The floor joists in your shed are an integral part of the structure’s soundness. They support anyone and anything that goes into your shed. With this in mind, it’s vital that the flooring be sound and able to support sufficient weight without problems. This is also a place where moisture seepage can cause problems, as most people tend to focus on roof leaks. Keeping potential moisture problems in mind while constructing your floor will prevent trouble and a lot of work later.
As far as joist spacing, the best is 16 inches on center. Since your floor joists will be holding weight along their length, closer spacing is vital. The recommended spacing is 16 inches on center to increase the load capacity of the floor and eliminate chances of bouncing.
Spacing can vary with the distance being spanned by the joists. There are calculators online that can be used to figure the spacing under varying specifications, such as the type of materials you plan for the floor. Building codes in your area should be checked and followed. Some might have different spacing requirements based on the width of the joist planks used. When you submit your shed plans for a building permit, be sure to specify your joist width and spacing distances.
Should I Use 2×6 or 2×8 for Shed Floor?
Most people use 2x6s for the floor joists. However, if you have a larger shed, meaning that the joists will stretch out over a longer distance, you may want to go to 2x8s. The maximum span recommended for 2×6 joists is nine feet, three inches, with a minimum bearing length of three inches at each end.
Also, if you plan to store heavy items, such as ATVs, welding equipment or lawn tractors, 2x8s will remove doubts about the stability of the flooring base. The grade of lumber you intend to use for the joists may come into play as well. Too much support is better than too little.
How Thick Should Plywood Be for Shed Floor?
If using plywood for the shed floor, you may want to consider putting the joists on 12-inch centers. Remember that shed floors usually have to hold a lot of weight. Even the best plywood can sag over time if the joists are too far apart. The recommended thickness for a plywood floor is 5/8 inch exterior grade plywood.
One source recommends pressure-treated exterior-grade CDX plywood. It is resistant to rot, even featuring plugs replacing knots in the wood to give it more moisture resistance and keep the surface smooth.
There is a type of plywood called tongue and groove, or T/G plywood. This is made with tongues and grooves at the edges that enable one sheet to fit into the next. While some sources state that this type of plywood is only good for eliminating squeaking between the sheets when weight is put onto it, others state that it adds strength to the flooring.
This is because the connections between the sheets help to distribute weight to other sheets and make a more rigid subfloor. It may tend to be harder to install, as the joints fit tightly. They don’t always go together easily. Keeping the flooring dry before installation will help with this problem. For information on using the strongest flooring material read my article What Are the Strongest Shed Floor Materials to Use?
Do the Floor Joists Need to Be Treated?
They don’t have to be treated, but since they will be the lowest wood in your shed, it would be rather foolish not to use treated lumber. Joists will last longer if they are treated. Even if your shed is raised to allow airflow and moisture drying underneath and you have protective barriers for wood parts that contact the ground or concrete, evaporation will cause moisture to rise and affect your joists. Read my article How High Should a Shed Be Off the Ground? to see if you need treated floor joists.
Does the Plywood for Floor Need to Be Treated?
Pressure-treated plywood is recommended for flooring or subflooring. There is a grade of plywood available called MR grade, which is made to be moisture resistant. It is a very popular grade of plywood and is even used for furniture.
How Do You Lay Out Floor Joists?
You won’t need a lot of tools to create your floor joists. You’ll need a level, tape measure, hammer or nail gun, speed square, chalk line and circular, miter or chop saw. For materials, you’ll need your joist lumber, 3-inch, 16 penny framing nails, and wood preservative. You may choose to use joist hangers with 3-inch exterior grade Simpson screws to attach them for extra strength.
Cut your frame according to the length and width of the shed, using all end cuts with wood preservative. Nail the frame together with the 3-inch framing nails, using three nails for each junction. Check to make sure your frame is square. A simple way to do this is to use the tape measure to stretch from one corner to the opposite corner diagonally. Then do the same with the other two corners. If the measurements match, your frame is square.
Now it’s time to cut your joists. One good idea is to just cut one and make sure it fits into your frame correctly. If it doesn’t, you can make adjustments to your measurements. Mark out the joist centers on the frame using your chosen spacing. Now you can fit in your joists, making sure the top edges are even with the top edge of the frame boards.
If using joist hangers, attach them with the Simpson screws, making sure that the center of the hangers matches the joist centers. Slide your joist ends into the hangers. You may need to tap them down a bit with a hammer. Attach the joists to the hangers, using the 3-inch nails. Angle the nails so they go through the joist and into the frame. I have another article What Size Floor Joist Spacing Should You Use? where I go into step by step detail on how to layout the floor joists if you need more help.
Planning the joists and subfloor carefully ensures that your shed will last a long time without worries. Materials are an important consideration, as the various types and grades of wood vary in strength. If you have questions about the best type of materials and best spacing for your joists, be sure to ask your supplier.