This is a relatively simple, elevated backyard fort I built for my kids.

I have a very small yard with no trees whatsoever. But we wanted a tree house or fort of some kind, so this is what I came up with.

It was loosely inspired by Calvin's tree fort in the comic strip Calvin and Hobbes. The design is intentionally simple and somewhat generic, which allows it to become a pirate ship, haunted house, rocket ship, or whatever.

A playhouse is just a play house. A fort is anything you want it to be!

To simplify the design and maximize the use of purchased materials, I tried to incorporate basic straight-from-the-store lumber dimensions as much as possible. For example, the floor is a 4-foot by 8-foot sheet of material, the posts are 8 feet tall, the roof is 10 feet wide, and so forth. All common sizes, so more bang for my buck with very little waste.

The goal was to make a solid, good-looking little treeless tree house/backyard fort that would last for many years. If you're interested in making something similar, hopefully I've provided enough information here to help you out.

Good luck, and thanks for taking a look!

Step 1: Supplies

Here are the basic supplies I used for this project:

Six cement blocks (I HIGHLY recommend the kind with epoxied-in metal post brackets. Although they are more pricey, they are much more secure and allow for easily adjusting post and block assemblies. More on that as we go along.)
Six 8-foot pressure treated 4x4 posts
Six 8-foot pressure treated 2x6 boards
One 4- by 8-foot 3/4" board OSB (same dimensions in exterior grade plywood is another option)
Approximately twenty 8-foot 2x4 boards
Four 10-foot 2x4 boards
Five 26" x 8' corrugated plastic roof panels
One box (50 pcs.) of roofing screws for corrugated panels (the kind with built-in rubber washers)
5 lb. box of 3" exterior grade screws
5 lb. box of 1 5/8" exterior grade screws
Two 50 lb. bags of general purpose sand
Gallon of waterproof sealer
Gallon of deck restoring paint

For the walls, I primarily used pine boards that were removed from old shipping crates (like large wooden boxes built on top of pallets). These came from a place I used to work that regularly threw them away. The longer pieces of trim around the walls I had to purchase. I also used a few pieces of miscellaneous scrap lumber that I had lying around.

If anything, get a few extra basic 2x4s. You'll need extra if you make any mistakes, but they will inevitably get used either way, whether for this project or some other!

fortcosts.xlsx
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Step 2: Layout of Foundation Blocks

I began by using the actual floor board as a template for laying out where to dig and place the post blocks.

I marked on the sheet of OSB where the posts would be passing through, taking account of the exterior floor beams that would be passing around the outside of the posts. These areas were cut out with a jig saw, according to the measurements noted in the photos above. (These photos are a recreation of this step, as I didn't take any originally.)

I positioned the floor board where I wanted the fort to stand in the yard, and spray painted through the cut out areas to mark the location for each post. With the floor board moved out the the way, I painted additional center lines extending through each marking to assist me in centering the post blocks once the holes for each was dug.

Each hole was dug a couple of inches over-sized all around to allow for some adjusting from any inaccuracy in the layout due to the ground being slightly sloped. Each block was placed on a layer of sand and leveled with all the other corresponding blocks using a straight 2x4 and a level. There was a lot of measuring, adjusting, and readjusting at this point to ensure that everything was level and in the correct place.

The open areas around the blocks were NOT back-filled with dirt until later on.

This step took almost an entire day. However, it was worth it because it allowed for smooth sailing the rest of the way.

Step 3: Posts to Blocks

With the blocks in position, I screwed all the posts in place with 3" exterior screws. For the screws nearest the ends of the post, I pre-drilled 1/8" holes for the screws to avoid splitting the post. Make sure you use screws that are rated for use in treated lumber.

One corner post was anchored it to the ground so it was perfectly plumb and locked in place. This was done with 2x4s that were screwed to the post about half way up, that angled out 90 degrees from each other and were screwed to stakes in the ground.

The other posts stood in place temporarily without any trouble I while moved on to the next step. If it had been a windy day, I would have had to anchor them down as well.

Step 4: Floor Frame

Working from the plumb, anchored post I added pressure treated 2x6s around the exterior of the posts to create the frame for the floor. These boards were attached with 3" exterior screws, with five screws into each post through each board. Holes were pre-drilled through the 2x6s and into the posts with a 3/16" tapered countersinking bit.

Beginning at the anchored post, these floor frame boards were attached so the bottom edge was 24" up from the bottom of the post. A level was used to ensure that all boards were level, and were pulling each individual post into plumb.

The 24" measurement stayed pretty consistent all the way around, due to my OCD nature while adjusting the height of the cement blocks in their respective holes. However, while the heights were near perfectly level with each other, a few of the post/block assemblies had to be tapped with a rubber mallet a few centimeters this way or that in order for the posts to be plumb and match up with the floor frame boards in the correct location.

With the outer frame in place, all the remaining joists were added. See diagram for additional tips and measurements.

I originally framed in and built a trap door that came up through the floor. Before adding the slide about a month after the fort was completed, the trap door was the only way in aside climbing up and over the walls.

While it was fun, I was always worried that the trap door was just handful of broken fingers waiting to happen. So out of safety concerns, I eventually decided to permanently screw it shut.

Step 5: Floor

At this point the floor board was just slipped into place over the floor frame. It went down without any trouble and I was quite pleased (read: pleasantly shocked) with how nicely it worked out!

I used 1 5/8" exterior screws to fasten this firmly to all the 2x6s underneath, through pre-drilled and countersunk holes.

Step 6: Walls

The first step to creating the walls was to put up a 2x4 frame all the way around the posts, just as I had done when framing the floor. The top of this was exactly 36" from the bottom edge of the floor frame.

Up to this point, I still had one post anchored to the ground in perfectly plumb position with a pair of 2x4s. All other posts were pulled into alignment with this one when I put together the floor, however the structure needed some internal squaring and stabilizing supports for when these boards were removed.

To do this, angled pieces of 2x4 were added all around connecting each corresponding pair of posts, like the one shown in the first photo. These are simply screwed through their faces to the posts. It doesn't matter which direction they are angled, as long as everything is square when they are installed. These internal pieces are the secret to making the entire structure solid and keeping it square. With these in place, all external supports were removed.

I then began adding the pine slats from the shipping crates all around to close in the walls. These were screwed in place through pre-drilled holes with exterior 1 5/8" screws. I kept the screws within about 3" from the board ends, so they would be concealed by the trim which was added last.

Once the walls were completed, I back-filled all the open areas around the cement blocks and tamped the soil down firmly.

Step 7: Roof

I deliberated for a few days on how do do the roof. I eventually settled on a simple, angled roof with corrugated recycled plastic panels.

I trimmed 3" off of the rear posts to get the desired angle. Two 10-foot 2x4 beams were installed centered across the tops of the front and back posts. These were screwed from the top down into the tops of the posts with 3" exterior screws.

An additional 10-foot 2x4 was added to each of these, both to serve as additional support to prevent sagging, and as a decorative touch. The ends of these were tapered slightly along the bottom edge as seen in the first photo.

Across the tops of the beams I added 2x4 rafters that were 82" long, 16" apart on centers. This length was chosen for looks. While I could have used full 8-foot rafters, they would have overhung quite a bit and just looked top-heavy. Each rafter was notched on a band saw to fit the slight angle where it joined the beams, and they were all screwed in place from the underside of the beams.

Across the tops of the rafters I added 1 1/2" by 1 1/2" strips to which the roof panels were attached. These were made from any remaining 2x4s and 2x4 scraps that I had at this point, which I ripped in half on a table saw.

The roof panels are "Suntop" brand corrugated foamed polycarbonate panels from The Home Depot. Each panel is 26" by 8'. The nice thing about these panels is that five of them install perfectly at 10 feet in width (each two corresponding panels overlap each other 2"). I trimmed off 14" from the length of each, and installed these with roofing screws (the kind with built-in rubber washers).

Step 8: Finishing

I painted the floor of the fort with Rustoleum "Deck & Concrete Restore 4X Deck Cover" paint shown in the photo. It goes on very thick and supposedly creates a hard, durable waterproof barrier. We'll see how it holds up over time. I put on two thick coats of the stuff.

Over the rest of the bare wood, I brushed on a coat of basic water proofer. I like that the wood is protected, but will still age naturally.

Step 9: Slide

As I mentioned earlier, the slide was a late but welcomed addition to the fort. We picked it up used for a few dollars, and it fit very nicely off the side of the fort.

To install the slide, I added a couple of 2x4s as support along the top rail to give me something to bolt it to. On the ground I dug out an inch or two of grass and dirt, and leveled out the area with sand. On top of this I added three large paver stones to support the bottom of the slide.