Choosing which floor joist spacing you should use will depend on the width and the span and what size of floor joist as well as the grade of wood you’re using. The most common floor joist spacing’s are 24 inch, 12 inch and 16 inch. The reason for these spacing is for a 4×8 sheet of plywood to fit without having to cut the sheet.
If you have ever used a full sized framing square its size is 24 inches by 16 inches which works out perfectly for laying out a 24 inch joist spacing or a 16 inch joist spacing. While using a 24 inch joist spacing will save a few dollars it’s also going to be the weakest choice. For the extra few dollars a 16 inch joist spacing will be much stronger and give you the highest weight capacity.
In this article I’m going to look at how to determine what size floor joist you’ll need and how wide you can make the floor using 2×6 joists, 2×8 joists and 2×10 joists. I’m also going to look at how the grade of wood will affect the size of joist you’ll need. I will also cover how to lay out the joist so the plywood fits on the centers and save you time and money by not having to cut plywood and add extra pieces.
What Size Floor Joist and Spacing Will You Need
For a structure or storage shed that’s up to 8 feet in width you can use 2×6 floor joists at 16 inches on center. If your structure is between 8 feet to 12 feet wide I’m going to recommend you use 2×8 floor joists at 16 inches on center. If the structure is between 12 feet to 16 feet wide then you would want to use 2×10 floor joists at 16 inches on center.
These sizes are for number 2 and better stamped material. The most common type of material is SPF which is spruce, pine and fir and is the industry standard for framing material. If you’re building a shed floor I recommend you use treated material with ¾ inch treated plywood for your flooring. This will cost more than none treated material, but you’ll never have to worry about rot or be concerned if the floor gets wet.
How to Lay Out a 16 Inch Joist Spacing
I’m going to be using the example of an 8×12 floor for a shed; the layout would be the same for any size you would only need to cut the box joist to the size of your building. If you need some help with building your floor on skids you can read my article “How to Build a Shed Floor on Skids” where I go into more detail. You can read the article here.
Layout For 8 foot x 12 foot Floor
I have attached a drawing showing the floor joist spacing for the 8 foot x 12 foot floor with 16 inch centers.
Step 1] Cut 2 box joists at 12’- 0” long.
Step 2] Cut 10 floor joists to 7’- 9“ long.
Step 3] Line up the 2 box joists and begin marking 16 inches from the left to the right, these will be the centers of the floor joists. Just connect your tape measure and mark the 16 inch centers, most tape measures will show the centers by having the 16 inch centers numbers in red.
Step 4] Measure back ¾ of an inch from each center point and draw a line and place an X to the right side of the mark. This is where you’ll line up the joist and nail it to the box joist.
Step 5] Nail all the joists into place.
Step 6] Using the first 4×8 sheet of plywood place it flush with the outside of the box joist and flush with the outside edge on the left side. Once you have flushed the 2 outside ends nail the plywood into place.
Doing it this way will ensure that the structure is square and you’ll notice that the plywood will line up in the center of the joist.
Step 7] Cut a 4×8 sheet of plywood in half [4’ x 4’] using one of the cut pieces to finish off the row.
Step 8] Start the second row with the [4’ x 4’] piece of plywood on the left side and then add the last 4×8 sheet of plywood.
Step 9] Nail the plywood down and place nails 6 inches apart on the joints and outside edges and nail the rest of the plywood at 8 inches apart.
This will give you the strongest floor and having staggered the plywood makes it even stronger. Now you’re ready to build the walls.
What Grade of Wood is The Strongest?
The grade of wood you’re using will determine the size of material and the joist spacing. The 3 main grades of lumber for framing are:
– Number 1 which is the best “construction grade”.
– Number 2 which is the most common framing grade.
– Number 3 which is utility grade.
The most common species of framing material are:
– Douglas Fir.
You probably won’t need to be concerned about the different grades and species for your framing needs. The standardized framing material is number 2 with is rated as standard and better and will be what lumber stores will be selling.
The other option is ruff sawn lumber; the lumber you buy in your local lumber store is referred to as S4S. This means the lumber is planed down on all 4 sides and its dimensions are typically 1 ½ inches in thickness and half an inch smaller in depth. So a 2×8 S4S is 1 ½ inches thick by 7 ½ inches deep.
Where I live I can buy ruff sawn lumber and it’s not planned down so a 2×8 would have an actual size of 2 inches thick by 8 inches deep. However most ruff sawn lumber is not graded and is commonly used for barns and other structures that don’t need to have S4S lumber. If you want to build with ruff sawn lumber you’ll need to talk to your local building inspector’s office for the requirements before using it.
Best Floor Joist Spacing For a Heavy Load Floor
Maybe you’re going to be installing heavy equipment or even a hot tub on the floor, a good joist spacing for supporting heavy loads is 12 inches on center. If you know what you’re going to be installing on the floor you can really strengthen the floor where the item is going to be sitting. You can do this by doubling the floor joist right under the legs or bearing points of the heavy item.
Another way strengthen the floor is by installing 2×8 or 2×10’s flat on top of the floor joists. This will disperse the weight over a larger area of the floor. Just make sure that the supports holding the floor joist are secured in place so they can spread the weight to the foundation or to the ground if you’re using skids.
What Joist Spacing is Best For Shed Floor?
If your only building a storage shed the joist spacing will be decided by the amount of weight the floor is going to be supporting as well the local building authority may have some bylaws that will require the shed to be built according to local building codes.
The most common floor joist will be either 2×6 or 2×8 and the joist spacing would be 16 inches. As I mentioned earlier in the article the size of the shed will affect the size of the floor joist. The wider the shed the bigger the joists will be.
To be safe use 2x6s if the shed is 8 feet or less in width with 16 inch on center and if there’s going to be a heavy load use 12 inch centers, and use 2x8s if the shed is over 8 feet in width if the load is going to be heavy use 12 inch centers for joist spacing.
Use 2x10s if the shed is over 12 feet in width. If your shed is wider than 16 feet than you’ll need to add an extra support in the center and still use 2×10 joist 16 inches on center and if the floor load is going to be heavy use 12 inch joist centers.
What Size Joist and Spacing if Your Insulating The Floor?
Many folks are using there sheds for more than just storage, some are using it as an office space, a workout place and more and more Women are turning their sheds into a “She Shed”. If you’re interested in learning more about making a she shed you can read my article “How to Make Your Very Own She Shed Getaway” here.
The most common joist spacing to insulate is 16 inches on center. To get the most out of the floors insulation you’re going to need to use R22 insulation’s. It’s 5 ½ inches thick and comes in pre-cut batts of 47 inches by 15 inches and is made to fit into the joist space at 16 inches on center.
I have added a drawing showing the details for insulation a shed floor with R22 insulation: Follow the steps below for step by step instructions.
You’re going to need to use 2×8 floor joists to insulate with R22.
Step 1] Nail the 2×2 strips [1 ½ x 1 ½ inch] to the bottom on each side of the 2×8 floor joists, remember to nail a strip on the inside only of the 2 outside 2×8 floor joists.
Step 2] Assembly the joists and box joists.
Step 3] Make some 3/8 OSB strips to lay on top of the 2×2 wood strips and attach with small nails or short screws.
Step 4] Install the batts of R22 insulation into the joist space [they should fit correctly if you used a 16 on center joist layout]. You may need to cut a bit off of one end.
Step 5] Install 6mm vapour barrier over the entire floor, staple to top of floor joists.
Step 6] Install the plywood floor and nail into place. If the weather is rainy you can tarp off the floor to keep the insulation from getting wet.
For more detailed information on insulating a shed floor, walls and ceiling you can read my article “10 Tips For Insulating Your Outdoor Shed” here.
What’s The Spacing For Joist Trusses?
Not as common for use in small structures and are mostly used in large wood frame buildings and houses. These are designed to span greater distances then normal floor joists and are engineered for the project. There joist spacing is typically 16 inches on center.
Do Pressure Treated Floor Joist Use Different Joist Spacing?
Pressure treated and treated lumber is only treated to protect against rot and it does not affect the bearing capacity or the joist spacing. Follow the same guidelines for regular not treated floor joists. Pressure treated wood is more expensive than non-treated wood, but the extra cost will mean the floor won’t rot and will last much longer than non-treated material.
What About 2×4 Floor Joists?
Good question… Not recommended unless the shed your building is small, you can use 2×4 joists for a shed that’s not over 5 feet in width and place them at 12 inches on center. Not recommended if you’re going to be putting anything heavy in the shed. Just not a good choice and for the little extra cost use 2×6 floor joists.
The floor is one area you don’t want to cut corners, you’ll notice I never talked about 24 inch joist spacing… I’ve build many shed and home renovations and the only place I would consider 24 inch centers is for non-bearing interior walls. Using 24 inch centers on the floor is a poor choice and the floor will sag between the floor joists.
Just use 16 centers on your floor joists and you won’t have any problems with floor sagging. If you’re going to be putting heavy items in the shed use the recommendations in the “Best Floor Joist Spacing For a Heavy Load Floor” section above. I hope you have found this article helpful and be sure to share it. Thanks for reading.