How to Build a 12x12 Island Deck
A Guide For Building An Isolated Platform
So the season has come. Spring is in the air and you're now ready to make good on that playhouse you promised to build for the kids. You take a look over your backyard and evaluate your options.

For most properties, an island platform to support their playhouse isn't necessary, as most sheds and playhouses are built on near level terrain. A few cinder blocks or layer of gravel is enough to flatten out the grade.

However, if you're planning on building on a steep grade or just want an elevated deck for your kid's house, a island deck is the ticket. The steps below are for a 12' x 12', but the same principles can be applied to any sized deck. A PDF plan of the below steps can also be downloaded.

Tools & Lumber that will be needed:

Reciprocating saw
Measuring Tape
Speed Square
Drill &/or Impact Driver
Circular Saw
Jig Saw (optional)
Miter Saw (optional)
Chalk Line (optional)
Compass (optional)

( 25 )   5/4” x 6” x 12'

( 6 )   2” x 4” x 12'
( 6 )   2” x 6” x 12'
( 12 )   2” x 8” x 12'
( 2 )   2” x 10” x 16'

Posts & Balusters
( 10 )     4” x 4” x 8'
( 72 )     2” x 2” x 36”

Constructing the Island Platform

Step 1: Digging holes

Four holes will need to be dug.  No matter the angle of the grade, the center of the holes should always match the bird's eye view of the left illustration.  Check what the frost line is in your area and dig down accordingly.  The holes should be approximately 12” in diameter.

Step 2: Installing the Posts

Unless you have a severe ground slope or want your platform to be higher, an 8' 4x4 should be adequate for a single post.  Place each 4x4 in the four holes and plumb them with a level and two furring strips.

After the post has been secured, fill the remainder of the hole with concrete, as depicted in the left illustration.  Repeat this four all four holes, as shown below.

Step 3: Notching the posts

After the concrete has dried, the notches for the beams can be made in the posts.  Use a water level to mark the bottom of where the notch will begin for all four of the posts.  Once all four of the posts have been marked, make the 1 ½” by 9 ½” notch in each post as depicted in the left illustration.

Step 4: Beams


From (2) 2x10x16s, cut two 13' pieces.  Install these pieces to the posts as depicted below.  Be sure to have matching cross measurements to maintain squareness.

Step 5: Joists

From (12) 2x8x12s, cut ten 11' 9” pieces and two at 12'.  Assemble these pieces together as depicted in the left illustration.  Maintain a 16” on center joist layout.   Be sure to have matching cross measurements to maintain squareness.

Step 6: Decking

From (25) 12' decking boards, cover the joists as depicted in the below illustration.  Space the decking boards apart by ¼”.

Step 7: Entry Ladder

From (1) 2x6x12, build the ladder shown below.  Note that the ladder stringers will need to be shortened or lengthened depending on the final height of your deck.

Step 8: Railing Posts

From (6) 4x4x8s, cut twelve of the piece shown in the left illustration.  Install these twelve posts to the platform as depicted below.

Step 9: Rails

From (6) 2x4x12s, cut and install the six pieces as shown in the left and below illustrations.

Step 10: Balusters

From (72) 2x2x36” balusters, cut seventy two 32” pieces.  Install these balusters to the rails as depicted in the left illustration.  Install these balusters 5” on center.
An island patio / deck with a white fence and a playhouse on top

March 20, 2017 — Paul Gifford