How to Build a Freestanding Treehouse, Step by Step

A freestanding treehouse is a tree fort on posts: a raised deck held up by four posts on concrete piers, with no tree anywhere near it. That is what we draw, because the structure then does not depend on a living tree that grows, sways and drops branches. The steps below follow our open tree fort, an 8x8 platform on four 6x6 posts with a guard and a rung ladder. Change the size in the designer and the numbers change with it.

The open tree fort has 64 sq ft of deck, 5 ft off the ground, with 4 posts. Materials come to about $2,898 with the safety surfacing (Home Depot prices read October 2026), and the build takes about 36 to 52 labor hours, roughly 2 to 4 working days for two people. If you only want the plan itself, open the open tree fort plan. If you are still deciding on a size, start with what a treehouse costs.

Before You Dig

Pick a Clear Spot

The use zone is the open ground around the fort. A child falling or jumping off lands there, so it has to be free of fences, sheds, house walls, trees and poles. The CPSC¹ home playground guidelines ask for at least 6 ft in every direction from any structure or obstacle, and 9 ft between two structures when the platform is higher than 30 in. Walk the spot with a tape before you stake anything.

Call 811

Step 1: Lay Out the Posts

Put a stake at each post center and run string lines between them, a few feet outside the stakes so the strings survive the digging. On the open tree fort the four posts form a square at the corners of the deck.

Square the Layout

Measure both diagonals from corner to corner. They must match before you dig a single hole. A layout that is out of square carries through every later step, and by the time the deck is on you cannot fix it.

Mark the Hole Centers

Drop a plumb line from the string intersections to the ground and mark each center with paint. Measure each center off the strings, not off the hole next to it, so a small error does not add up across the layout.

Step 2: Dig the Footings

Each post sits on a concrete footing. Dig at least 12 in below the undisturbed ground or below your frost line, whichever is deeper (the Deck Construction Guide, DCA6, sets that rule for deck footings, and it is the one our designer uses). Your building department knows the local frost depth. The footing also holds the fort down in a wind: the designer works out how hard each post can pull up and makes the footing wide or deep enough that its own weight holds it.

Pour the Footings

Pour concrete to the bottom of each hole and bring it up as a pier to 2 in above the ground. Set a cast-in post base with a standoff in the wet concrete, at least 3 in from the edge of the pier, so the wood never sits on the concrete. The plan uses a Simpson CBSTQ base, rated for uplift and for a sideways push. Keep the bases lined up on the string lines as the concrete sets.

Why the Post Does Not Go in the Ground

It is tempting to drop the post straight into the hole and backfill. We do not draw it that way. Simpson Strong-Tie, a connector maker, advises against burying deck posts because of the risk of rot, so the maker’s advice is to use a base. Our footing is a pier with a post base for that reason, and it is why the next step uses a post on a base, not in the ground.

Step 3: Set the Posts and Beams

The posts are 6x6. The DCA6 deck guide sets 6x6 as the minimum for a deck post, and it keeps the post in range of its loads. For a fort at 5 ft, the vertical load is not what limits you, the sideways load is, which is why step 5 has braces.

Choose the Right Wood

Use pressure-treated lumber. The DCA6 guide specifies treatment for above-ground use (UC3B) for the frame and ground-contact treatment (UC4A) for anything that touches the soil. Brush a preservative on every cut end, because cutting opens the wood to the part the treatment did not reach.

Post to Beam

Set the posts in the bases, plumb on two faces, and brace each one temporarily with stakes and 2x4s in two directions: a post on a base can rotate until the knee braces are on. Then fix the beams. A two-ply beam sits in a notch cut in the top of the 6x6, bearing on it fully, with two 1/2 in through-bolts (DCA6 Figure 8A); a three-ply beam sits in a post cap. The DCA6 guide prohibits fixing a beam to the side of a post without a notch.

Step 4: Frame the Joists and Deck

Joists run across the beams at 16 in on center. The DCA6 table for Southern Pine at that spacing allows a 2x6 to span 9 ft, a 2x8 11 ft 10 in and a 2x10 14 ft, so the 8 ft platform of the open tree fort is well inside the range for a 2x6 or heavier. The designer picks the joist size for the size you draw.

Joist Hangers and Spacing

Hang each joist from the beams with a joist hanger, or sit it on top and tie it with a framing tie, and nail the hanger to the manufacturer’s pattern. Keep the joists straight with blocking at mid-span.

Decking

Lay 2 in decking across the joists, fixed with two 8d threaded nails or two number 8 screws at each joist, with about 1/8 in between boards so water drains (the DCA6 guide). Let the boards run long and trim them to the edge with a saw once they are down.

Step 5: Brace It Both Ways

A freestanding fort has no wall to lean on, so the bracing does the work. Fix 2x6 knee braces at 45 degrees from each post to the beam and to a doubled joist over the post, through-bolted at both ends, so the structure is held in both directions.

How the Bracing Is Sized

The DCA6 guide leaves the sideways stability of a freestanding deck out of its scope, so there is no table to copy. The designer calculates it instead: the wind on the side of the fort from the pressures in the IRC tables, at the wind speed you enter, or a 200 lb push at the top of the guard, whichever is greater. Each brace, its bolts, the posts and the post bases are checked against the NDS wood design rules and the maker’s ratings, and the plan lists the leg length and the number of bolts that pass.

Check Every Brace

Tighten every bolt, check the braces do not loosen when you push the deck sideways, and revisit them after the first few weeks of use, because wood moves as it dries.

Step 6: Build the Guard

The deck of the open tree fort is 5 ft above the ground, which is above 30 in, so it needs a guard on every open side. Our planner draws the guard at 38 in, which is the higher of the IRC minimum (36 in) and the CPSC¹ figure for school-age children.

Posts and Balusters

Fix the guard posts to the deck framing with through-bolts, not nails or screws, and fix the rails between them. For balusters, the gap is 3 in gap at most. The IRC sphere test is 4 in, and we use the tighter figure for children.

No Gaps Between 3-1/2 and 9 In

A gap of 3-1/2 to 9 in is an entrapment hazard for a child’s head, so it is not allowed anywhere on the fort, including between the deck and the guard. When the deck is more than 48 in off the ground, the CPSC¹ wants a barrier with no horizontal rails a child can climb. Our open tree fort uses vertical balusters for that reason. The guide to railing height covers the numbers in detail.

Step 7: The Way Up

Our open tree fort uses a rung ladder. The CPSC¹ guideline for public play equipment puts the angle of a rung ladder at 75 to 90 degrees, at least 16 in wide for school-age children, with the rungs no more than 12 in apart vertically. A rung ladder is the simplest access for a short deck, and it keeps the footprint small.

Fix the Ladder

Fix the ladder rails to the deck framing and to a footing at the base, so it cannot move when a child climbs it. Space every rung evenly, including the top one, and check that the angle is within range.

Hand Holds at the Top

A ladder with no handrail needs a hand support where the climber moves from the ladder to the deck, so the climber has something to hold while stepping across (CPSC¹ 325). Add grab bars or posts at the top, set where a child’s hand falls.

Step 8: Add a Roof (Optional)

A roof turns the deck into a shelter and keeps off sun and rain. It adds weight and wind load to the same four posts, so the bracing matters even more. We draw it as a separate plan, the fort with a roof, with half walls and a single-slope roof, and the designer re-checks the structure when you add it. Wait until the open fort is solid and bring the roof on as the second stage.

Step 9: Spread the Safety Surfacing

The surfacing under and around a fort is a safety part of the build, not a finish. The CPSC¹ guides ask for 9 in of loose fill kept up all the time, over the fort and 6 ft beyond it on every side, and rate each material for a fall height: wood mulch with no CCA-treated wood to 7 ft, wood chips, engineered wood fiber and shredded rubber to 8 ft.

How Much and Which Kind

Loose fill packs down by about a quarter, so the plan has you put in 12 in to keep 9 in. For the open tree fort that is playground wood mulch delivered in bulk, about $1,120, and it is in the materials total. Dig out the area or edge it so the fill stays where it is needed.

Keep It Up

Mark the 9 in level on the posts, rake the fill back under the ladder and the slide every month in the play season, and top it up each year. Do not put the fort over grass or dirt alone, and never over concrete.

Common Mistakes

• Skipping the diagonals. A layout out of square can not be fixed once the deck is on.
• Burying the posts. The post base on a pier lasts longer than a post set in the ground.
• Fixing the beam to the post face. Notch it or use a cap.
• Leaving out the bracing. The posts stand on bases that let them rotate, so the knee braces are what keep the fort upright.
• A gap between 3-1/2 and 9 in. It is an entrapment risk, so close it.
• No safety surfacing. A fall onto dirt or grass is far worse than a fall onto 9 in of mulch.

What the Plan Adds

The free designer shows the fort on screen with the checks and the priced materials list. The paid plan, $29, draws it as sheets you can take to the lumber yard and to your building department. If you would rather have a tree-free design of another type, see treehouse without a tree or browse the tree fort plans.