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  • The Ruthless Maker

How to Build the Foundation for a Raised Play Fort - One Room Challenge; Week 2 Update

This week has been all about designing the raised play fort, ordering and purchasing material and building the play fort foundation. Here are some inspo pics for the playhouse. I knew that I wanted modern lines, with a gabled roof, black exterior, something that would age well (design wise) and blend in with the forest surroundings.

Here is the design I came up with. The play fort will be three sided, and stand four feet off the ground, with a slide and rock climbing wall. The roof will be gable style and be made out of corrugated plastic roofing panels. The walls will either be cedar boards or paneling in black. In front, there will be a slide, and on the side a rock climbing wall. There will likely be stairs too but I haven't added them into the design just yet.

This week I focused on building a strong foundation for the play fort.

The play fort will have a 96" x 99 (8' x 8'3") raised base that will stand four feet off the ground. I chose this size to keep with conventional lumber lengths (8' length). This reduces unnecessary cutting, and since I've never built anything like this before, I wanted to keep the build manageable.


The height of 48" I chose because I wanted the option to add on a slide. Most off the rack slides are spec'd for a 48" play deck.


Since my kids will be playing on this structure, safety is very important. I did a lot of research on how to make a stable foundation for the base of the playhouse. Here are the options I explored:


1. Deck blocks - this would be the easiest approach. Deck blocks sit above ground and there is a slot for the 4x4 post to sit in. Deck blocks don't offer much side to side support since the post is not secured to the concrete deck block and the deck block is not secured to the ground.

2. Deck piers - this involves digging a four foot deep hole, placing a Sono tube (its like a giant toilet paper roll that comes in different sized diameters), filling it entirely with concrete and then sinking a brace on top while the concrete is still wet. The post is set into the brace once the concrete is dry and then secured.


3. Post in concrete (fence post method) - this involves digging a four foot deep hole, adding some gravel at the bottom, setting in your fence post and then filling the hole up with concrete.


*The reason we dig to four feet is to get to the frost line. If you do not set your foundation below the frost line, then when the ground freezes in the winter your foundation will shift. Frost lines vary depending on where you live. I live an hour north of Toronto, Canada, if you live where its warmer you likely will not have to dig this deep. There are tables on the internet you can look at that will tell you your frost line*


I decided to go with the fence post method (post in concrete). I felt it was good balance between difficulty level to install and sturdy foundation.


First, I marked out my post locations. I did this by laying out the 2x8x8s that I would use to make my base and using a speed square to square up the edges. I staked the corners and used spray paint to mark out my lines.


Next, I dug the holes using a spade, pick ax and a post hole digger. The holes diameter is 12" and depth is 4'.



With the holes dug, I put 0.5" diameter gravel in the bottom of each hole. This 30kg bag was $7.20 at Home Depot, and one bag is plenty. I had bought 3 bags, but ended up using just 1/2 a bag.


Then I built temporary post braces using the 2x8x8s. I squared these up then placed a 4x4 post in each hole I dug and tacked it to the brace using a wood screw. Using a level, I made sure that each post was plumb (vertically straight) and secured them to the temporary post brace before dumping 2x bags of post concrete into each hole and adding water (follow the instructions on the bag), I used Sakrete Quick Post. Be sure to get the concrete mix all around the post.

Couple of tips and lessons I learned are:

  1. Read the instructions carefully. On three of the holes I didn't realize I was supposed to simultaneously pour the concrete powder and the water into the hole at the same time. This resulted in a soupy goop on the top and it took a while for the water to soak down. On the last hole, I actually mixed water and concrete powder simultaneously and it set up in under 10 mines. Obviously the method may vary depending on the type of concrete you buy, so be sure to follow the instructions on the bag you buy.

  2. Even pressure treated posts can rot in concrete. I learned this after I had already set my posts. The way to slow down the process is to ensure you have concrete that extends past the soil line, and you angle the concrete to slope away from the post to allow for water to drain. I actually have ordered 3 more bags of concrete to further fill up my holes so the posts are not in contact with the soil. I sure am glad I learned this before I through soil in to fill the rest of the holes.

Now that I have the posts set in concrete, my next job will be to add in my rim joists and start building out the platform! This will be my goal for next week of the #Oneroomchallege!


For more details an step by steps, you can check out my ORC highlight bubble on my Instagram profile @theruthlessmaker. Also, don't forget to check out some of the other One Room Challenge projects other designers/builders are doing. There are definitely some cool rooms being transformed.


Thanks for following on with me so far!


~The Ruthless Maker~










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