the footings

Position the footings according to your plan. Then dig footing holes with a post hole digger and set 8-in. dia. cardboard forming tubes into them. To establish the correct footing height, measure down from the string the thickness of your 6×6 beam plus the floor thickness, mark the tubes and cut them to this length with a sharp utility knife. Then place them back in the holes, wedge them solidly so the beams will center on them, and fill them with concrete. Set 1/2-in. x 12-in. anchor bolts into the center of each and smooth the tops.

Photo 5: Position and cut the beams

Transfer the layout lines to the footings by plumbing down from the string and marking the concrete piers. Connect the marks with a chalk line. Cut the 6×6 beams to length, allowing an extra 3 in. on the side wall beams so they’ll fit into the existing foundation (Photo 7)

Photo 6: Mark and drill for anchor bolts

Lay the outer beam on the footings and measure from the chalk line to the center of the anchor bolts. Transfer these measurements to the 6×6 beams. Drill 5/8-in. holes through the beams at these marks. Drill 1-in. holes 1/2 in. deep to recess the washer and nut.

Photo 7: Anchor the beams

Bolt the outer beam to the footings. Use a masonry blade on a circular saw to cut a pocket in the block foundation to support the end of the 6×6 (or pour another footing if the foundation is concrete). Spread pea gravel under the beams. Connect the side beams to the outer beam with 3/8-in. x 10-in. lag screws.

Begin with a square and level foundation.

To quickly double-check the dimensions on your plan, mark the post locations on the 2×4 bottom plates and cut the plates to length (Photo 1). Use the dimensions from these plates to set up the batter boards and strings (Photos 2 and 3). It’s best to align the strings with what will be the outside edge of the 6×6 foundation beam. (Remember to deduct 1 in. from the total length of the front wall and 1/2 in. from each side wall so the walls will overhang the beams by 1/2 in. as shown in Fig. C.) We aligned our strings with the inside edge of the 6×6 because the concrete patio was in the way of setting up batter boards. Take your time getting these strings perfectly level and square.

Our shed is held up by 6×6 treated beams bolted to 42-in. deep concrete piers, which are appropriate for the frost depth in our area. Your building inspector will tell you how deep to go in your region. We rested the end of the 6x6s on notches cut into the garage foundation. If you can’t do this, add piers at these locations. About a week before you plan to dig, call the utility companies to locate buried wires or pipes. Use 8-in. dia. cardboard forming tubes for the piers. They allow you to easily establish accurate pier height (Photo 4). Figure one 60-lb. bag of concrete mix for every 1-ft. length of 8-in. dia. tube. After pouring the concrete, position the anchor bolts 2-3/4 in. from the string so they’ll end up in the center of the 6×6 beam.

Tip: Before the concrete hardens, double-check that all the footings are level with each other using a long, straight board with a 4-ft. level on top.

Then let the concrete harden overnight. Before you start on the 6×6 beams, dig out enough soil to allow space for 3 in. of gravel under the beams and 5 in. under the concrete patio block floor for good drainage.

Next, you’ll cut the 6×6 beams to length and bolt them down (Photos 6 and 7). Mark the beam locations on the footings (Photo 5) and cut the 6x6s to align with these marks. Remember to add the amount that will rest on the garage foundation onto the shorter end beams (Photo 7). Paint wood preservative onto the cut ends of the beams before you bolt them down, and countersink the nuts and washers so the walls will slide on easily.

Step 3: Frame the walls and roof

Photo 8: Open up the garage rafters

Pry off the soffit and fascia material in the area of the shed. Remove full pieces if possible. Then cut and replace them after the shed is built.

Photo 9: Make a rafter pattern

Mock up the walls and rafter to make patterns. Position the 4×4 posts at the wall locations (overhanging the 6×6 beams by 1/2 in.), plumb them in both directions with a level, and brace them with 2x4s. Make a rafter pattern by cutting the wall angle on the end of a 2×6, sliding it under the overhang, and temporarily securing it to the wall. Use a taut string to align the 2×6 with the existing roof and temporarily screw or nail it to the posts. Mark the 2×6 along each side of the 4×4 posts.

Photo 10: Mark the post heights

Mark the posts along the underside of the 2×6. Use these marks to establish the wall height and length of the 4×4 posts. Take the 2×6 and posts down.

Photo 11: Make a rafter cutting pattern

Mark the 2×6 to make a pattern rafter. Draw the seat cut (where the rafter rests on the top plate) at a right angle to the post marks. Then use a Speed square to transfer the “plumb cut” angle to the end of the rafter, 11 in. beyond the seat cut for the overhang.

Photo 12: Cut and notch 4×4 top plates

Cut the outside 4×4 top plate 2 ft. longer than the outside 2×4 bottom plate to support the overhanging rafters. Cut the end 4×4 top plates 1-3/4 in. longer than the end 2×4 bottom plates to fit into the notch in the outside 4×4. Align the 2×4 bottom plates with the 4x4s and transfer the layout marks. Cut a 1-3/4 in. x 3-1/2 in. notch in each end of the front 4×4 to accept the side 4x4s.

Photo 13: Assemble the walls

Cut the 4×4 posts to length and assemble the walls. Pound two 6-in. pole barn nails through the predrilled 4×4 top plate and into each 4×4 post. Drill two 1/8-in. pilot holes through the 4×4 top plate at each post location and pound 6-in. pole barn nails through the 4×4 into the posts. Secure the treated bottom plate to the 4×4 posts by nailing two 16d galvanized box nails into each post.

Photo 14: Erect the walls

Lift the front wall onto the 6×6 beam. Position it to hang over the 6×6 beam 1/2 in. on the front and ends. Nail the bottom 2×4 plate to the 6×6 beam with two 16d galvanized nails near each post. Hold a 4-ft. level against the 4×4 corner post and push or pull it until it’s perfectly vertical while your helper nails a 2×4 diagonally across the wall to temporarily brace it. Slide the side walls into the slots in the siding and fit the 4×4 top plates into the notches. Nail the bottom plates to the 6×6 and connect the top 4x4s with two 6-in. pole barn nails.

Photo 15: Nail up the rafters

Mark and cut the rafters using the pattern rafter (Photo 11) as a template. Position the overhang rafters 3/4 in. back from the end of the overhanging 4×4 top plates to allow room for trim. If they don’t land alongside an existing rafter, secure the tops as shown in Photo 16. Position the remaining new rafters alongside the existing rafter tails and connect them with three 16d nails every 12 in. Toenail the rafters to the 4×4 top plate with three 8d box nails.

Photo 16: Add joist hangers

Support the tops of the rafters that don’t align with existing rafters with metal joist hangers. First nail a 2×6 block between two rafters and into the top plate if possible. Notch the rafter to fit into the metal 2×6 joist hanger. Pound special 1-1/2 in. joist hanger nails into every hole in the metal joist hanger

Next, mock up a wall and rafter to make exact patterns rather than using math (Photos 9 and 10). After you’ve marked where the rafter crosses the 4×4 top plate (Photo 10), subtract 5 in. (1-1/2 in. for the 2×4 bottom plate and 3-1/2 in. for the 4×4 top plate) from the high side to arrive at the length of the 4×4 wall posts (Photo 10). Cut all the posts off square at this length. Then assemble the walls and brace them plumb and square (Photos 13 and 14). To tie the side walls to the house, we cut out a slot in the siding and slid the 4×4 wall post against the sheathing. Then, since there were no studs in the existing wall at these locations, we added some from inside the garage to provide solid wood for attaching the walls. After nailing the walls in place, use a handsaw to cut out the bottom plates that run across the door openings.

Cut out a rafter pattern (Photo 11) and set it up on the walls to check the fit. This long rafter will need two “bird’s-mouth” cuts, one to fit over the 4×4 top plate of the long front wall and one for the short 4×4 beam that holds up the lattice-covered overhanging section. Adjust the fit as needed and use the pattern to mark the remaining long rafters. Make a second pattern for the shorter rafters.

Sight along the top of the front wall to make sure it’s straight before you secure the rafters.

Figure C: Finish Details

Note: You can download Figure C and enlarge it from the Additional Information below.

Step 4: Finish the roof

Photo 17: Install roof sheathing

Cut the 3/4-in. CDX plywood and nail it to the rafters with 8d box nails. Temporarily support the sheets with 2×6 blocks nailed to the subfascia. Align the sheets so the 8-ft. side is perpendicular to the rafters and stagger the joints on the 4-ft. ends. Space the sheets 1/8 in. apart.

Photo 18: Nail on roof trim

Notch the 1×6 trim to fit over the 4x4s and miter the ends. Nail the 1×6 trim to the rafters and subfascia with 8d galvanized finish nails. Use your square as a straightedge to align the 1×3 drip edge with the surface of the plywood before you nail it in.

Photo 19: Shingle the roof

Staple no. 30 roofing paper to the plywood. Start at the bottom and overlap each sheet 4 in. onto the one below. Install the shingles according to the instructions on the shingle wrapper. Snap chalk lines to ensure the slots in your new shingles align with those in the existing shingles.

With the rafters in place, nail down the roof plywood (Photo 17) and install the soffit plywood and fascia boards (Photo 18). Match the soffit and fascia details to those on your house.

When you cover the roof with roofing paper (Photo 19), tuck the new paper under the existing shingles and paper. Plan the shingle installation and chalk the lines so the old and new shingles line up (Photo 19). Fasten the shingles with 1-in. galvanized roofing nails according to the instructions on the wrapper. When you get to the top, carefully lift the old shingles and remove the first row of roofing nails with a flat pry bar to allow the new shingles to slide underneath.

Step 5: Finish the exterior and floor

Photo 20: Frame the end walls

Fill in the end walls with a 2×4 top plate and short 4x4s angled to fit the roof pitch. Add 2×4 nailers to support the soffit.

Photo 21: Frame and install siding

Frame between the posts with 2x2s nailed flush with the inside surface of the 4×4 posts. Cut angled windowsills and drip caps on a table saw to fit between the posts at the top and bottom of each section. Nail the drip cap to the 2×4 bottom plate and screw the windowsill to the 2×2 frame. Fill each section with siding nailed to the 2×2 frame. Drill pilot holes for the nails to keep them from splitting the siding.

Photo 22: Trim and hinge the windows

Trim the barn sash to fit in each opening, allowing about 1/8 in. on the sides and bottom and 1/4 in. on the top. Screw a 3-in. screen door hinge to the top of the sash.

Photo 23: Hang the windows

Screw a 2-5/8 in. x 1-1/2 in. cedar board to the top of each window opening. Screw the hinges to this board. Nail 3/4 in. x 1-1/4 in. cedar stops to the top and bottom of the opening and 1/2 in. x 1-1/4 in. stops to the sides. Position the stops flush with the inside edge of the 4×4.

Photo 24: Trim and mortise the swinging door

Sand the door until it fits into the opening. Allow a 1/8-in. space on the sides and top. Cut enough from the bottom to allow at least 3/8-in. clearance above whatever flooring material you decide to use. Outline the 4×4 butt hinges with a utility knife and chisel mortises so the hinges will sit flush with the door. Screw the hinges to the door.

Photo 25: Hang the swinging door

Shim the door opening and mark the hinge positions on the 4×4 post. Remove the door and chisel the hinge mortises in the post. Screw the hinges to the post and nail 1×2 stops to the 4×4 door frame. Complete the installation by installing the lock according to the instructions included.

Photo 26: Build and hang the sliding door

Construct a sliding door by screwing 1×6 tongue-and-groove siding to a 1×4 frame. Attach a 1-1/2 in. x 2-in. strip of cedar to the horizontal 4×4 with 3/8-in. x 4-in. lag screws and mount the aluminum pocket door track to it. Hang the doors from pocket door wheels that ride in the track and snap into brackets mounted to the top of the door. Cover the track with a 1×4 cedar board.

Next, fill in the triangular sections above the wall on each end. Then close in the shed by filling the spaces between the posts (Photo 21). With our shed design, you don’t have to install door and window jambs, but you do have to cut angles on the 3/4-in. x 2-3/4 in. drip cap that fits below the siding, and on the 2×4 windowsill. Use a table saw for these cuts and to rip a 2×4 down to 2-5/8 in. for the pieces above the windows (Fig. C). Photos 21 – 23 show how to install the siding and windows. For our floor we laid 18-in. square concrete patio pavers over a level bed of pea gravel. Cut the pavers along the walls and at the doors with a dry-cut diamond blade mounted in a circular saw. We also used the pavers to build a ramp into the yard from the sliding door.

Install the doors after the floor. Photos 24 and 25 show how to hang the entry door. Construct the sliding door by driving 1-1/4 in. galvanized or stainless steel screws through the backside of the 1×6 tongue-and-groove boards into a 1×4 cedar frame. Fig. D and Photo 26 show this detail and how to install the heavy-duty pocket door hardware and three-wheel rollers that suspend the door. Mount a 2×4 open bar holder bracket to the 6×6 to keep the bottom of the door from swinging out. We installed a barn door latch on the outside to secure the door.

Finally, you can customize the shed for your hobbies. To provide a separate planting area inside the shed, we built a wall of 2x6s spaced 30 in. apart and covered on the back with 1/2-in. plywood. Metal shelf standards and clips support 2×6 shelves for pots and planting supplies. We created a narrow, hollow space next to the windows to allow access to the window latch. Use the lattice-walled overhang to store a gas grill or to hold a potting bench like ours. Or use it as a vine-covered sitting area. Cut the cedar lattice panels to fit between the 4x4s and secure them with 1x2s nailed on each side. Make the bench top by screwing a 2×4 ledge to the 4×4 posts and cutting 2×6 cedar boards to rest on top.

To retain the natural look of your cedar shed, apply two or three coats of a top-quality clear exterior finish. Paint the door and exterior trim with alkyd primer and two coats of top-quality exterior latex paint.

Figure D: Sliding Door Details

Note: You can download Figure D and enlarge it from the Additional Information below.

Additional Information

Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

  • Circular saw
  • Corded drill
  • Drill bit set
  • Drill/driver - cordless
  • Extension ladder
  • Hammer
  • Level
  • Line level
  • Miter saw
  • Posthole digger
  • Sledgehammer
  • Socket/ratchet set
  • Speed square
  • Stapler
  • Stepladder
  • Table saw
  • Tape measure
  • Wheelbarrow
  • Wood chisel
Dry-cut diamond blade,
String line