12x20 Lean-To Storage Shed Plan

You can buy the 12×20 lean-to shed plan now, or open the free 12×20 lean-to shed plan PDF first to see how the drawings look. The door is in the 20 ft front wall, so you open it facing the full width of the shed.

12×20 lean to shed plans: the numbers for a site check

The roof falls to the back, so the low 6′ 8″ wall is where all 20 ft of water ends up. Plan where that water drains before you set the skids.

Laying out the pad

Check with your permit office first for size, height and setback rules. Then call 811, the free call-before-you-dig line, a few working days before any stake goes in so buried power, gas and water lines get marked.

Stake the corners, 20′ 0″ along the front and 12′ 0″ along the sides, and move one stake until both diagonals read 23′ 3 7/8″. Strip the sod and roots over a 22′ x 14′ patch, a foot past the shed on every side so rain runs away from it. Digging knocks stakes around, so reset them and check the diagonals again.

The stone goes down 4″ deep, tamped in two layers with a hand tamper, then raked level and checked with a long straight board and a level in several directions. The plan says a pad level within 1/4″ over its length saves hours later.

On a sloped yard you stretch a string with a line level from the highest stake to each of the others and measure down at each. A few inches of fall gets made up with more stone on the low side, tamped every 2″. More than that and you either dig the high side down or set solid concrete blocks on compacted stone under the skids on the low side and shim them with pressure-treated shims.

How the 12×20 lean-to shed goes together

Building 20 ft long from shorter boards

Nothing in the frame comes in a 20 ft length, so the plan splices. Each skid is two pieces, 8′ 0″ and 12′ 0″, butted on the gravel with the joint centered under a floor joist and scabbed on both sides with a 3 ft pressure-treated 2×4, six nails per scab. The joints in neighboring skids stay 4 ft or more apart, and the pieces alternate ends from one skid to the next.

The rim joists are cut as 9′ 4 3/4″ plus 10′ 7 1/4″. Both rims break over a doubled joist, so each rim end gets three nails into a full joist. Wall plates are spliced the same way, and the second top plate is cut so its joint lands on a stud and well clear of the joint underneath, which means each layer covers the other’s.

The floor

The four skids run along the 20 ft side, 3′ 10 13/16″ on center, with the outer two flush with the floor edges. They have to be ground-contact lumber (UC4A or higher), laid crown up so the weight of the floor flattens them.

You build the frame right on the skids. Once it’s nailed it weighs about 530 lb and racks out of square if you lift it, so nobody carries it. Clamp the two rims together and mark the joists on both at once. Stand each rim on edge over an outer skid, stand the 11′ 9″ joists between them on the marks, and nail through the rims with three 3 1/2″ nails per joist. Push the long corner along the skids until both diagonals read 23′ 3 7/8″, and only then toe-nail it down at every crossing.

Eight sheets of 3/4″ tongue and groove plywood cover the deck. A sheet only covers about 47 1/2″ once its tongue sits in the next groove, so three rows come to 11′ 10 1/2″ of the 12′ 0″ frame. The missing 1 1/2″ along the back rim is made up with a ripped 1×4 strip glued and screwed down, which the back wall’s bottom plate covers later.

Raising the four walls

Every wall is framed flat on the deck, tipped up as a bare frame and braced plumb. Siding waits until all four stand and the corners are nailed.

At 20 ft the front and back walls are too long for two people to stand up in one piece, so the plan splits each into two sections with their own plates and a joint stud at each end. You raise the first, then the second tight against it, clamp the two joint studs together and nail them every 12″. The full-length second top plate goes on after all four walls stand and bridges the joint, lapping it by at least 4 ft on each side.

The front wall goes first, while the whole deck is empty. You lay it with the bottom plate set back 3 1/2″ from the front edge, because the wall swings out to the edge as it tips up on the inner edge of that plate, and it lands flush. Before the lift you drive two or three nails part way through the bottom plate so it hinges instead of sliding, and nail two 12″ stop blocks to the outside of the rim, 2″ above the deck.

Nobody stands on the ground in front of a wall going up. A wall that passes plumb falls outward, off the deck. Pick a calm day too, because a bare frame catches wind like a sail. The plumb braces are 12 ft 2x4s running out from both ends to stakes about 5 ft away, so the deck stays clear for the next wall.

The sloped side walls

The side walls follow the roof, so each has ten studs, every one a different length, from 6′ 5 3/8″ at the back to 9′ 3 3/16″ at the front. Each stud top gets a 14.0 degree bevel through the 1 1/2″ thickness, square across the face, with the long point facing the tall front end. The sloped top plate is 11′ 8 15/16″ along either edge, and both ends are plumb once it is up.

Mark the sloped plate from its own column in the stud table. The marks spread out along the slope, and if you copy the bottom plate marks instead, the last stud ends up 4 3/16″ off plumb. Each side wall goes up between the standing front and back walls. Its plates are 1/4″ shorter than the gap so it tips up between them. Push it tight to the front wall and leave the gap at the back corner, where you shim it when you nail the corners.

A diagonal brace stays tacked across each wall until just before that wall gets its siding. The plumb braces only stop a wall leaning in or out. The diagonal is what stops it racking along its length.

Joining the corners

Plumb each corner both ways, then nail through each side wall’s end stud into the front or back wall’s end stud, 3 1/2″ nails every 12″, within 3/4″ of the side wall’s outside face. A lean-to side wall has no level plate to lap, so each of the four corners also gets a twist strap on the inside, nailed to the sloped plate and end stud above and to a corner backer below.

Siding then goes up one wall at a time, each sheet hung from a straight 2×4 ledger screwed to the floor frame. A 4×8 to 4×10 sheet of 5/8″ T1-11 weighs about 60 to 80 lb, and its top is nailed from a ladder, so this is a two-person job.

The double door and the window

The door opening is 5′ 0″ wide and 6′ 4 1/2″ tall, running from 7′ 6″ to 12′ 6″ measured from the left corner, with a king stud and a trimmer on each side and a doubled 2×8 header with 1/2″ plywood between the plies. The bottom plate runs straight through the opening while the wall goes up and gets cut out flush with the trimmers once the siding is on, which gives a 6′ 6″ opening height.

You build two leaves, each 2′ 5 5/8″ wide and 6′ 5 1/4″ tall, from two new sheets of T1-11, because the piece cut out of the opening is too short. A 1×4 frame of two stiles and three rails goes on the outside of each, then two Z braces cut at 29 degrees on the miter saw. Both braces rise from the hinge side toward the latch side, so they carry the leaf’s weight into the hinges. Each leaf hangs on three T-strap hinges against a 1×2 stop under the header.

Only one leaf is the working door. The other is fixed with a barrel bolt at the top and one at the bottom, and a 1×2 astragal on its inside face gives the active leaf something to close against. The active leaf swings out, and the latch goes on it.

The window is a prefabricated unit for a 36″ x 36″ rough opening (units are usually 1/2″ smaller), in the back wall from 8′ 6″ to 11′ 6″ measured from the right corner, which is the building’s right when you stand behind the shed. Its sill sits 2′ 10″ above the bottom plate. Tape the opening before the window goes in, sill first, then the sides, then the head. A drip cap goes over the door and window casings.

What is on the 12×20 lean-to shed material list

The shopping list is split by stage, and each lumber line names the board length that wastes the least. The plan suggests adding 2 or 3 spare studs for bad boards. The list gives quantities only, no prices.

• Foundation and floor: eight pressure-treated 4×4 skid boards (four 12 footers and four 8 footers), two 14 ft 2×4 boards cut into the eight 3 ft scabs, 21 pressure-treated 2×6 boards for joists and rims (two 10 footers, 19 12 footers), 8 sheets of tongue and groove plywood and three 1×4 filler strips.
• Walls: 2×4 studs and plates in lengths from 8 to 14 ft, a doubled 2×8 front header, a doubled 2×6 back header, and T1-11 siding in 4×8, 4×9 and 4×10 sheets, 16 sheets for the four walls. No 4×9 at your store? The list says buy 4×10 and cut it to the same marked height.
• Roof: 18 2×6 x 14 ft boards for the 16 rafters and 2 fly rafters, 2×6 blocking, 11 sheets of 7/16″ OSB, 2 rolls of 15 lb felt, 10 bundles of shingles and 10 pieces of drip edge.
• Fascia, door and trim: 1×8 fascia (the rake pieces are 16 footers), 1×4 for corners, casings, stiles and rails, 2 more sheets of T1-11 for the door leaves, 6 T-strap hinges, a handle and latch, 2 barrel bolts and the prefabricated window.
• Hardware: about 31 lb of 16d galvanized framing nails, 28 hurricane ties, 4 framing angles for the end rafters, 4 twist straps, 11 tubes of construction adhesive, 8 ground anchors with 48 structural screws and 6 gallons of exterior primer and paint in one.

What fits in a 12×20 lean-to shed

The front is the tall side, 9′ 7 1/8″ with plates, and the back is 6′ 8″ with the window. Front to back the clear span inside is 11′ 5″, so check any shelf depth or bench length against that and the 20 ft width before you build them.

For a mower or a wheelbarrow, measure it against the 5 ft opening, and remember the leaves hang inside it and the astragal sticks out 3/4″ behind the fixed leaf. The deck is 9 3/4″ above the gravel and the plan has no ramp, so plan one separately if you’ll be rolling things in.

How the 12×20 lean-to roof is framed

A single-slope roof has no ridge, so it pushes nothing outward, and the hurricane ties hold it down in wind. Each rafter gets a tie on the inside face of both walls. The two end rafters, where the side walls cover that spot, get framing angles instead, and they are also nailed up through the sloped side plates every 16″.

For the 11′ 5″ clear span the plan uses 2×6 rafters at 16″ on center, in #2 spruce-pine-fir or Douglas fir-larch, assuming about 30 psf of ground snow. Snow loads and the span table differ from place to place, so check the rafter size with your local permit office before you buy. Typical span tables list Southern yellow pine #2 at about 11′ 2″ in a 2×6, short of this span, so in Southern pine the plan says to go up one size.

Each rafter hooks over both walls with a birdsmouth: a 3 1/2″ level seat and a 7/8″ plumb heel, about 15% of the rafter’s depth. The plan says never to cut deeper than a third. Before you cut them all, measure from the inside face of the front plates to the outside face of the back plates at both ends and in the middle. It should read 11′ 8 1/2″ every time, held level, not along the slope. If the middle is off, push or pull the wall there and hold it with a temporary 2×4 across the plates until the rafters are in.

Cut one pattern rafter from a straight 2×6, try it on both walls at each end of the shed, then cut the other 15 and the 2 fly rafters. The front birdsmouth is the easy one to cut backwards, so it has its own drawing. The fly rafters have no birdsmouth. They sit 2 1/2″ outside the end rafters on 2×4 lookout blocks and carry the rake fascia.

Venting, felt and shingles

The blocking over both walls is ripped to 4 3/4″ tall and drilled with 1″ vent holes. Air comes in through a 1/4″ gap in the back soffit, rises up the rafter bays and leaves at the front. Staple the insect screen under the rafter tails before the soffit strips go on.

At 3/12 the shingle makers ask for double felt: a 19″ strip along the low eave, then full 36″ courses each lapping the one below by 19″, with 4″ end laps. The drip edge goes under the felt at the low eave and over it on the rakes and the high front edge. Then come 33 courses at 5″ exposure, about 259 shingles or 10 bundles including the starter, with the last course trimmed flush at the front and sealed with roofing cement.

Ground anchors and wind

The PDF gives wind figures for this shed. Empty on loose skids, it slides at about 10 psf of wind (gusts near 55 mph) and tips at about 22 psf. Once the roof is on, you anchor it before you fill it.

The plan uses 8 screw-in auger anchors, 30″ or longer and galvanized, 4 along each of the two outside skids. Each goes in tight against the skid until its eye sits about 1″ above the ground, with a galvanized strap running up the skid and onto the rim joist. Four structural screws go into the skid and two into the rim. The plan also says to check your local permit office for what your area asks.

Ladders and roof safety

The high edge is about 11′ 1″ off the ground, so the tools list calls for a 20 ft extension ladder plus two 6 ft step ladders for the walls and rafters. Set a ladder 1 ft out at the foot for every 4 ft up, tie the top to a rafter or the fascia, and let it reach 3 ft or more past the edge. Look up before you raise a ladder or carry a sheet, and keep yourself, the ladder and the sheets at least 10 ft from any overhead power line.

On the roof, a 2×4 screwed flat a foot above the eave gives you a foot stop, and nobody works up there alone. Don’t climb with a sheet in both hands: stand it on its long edge and hand it up to a helper on a step ladder. Shingle bundles weigh about 60 to 80 lb, so carry half a bundle at a time or ask the supplier whether they will deliver onto the roof.

Free 12×20 lean to shed plans PDF vs the full plan

The free PDF is 7 pages. It walks the five stages (site, floor, walls, roof, trim) with 3D views of the same model, so you can see the drawing style and the finished shed before you pay. The full 46-page plan adds the shopping list, cut list and cutting plan, stud-by-stud wall drawings with position tables, every sloped stud length, the rafter marks, piece maps for the deck, siding and roof, the door build and pro tips on most steps. It has 18 build steps from ground prep to paint, plus a checklist page to tick them off. The PDF’s numbers come from one 3D model, and it lists 87 automatic checks that all pass, plates plus studs matching the wall height among them.

Before you build a 12×20 lean-to shed

Rules for sheds come from your town, and from your HOA if you have one. Size, height, distance to the property line and whether you need a permit at all vary, so check your local permit office before you buy lumber. Ask the same office about snow and wind, since heavy snow or high wind can call for closer rafters or larger rafters. This is common small-shed framing, not an engineered design, and the disclaimer on page 2 says so.

To keep it dry: raise a low skid with gravel or treated shims, never untreated wood. Keep 6″ or more between the siding and the soil or mulch, and keep the drip edges clear. Each spring, look for lifted shingles, re-caulk the trim, touch up the paint and check that the skids are still level and dry.

Tools the plan lists

The plan assumes a circular saw with a rip guide, a drill/driver and a framing hammer. On top of those come a miter saw for the door braces and trim, a reciprocating saw or oscillating tool for the flush cuts at the door and window, two 24″ bar clamps for marking the rims together, a 3 lb sledge for the brace stakes, and a shovel, rake and hand tamper for the gravel.

Related plans

• 2 ft shorter: the 12×18 lean-to shed plan. 4 ft shorter: the 12×16 lean-to shed plan.
• 4 ft longer, same depth: the 12×24 lean-to shed plan.
• 2 ft deeper, same width: the 14×20 lean-to shed plan.
• Same footprint with a two-sided gable roof: the 12×20 gable shed plan.
• Every size on the lean-to shed plans page.