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Every backyard playhouse, fort, and clubhouse starts with the same problem: a level, solid platform on unlevel ground. This plan is that platform — an 8-foot-square deck on 4x4 corner posts, framed in 2x6 with joists on layout, diagonal corner braces, and 2x4 cross-bracing below. It's deliberately built like a real deck, because kids load structures like a demolition crew.
The honest line in the plan deserves quoting: 'it will be a miracle if it's square without adjustment.' Expect to rack it, expect to shim, and get the frame level and square BEFORE the diagonal braces go on — the braces lock in whatever geometry exists when they're fastened, correct or not.
The cut list carries the swing-set option: two of the four posts run 96" instead of 47", becoming uprights for a swing arm later. Decide before cutting, because it changes which posts land where. All screws are exterior self-tappers, everything gets paint or stain, and once the platform is done, the walls that come next are a separate (and easier) project — this deck is the part that has to be right.
| Item | Qty |
|---|---|
| 4x4 post, 8 ftThree if building the swing-set option. Ground-contact treated. | 2 each |
| 2x6, 8 ftFrame, joists, and decking. Eight can be 92-5/8" studs if cheaper. | 29 each |
| 2x4, 8 ftUnder-deck cross bracing. | 4 each |
| Exterior deck screws, #9 x 2-1/2", self-tapping | 150 each |
| Exterior deck screws, #10 x 3", self-tappingThe framing screws. | 100 each |
| Exterior paint or stain | 1 gallon |
Overall cost: $$$. Lumber prices vary by region, species, and season, so we give a band rather than a figure we would only be pretending to know.
| Part | Qty | T × W × L |
|---|---|---|
| Posts4x4Swing-set option: two at 47" and two at 96". | 4 | 3 1/2" × 3 1/2" × 47" |
| Shorter side boards2x6Includes the joists. | 8 | 1 1/2" × 5 1/2" × 92 1/2" |
| Longer side boards2x6 | 4 | 1 1/2" × 5 1/2" × 96" |
| Diagonal corner supports2x6Both ends mitered 45°. | 8 | 1 1/2" × 5 1/2" × 12" |
| Deck boards2x6 | 16 | 1 1/2" × 5 1/2" × 96" |
Some parts are longer than a 8 ft board, so there is no honest board count to show here. Open the board plan to pick a longer stock length — it names every part that does not fit.
2 boards
10 boards
Assumes 8 ft stock, a 1/8" saw kerf, and buying each part at its own width. Change the board size or kerf on the full board plan →
Lay the 4x4 posts flat and screw the first side boards to them with 3" screws, per the diagram. Building the swing option? One post per side is a 96-footer — decide which end of the playhouse faces the swing before this step, not after.
Attach the remaining side boards to the posts with 3" screws, forming the 8x8 rim. Now the real work: level the frame on the actual ground (dig or shim under posts — dirt moves, framing doesn't) and square it by matching diagonals.
The plan promises it won't be square on the first try, and the plan is right. Take the time; every later piece inherits this geometry.
With the box square AND level — verify both again — screw the eight 45°-mitered braces across the corners. These freeze the geometry: they make a square frame permanent and a crooked one permanent too. That's why they come after the fight, never before.
Mark the joist layout on the long sides — even spacing, roughly 16" centers — and screw each joist through the rim with 3" screws, three per end, tops flush with the rim top. Sight down the tops: a proud joist telegraphs a hump into the deck.
Screw the sixteen deck boards across the joists with 2-1/2" screws, two per crossing, small even gaps for drainage. Start flush at one edge and check remaining spacing every few boards so the last one isn't a sliver.
Sand the whole surface and every arris — this deck hosts bare feet, crawling knees, and eventually someone's sleeping bag.
Screw the 2x4 cross braces under the deck per the diagram, into the framing — they stop side-to-side sway once kids discover the deck is also a stage, a ship, and a trampoline.
Paint or stain everything, then build whatever the platform was for: walls, railings, the swing arm. The hard part — flat, level, solid — is done.
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