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Kids Treehouse Plan - Part Three

Kids Treehouse Plan - Part Three

Kids Treehouse - Building the Deck

Welcome back!  Here we are on part three of your backyard treehouse build.  In this step, we really start to see the fruits of all your hard work.  If you are here for the first time, be sure to check out part one and part two of the backyard treehouse plans as they discuss how to properly prepare for your treehouse project before any building is started, how to prepare the building area and finally, how to correctly place the posts that will be the primary supports for your treehouse deck.  In the previous posts, I also discuss in detail all the tools and building supplies I recommend you have in order to complete the treehouse build properly and efficiently. 

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

Suggested Tools:

  • Saw
  • Cordless drill/driver or nail gun
  • Measuring tape
  • Speed square
  • Pencil or marker
  • Safety googles and gloves

Building Supplies:

  • 4 - 2”x6”x10’ pressure treated lumber for beams - Length depends on your specific design.  My design required 10’ long 2x6’s.
  • 11 - 2”x6”x10’ pressure treated lumber for joists - Same comment as above.
  • 22 - 5/4”x6”x10’ pressure treated pine deck boards.  Adjust the length per your design or use an alternate material for a different look.
  • 1 - 2’x2’ plywood.  This will be cut into smaller sections to sandwich between the beams where they attach at the posts.
  • 3” deck screws or equivalent nails - I like square or star drive screw heads over a standard phillips head because they do not strip nearly as easily.  You will need a large box of these as you will use a lot of them.  You could drive 20+ of these into every deck board so you will go through at least a couple hundred on the decking alone.  You will also need some for the beams.  Any leftovers will be used in the next phase of the treehouse build when you start framing walls.
  • 4 - Post Cap/Base for attaching the beams to the top of the posts.  10d nails or #9 x 1.5 strong drive screws.
  • 22 - Hurricane ties for joist hangars.  8d nails or #9 x 1.5 strong drive
  • 1 Box of #9 x 1.5” Strong Drive Screws for post cap/base and hurricane ties
  • 8d and 10d nails as an alternate to the strong drive screws above

Step 1: Install the beams

Now that your posts are properly placed and the concrete has set, the next step is to add the beams over the posts.  First, you will need to place the post caps on the top of each beam and secure them with the appropriate nails or screws.  Next, you will take the beams and place them inside the post caps.  There should be two 2x6 boards placed inside each post cap to form the beam for the front and back of your deck.  It is likely that your two 2x6’s will not completely fill the post cap space.  It is important that your beams fit tightly together in the post cap and one way to do this is to cut a piece of 1/2” plywood to sandwich between the two 2x6’s.  The plywood does not need to run the entire length of the beam.  It only needs to be long enough to fit within the the post cap however, I would cut it so that it extends a few inches in either direction outside of the cap.  Once the plywood is between the beams, drive a few screws or nails into each beam and into the plywood on either side.  Continue the process by securing the post cap to each side of the beam using the screws or nails that are appropriate per the post cap specifications.  

Treehouse freestanding deck on beam

Step 2: Install the joists

Now that your beams are in place and properly secured, it is time to start hanging the joists.  The joists will run perpendicular between each beam and will be attached to each beam with a joist hangar.  The spacing between joists is determined by the length and dimensional size of your joists.  If you are building your kids treehouse according to my plans, you will space your joists 12” apart on center.  On center means from the center of one joist to the center of the next one is 12”.  I wanted to be sure that I had a joist flush with each end of the beams so your joist spacing may need to be adjusted slightly to ensure even spacing of the joists between the two ends.  I also had a tree that I wanted to build my treehouse deck around so I had to adjust my spacing a little bit so that I had a joist on either side of the tree trunk.  Place your first joist hangar at the end of one beam and the next joist hangar at the end of the beam directly across from the first.  Attach the joist hangars to the corresponding beams with the appropriate screws or nails.  Once attached, insert your first 2x6 joist into the hangars and secure the hangar to the joist with the same screws or nails you used to attach the hangar to the beam.  From the center of your first joist, measure 12” down the beam and mark the spot.  This spot will mark the center of your second 2x6 joist.  Go to the other beam and mark the same measurement.  Attach two more joist hangars at these marks remembering that the center of the joist hangar should be aligned with each mark.  Drop in the 2x6 joist and attach as you did the prior one.  Continue this process until all joists are hung.

Treehouse freestanding deck joists
freestanding treehouse deck joists

Step 3: Mulch (optional)

At this point, your treehouse support structure is complete.  If you have not put mulch down, I would do that now.  You should at least add the mulch in the area under the joists as it is still fairly accessible.  Dump in a nice thick layer of mulch and spread it around evenly so that all of the landscaping cloth is covered.

Step 4: Install the decking

All you have to do now is add the deck boards and you will finally have something you can actually stand on!  The deck boards will run parallel to the beams and across the joists.  The deck boards should start and end with little to no overhang on both the beams and the end joists.  In other words, the deck boards should not overhang the beams or joists by much.  If they do overhang, you can cut them before installing them or you can install them all and then cut off the overhang later if you have a circular saw or jigsaw.  Any overhang will need to be notched later when we add the railing posts.  

When installing the deck boards, it is important to keep a few things in mind.  First, you want to make sure that they are installed straight.  If your first board is crooked by 1/2”, that will compound as you add more boards.  Take measurements and make adjustments as you go so that when it comes time to install that last board, you do not have to rip cut it at some strange angle in order to cover the last few inches of your deck.  Second, you want to allow a proper and consistent space between each deck board.  If you are using the pressure treated deck boards from my treehouse plans, then you should butt the boards together because as they dry, the boards will shrink and small gaps will appear.  If you decided to go with composite decking, then follow the manufacturers guidelines on how much space should be left between the decking boards when installing.  Make yourself some kind of jig or find an object that is close to the recommended gap so that you get an accurate space for every board.  

kids treehouse decking

Before installing any decking, measure the actual width of your deck between the outside end of each beam.  Then, measure the width of your deck boards.  Divide the total width of your deck by the width of an individual deck board plus any required spacing (deck width / (deck board width + deck board spacing).  Hopefully, you come out with a whole number or something close to a whole number.  If not, you will either need to adjust the placement of the first and last deck boards so that they overhang the beams as little as possible or you will need to rip cut the first and/or last deck board so that they end flush with the beams.

Lay the first board across the deck so that it lays along the length of the outside of the beam and the ends are flush with the end joists.  Measure from the inside of the deck board to the outside of the opposite beam.  Do this for both ends of the deck board and make adjustments until your measurements for both ends of the board to the opposite beam are equal.  This ensures that your board is straight.  Drive two nails or screws through the decking and into the first joist.  Drive two more nails or screws into the other end of the decking and then confirm that you are straight again by taking the same measurements as before.  If your measurements are equal, drive two screws/nails into the decking at every joist.  Place your second deck board on the deck and butt it up against the first one (if using pressure treated lumber) or spaced appropriately if using composite.  Screw or nail this board in place with two screws/nails at every joist and repeat the process until the deck is completely covered.  Remember to confirm that your decking is straight after attaching every two to three boards and make any necessary adjustments as you go so that your last deck board ends flush or close to flush with your last beam.

Step 5: Relax

Once all of your decking is attached to the supports there is one more critical step.  First, grab a comfortable chair and place it on top of your new deck.  Second, grab a cold refreshing beverage of your choice and bask in the glory of all that you have accomplished to this point.  Your kids treehouse deck is now complete.  Next, we are going to frame the treehouse walls, add the roof support structure and attach the siding and roofing materials.

Kids Treehouse Plan - Part Four

Kids Treehouse Plan - Part Four

Kids Treehouse Plan - Part Two

Kids Treehouse Plan - Part Two