The main function of your sheds foundation is to transfer the weight of your shed and its contents to the ground.
But it also serves several other important functions.
- It provides a way to level your shed if necessary
- It protects your sheds floor against moisture
- And it protects your shed against termites
- When necessary can protect your shed from movement resulting from frost heave, water or wind.
4 General Types Of Shed Foundations
- Simple skid foundation on earth, gravel or concrete blocks
- Wood and concrete pier foundation
- Concrete slab which serves as both a foundation and floor
- Floor-less foundation when you want to build a shed without a floor
Simple Skid Foundation
A simple skid foundation consists of two pressure treated runners laid parallel on the ground.
This is a lot cheaper than a concrete slab and has the added benefit of keeping your shed portable should you ever want to move it in the future.
Pressure treated means that the skids are rated for long term ground contact and are resistant to water rot and termite damage.
Pressure treated wood is usually some shade of green as a result of it’s chemical treatment and is labeled with a tag to identify it as being pressure treated.
See Table 2 and Figure 2
Cut your skids to length using measurement “B” From Table 2 in your shed plans. Don’t trust the factory cuts because large pieces of dimensional lumber are usually ¼ to ½ inches longer than stated.
Select a location with adequate drainage then clear and level the building area.
You can lay the skids directly on the ground, on concrete blocks, or on a bed of gravel.
Their spacing will differ depending on the width of the shed you’re building. The measurement for the skid spacing is measurement “D” in Table 2, and is shown on Figure 2: Floor Dimensions and Layout.
If your site is fairly level you can lay the skids directly on the ground.
If your site is not level then place concrete blocks under each skid every 4 feet or less and build the low points up with more blocks and wedges until the skids are approximately level. 
Dig the ground out to provide a stable base for the blocks or skids where necessary.
Don’t worry about getting the skids perfectly level at this point because you’ll make the final level when the floor frame is complete.
If drainage is a problem you can dig a trench under each skid about 12 inches wide and 12 inches longer than the length of your shed, fill it with gravel and place your skids on top.Connecting the skids together
As long as you support the joints well you don’t need to connect the skids together because they are not structural. Their only purpose is to transfer the weight of the shed to the ground. But if you want to connect them together you have 2 options.
- You can use pressure treated 2×4 splices about 48 inches long on either side of the joint, nailed with 16d nails or screws or bolts.
- Instead of using 4×4’s you can sandwiched two 2×4’s or 2×6’s together and offset the joint 48 inches and tie them together with 16d nails or screws or bolts.
The second option is better and easier if you want the extra strength.
Shed Tie Downs
If you expect to have a problem with your shed moving from frost heave, water or wind, or if it’s required by building codes in your area you can tie your shed down.
I’ve already covered shed tie downs in these other posts…
- Why you might need to tie your shed down
- Cheap home made shed tiedowns
- Frost heave and your shed foundation
If you need to then just skip over to my shed tie down posts to see what you need to do before you begin the next step, which is building the floor.
Wood And Concrete Pier Foundation
The location, number, size and depth of the piers might be dictated by building codes. It will also be a function of the weather in your area. You might need to dig down below the frost line to get the best results.
In the absence of building codes and with moderate weather you should have a pier at each corner, about 12 inches in diameter and going down 24 to 36 inches into the ground. In addition you will need a concrete block support every 4 feet or less in between the piers.
Dig Holes
Lay your skids out, level and square them as above. But don’t put a support block at the end of the skids where you will place the piers. Mark the ground where you will dig your holes about 6 to 8 inches from the end of the skids.
Move the skids out of the way, dig your holes and put the skids back in place. Measure from the bottom of the skid to the bottom of each hole and cut a pressure treated 4×4 pier 4 inches less than this measurement. This will allow enough room for concrete to flow under the bottom end of the pier to prevent wood to earth contact. Just an extra precaution.
Install Piers
Place the 4×4 piers into their holes and secure them to the skids. You can use a metal mending plate on each side, or a metal strap going over the top of the skid attached to either side of the 4×4 pier. Or you can use a specializes metal Simpson tie if they are avaliable in your local store.
With the 4×4 piers hanging down in the empty holes, re square and re level the skids. When they are correctly positioned then fill the holes with concrete up to ground level and let them dry for a day or two.
Finished Foundation
Now you have a solid foundation to build your floor on.
Before you sheet your floor you can add some metal straps or H25’s to tie the skids to the floor joists for extra security.
Foundation for floor less shed
You don’t necessary need a floor in your shed as long as you have a suitable foundation. Here are 2 foundation options if you want to build without a floor.
- Pressure treated wood with post and concrete piers
- Concrete stem wall
It’s important that you build the foundation tall enough to keep the siding away from the ground where moisture and termites will damage your shed. I recommend at least 4 inches of distance between the ground and any untreated wood. Like the bottom edge of the siding.
One way to accomplish this is by increasing the stud length. This will reduce the lower siding overhang and make your walls taller. I recommend a minimum of 1 inch lower siding overhang to prevent water from seeping under the bottom plate.
This means your foundation needs to be at least 5 inches above ground level.
How deep you go will depend on building codes, frost level and if you will have animals trying to dig under your foundation to get in or to escape.
Pressure treated wood with post and concrete piers
Layout perimeter
Lay out pressure treated 4×6’s to make a wooden perimeter frame the same size as the shed. Turn them so they are 6″ tall. Pull a tape measure diagonally across to make sure the frame is square.
This is an example of a foundation frame for a 12×16 shed.
Install post and piers
Mark where you want your post and piers to be. Remove the pressure treated wood perimeter frame and install the piers every 4 to 6 ft, as described previously.
And make sure it’s level and square then secure the wood perimeter frame to the uprights with galvanized metal straps.
Don’t worry about tying the individual perimeter pieces together because once you tie the shed in, that will tie all the foundation pieces together.
Attach shed
Attach the shed walls to this perimeter frame with 3 inch nails or screws through the bottom plate and galvanized 8d nails every 8 inches through the siding overhang.
This graphic shows a 5 1/2 inch tall frame with 1 inch siding overhang and 4 inch gap from ground level.
Got animals?
As an option you can install more pressure treated wood below ground level to keep animals from digging under the walls.
Concrete stem wall
Another option is to build a concrete stem wall.
This can be a footer with a concrete block stem wall or a mono pour with the footer and stem wall made at the same time.
Build the outside of the stem wall the same size as your shed.
Attach with j-bolts
Attach the shed walls to the stem wall with j-bolts embedded in the concrete 12 inches off each corner then 48 inches on center.
The top of the stem wall should be 5 inches above ground level minimum, which makes a 4 inch gap from the ground to the bottom of the siding when using a 1 inch siding overhang.