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 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.
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.
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.
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.
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.
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
This is an example of a foundation frame for a 12×16 shed.
Install post and piers
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.
This graphic shows a 5 1/2 inch tall frame with 1 inch siding overhang and 4 inch gap from ground level.
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.
Build the outside of the stem wall the same size as your shed.
Attach with j-bolts
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.