Building a Sub Floor for Your Shed

Building a sub floor on an existing or recently poured concrete slab is a task that is usually done by professional builders. With these easy plans in this tutorial, you’ll learn how to do this easily and the right way. Whether it’s a small shed or gazebo, this article describes the correct method for installing a sub floor.

After installing a foundation, the next step is to build a sub floor. Some prefabricated sheds or gazebos come with their own sub floors, but if they don’t, here’s what you will need.

Typically a small sub floor will only require 2×6 joists but if you’re going to put a lot of weight on it, then use 2×8 or larger joists. Joist hangers and hurricane clips are needed to help secure the floor down during high winds. Check you local building department for information on this. ¾ inch tongue and groove plywood is the standard on flooring, but you can go larger if you need to. Just don’t go any smaller than ¾ inch.

The first step is to cut the header joists. These are the exterior joists that hold the interior ones in place. Basically make a square on the slab with the 2×6. Once you have those cut nail the ends with galvanized 10d nails. Pull a diagonal measurement from corner to corner. Do this for both sides. The measurement should equal the same, if not, then you will need to adjust them until the measurements are the same. When it is square, attach to the slab using corner brackets and tapcons.

Now you ready to pull a layout for the floor joists. Hook the outside of the header joist and pull a layout 15 ¼ inches and so one. Subtract ¾ inch from the typical 16 inch layout. This way your plywood will extend to the outside and end up in the middle of your joist. Square down with a speed square on the inside of the header joist. Attach the joist hanger brackets on your layout.

Cut the 2×6 joists to fit and place into the joist hanger brackets. Nail with 10d galvanized joist hanger nails. Repeat this step until all the joists are in place and nailed properly. Now you’re ready for plywood.

Caulk the top of the joists with sub floor adhesive or liquid nail. This prevents any squeaking in the future and hold the plywood tight to the joist so it does not buckle. Lay down the first sheet along the outside edges. Make sure it is square with the outside and nail or screw the sub floor into place using 8d nails or 2 ½ inch decking screws. If the sub floor is going to be exposed to the elements, use galvanized. Nail or screw down with a 6 inch space between nails or screws on the seams and 12 inches everywhere else. Trim the plywood so that the joints are staggered every 4 feet. Leave a 1/8 inch gap between all pieces to prevent buckling and allow for expansion and contraction.

That’s it! You should now have a great looking sub floor for your shed. Paint or stain to your color choice, or simply leave it as is. It’s your choice so be creative! Enjoy!