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Kids Treehouse Plan - Part Two

Kids Treehouse Plan - Part Two

Preparing the Building Site

Welcome to part two on how to build a kids treehouse in your backyard.  In this portion of my treehouse plans, I will explain how to prepare the area where you plan to build and how to layout and install the support posts for the treehouse deck.  At this point, you should have received the appropriate approvals from your homeowners association and your local code enforcement agency as discussed in part one.  Hopefully, you have also acquired all the necessary tools and sketched out your design and documented the measurements.  

Here is what you will need in order to complete this step of the backyard treehouse:

Suggested Tools:

  • Post hole digger or shovel
  • Level and post level
  • Cordless drill and screws or hammer and nails
  • Measuring tape
  • Saw
  • Speed square for marking cut lines
  • Pencil or marker
  • Landscaping tools necessary to clear and level the area where the treehouse deck will be located
  • Mason twine for batter boards
  • Safety goggles and gloves

Building Supplies:

  • 4- 4x4 posts - Length depends on your specific design.  I bought 2 4”x4”x8’ posts and cut them in half so I could bury the post 2’ in the ground and have 2’ above the ground.
  • 8 - Bags of fast setting concrete - I used 2 bags per post hole but the number of bags you will need may vary depending on the depth of your holes.  See bags for recommendations.
  • 2 cubic feet of gravel - This will be used in the bottom of the post holes.
  • 1x2’s for batter boards (see step two for details) or 2x4’s for locating post locations.  2x4 length will be dependent on the size of your deck but mine were 10’ long.  If you go with batter boards, you will need enough lumber to make eight.  If each leg is 18” and the cross bar is 18” then you will need about 36’ of 1x2’s.
  • Landscaping cloth (optional)
  • Mulch (optional)

Treehouse Deck - Freestanding Version

My treehouse design is based on a freestanding deck that is made up of 4x4 posts buried a couple feet in the ground and anchored with cement.  I chose this design for our kids treehouse for a few reasons.  

 freestanding treehouse deck

First, I feel that it is much safer to build a deck just off the ground rather than placing it high up in a tree.  This way, if someone was to fall or jump off of it (parents of boys will understand this) they would not be falling/jumping from very high and would likely not be hurt, at least not too badly.  Another safety issue corresponds to the actual construction process.  I felt it would be safer and much easier to build close to the ground rather having to scale a ladder and tree throughout the entire build.

Second, I was not confident that I had a tree that was robust enough to handle a treehouse of the size I was wanting to build.  I have quite a few oak trees in my yard and they probably are capable of holding a treehouse but I did not want to chance damaging the tree or risk losing the treehouse if the tree was not strong enough.  Also, I live in an area that is prone to strong thunderstorms, tropical storms and hurricanes and I did not want to lose the treehouse in a storm or have my treehouse end up in my neighbors pool.

Third, a treehouse on stilts versus a treehouse in a tree would allow me to increase my square footage significantly.  The total area of my design is 100 square feet and I would not have been able to put this into a tree.  This amount of square footage allows for a small house to be built and still provide some room for porch areas.

Step 1: Clear and level the ground

Because the kids treehouse is just off the ground, it is important to properly clear the area where the deck will eventually be built.  You will need to remove all plants and bushes in the area especially those that will end up covered by the treehouse deck.  You will also want to trim any bushes or trees that are in the immediate area in order to give yourself some room to work.  Level and clear the area as best you can.  You may want to consider installing some landscape cloth and a thick layer of mulch once you have your posts set in order to keep weed growth to a minimum.

Step 2: Mark location of deck posts

Once the area is cleared and level, it is time to start laying out where your deck posts will go.  If you plan to bury them in the ground, make note of any nearby trees, bushes, sprinkler systems or potential plumbing hazards that you could encounter.  It is really important to ensure that your posts are square to each other so if have to move one because of tree roots, then you will need to shift them all in order to maintain squareness.  There are a couple methods that you can use to mark the spot for each post and be assured that they will form a square once they are all installed.

- Batter Boards and the 3-4-5 method -

The first method, which is what the pros use, is called batter boards.  Batter boards are essentially some 1x2’s nailed or screwed together in sort of an “H” shape.  You will need two of these batter boards for each post.  What you will do is hammer a batter board into the ground a few feet beyond the area you expect each corner post of the deck to be placed.  The second batter board in the same area will be hammered into the ground at a right angle to the first.  Once you have done this at each corner, you will then tie a string from one batter board to another batter board in the opposite corner so that the strings eventually form the layout of your deck.  Notice that the strings will cross each other near each corner.  This crossing of the strings is the location of where your posts will be once you have confirmed that the strings form a perfect square.  How do you know if the strings form a perfect square?  Well, reach deep inside your memory bank back to middle school math class where you were taught pythagoreans theorem.  This is essentially how you find the length of each side of a triangle.  For our purposes, let’s just refer to it as the 3-4-5 method.  What you will do is measure three feet down one of your strings and mark the spot.  Then from the same corner, you will measure four feet down the string going in the other direction and mark that spot.  You will then measure the distance between the two marks on your strings and the resulting measurement should be five feet.  If it is not five feet, then adjust the two batter boards in that corner so that you eventually reach the five foot measurement.  You will then repeat this process in each corner so that you eventually have a perfect square formed and that each string crossing is now the location of your posts.

 laying out the kids treehouse

- An alternative method -

The soil in the area where I wanted my treehouse was too sandy and loose to properly hold the batter boards and I had some shrubs that were in the way to do this properly so I needed to devise another method to locate the proper position of the posts.  Since my treehouse deck was going to be 10x10, I bought four 10’ 2x4’s and attached them together in a square.  I then used the 3-4-5 method to square the 2x4’s and marked the location of the posts accordingly.  I kept the 10’ 2x4 square in place while I dug the post holes and inserted the posts to act as a guide and ensure proper positioning.  

Step 3: Install the posts

Now that you have the location of where your posts will be located you can start digging the post holes.  The holes should be dug below the frost line if that is an issue in your area.  You will want to dig your holes several inches deeper than you want the pole buried as you will need to add a few inches of gravel to the bottom of the hole in order to allow for proper drainage of water away from the bottom of the post.  Once the gravel is in the bottom of the hole, add the post and your first bag of concrete, paying close attention to make your post level and the post faces are squarely facing the location of the other posts.  Follow the directions on the concrete bags to add the proper amount of water and the number of additional bags that are needed to properly fill your post hole.  Continue this process for the other three posts.  Again, pay close attention to making sure the posts are level vertically and horizontally.  Allow the concrete to dry according to the directions on the bags before moving on to the next steps.

 kids treehouse deck

Step 4: Install landscaping cloth and mulch (Optional)

At this point, it would be a good time to lay down the landscaping cloth that will prevent weeds from growing underneath and around your treehouse.  If your treehouse deck is low, it will be difficult to get underneath of it once you add the joists and decking so doing this step now will save you a lot of effort.  Lay the cloth according to the instructions.  You can cut holes in the cloth to fit over your posts which will give you a tight fit and further prevent weeds from growing around the posts.  You can add the mulch at this point as well.  If you do not add the mulch now, I would recommend doing it after you attach your joists but before you add the decking.  You will be able to dump the mulch between the joists and reach in to spread it around without your decking in place.  Once the decking is there, you would have to crawl underneath the treehouse deck to get the mulch in.

So while you are waiting for your concrete to dry, I am going to prepare part three for your backyard treehouse plans.  In the next step, your treehouse project really starts to come together as we add the floor supports and the deck boards.  It will be exciting for both you and the kids to actually have something they can stand on!

Kids Treehouse Plan - Part Three

Kids Treehouse Plan - Part Three

Kids Treehouse Plan - Part One

Kids Treehouse Plan - Part One