How to Put a Foundation Under an Existing Shed

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Ever gone to your utility shed to take out the lawn mower only to have your foot go through the plywood floor? Looked at your tool shed and noticed it’s got a decided lean from a frost heave or recent rainstorms? What do you do? How to put a foundation under an existing shed? Do you have to take it apart? Hire a contractor? Get a permit? What are your options?

The good news is that it is possible to put a foundation under an existing shed. Depending on its size, it could be easy to do too. Also, a great opportunity to clean out and organize your shed!

 

What You’ll Learn In This Post

 

When to Put a Foundation Under an Existing Shed?

  • No Foundation. The shed is on the ground
    Many backyard sheds just rest on the ground. They may have been set down on gravel, but over the years it has disappeared. Many steel box sheds have a plywood floor attached to braces. Time and Mother Nature though have rotted the floor or shifted the structure off level. This makes it difficult to open and close doors.

  • Some sheds are built on skids. The ground settles or shifts
    Some homeowners have used skids to create a raised level platform for their shed to sit on. Over time, the ground settles or shifts, wood rots, and you have the same problem of having no foundation.
  • Existing foundation is sinking
    You didn’t build on bedrock and your existing foundation is sinking. Again making doors/windows difficult to open or close. It could also be enough of a tilt to put the structure itself at risk of collapse.
  • Existing wooden (untreated) foundation is starting to rot
    A lot of older wooden sheds rest on wooden foundations. Moisture and time have caused the wood foundation to begin to rot. It’s only a matter of time before your whole shed is rotten or collapses.

how to put foundation under existing shed

What are the Foundation Options?

Concrete blocks foundation is a possible solution and good for bearing weight. Concrete blocks are also small enough to move around under a shed wall. These are available at most building stores or even second hand. Patio stones are another possible solution. They create a solid floor or foundation style support for a light structure.

Deck blocks have become a common solution for supporting sheds. They are a denser more solid concrete than cement blocks and can carry a lot of weight. I use them at the corners of my sheds and across the middle depending on shed dimensions.

The final two solutions need more work but create a more solid and permanent foundation. They are a poured concrete slab or a foundation wall with a footing.

Keep in mind that the more work involved, the greater the need to ensure you have all the necessary permits. You don’t want a fine or to have to tear out all your hard work.

How to Put a Foundation Under a Shed

Every shed is unique. So every solution will have some differences, but also some similarities. The size of the shed and the number of windows and doors affect your choice.

The building materials used and space accessible around the shed also impact what you do. Your budget and your experience and that of your helpers, also determine your options.

The first thing you need to do when putting a foundation under a shed is to lift or move the shed. The smaller the shed, the easier this could be.

Step #1 Lift or Move You Shed

How to Jack Up a Shed

Small Shed

If the shed is small (8’x8’) and sitting on the ground or skids, you’ll need to get a lever under the wall. I use a concrete block as a fulcrum for my lever. Have some wood blocking available to put under the wall when it’s lifted.

You Will Need

  • 2×4 or 2×6 levers
  • Concrete patio blocks or treated woodblocks as spacers

Step #1 Remove all items from your shed

Step #2 Use lever and block to lift one corner.

Step #3 Have your helper slip in the blocks underneath.

Step #4 Move to the next corner.

Only use this method if one or two people are going to be able to lift it with little effort. Don’t put much pressure. Be careful because if the lever breaks you’re going to be hurt.

Large Shed

If your shed is larger (10’x20’) and you want to put a foundation under it, it will take more planning and effort.

You Will Need

  • Power Jack Puller
  • 4×4 blocks
  • 3 1/2 inch high cinder blocks

Step #1 Slide two 4×4″ about 2 1/2′ from center on each side of the shed

Step #2 Next, lay one 4×4′ underneath the two 4×4’s

Step #3 Dig out from under that, set the jack in the center and put a cement block under the jack.

Step #4 Lift the shed up enough to slide concrete block under each corner.

Step #5 Go to other side and do the same.

Step #6 Slide a 4×4” dead center and place the jack under 4×4” and lift up again until you can add cinder block to the 2 corner blocks.

 

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lifting shed

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Images courtesy Shed to House

How To Move You Shed

There are different ways to move a backyard shed. One is using metal or plastic pipes,

Another method – using a heavy-duty dolly

Step #2 Build a Shed Base

Block or Patio Stone

After the shed is lifted, you can install your foundation.

Don’t forget to put a sill gasket or strip of plastic between the foundation and the wall. This will help prevent the movement of moisture.

I helped a buddy put a foundation under a small shed not too long ago; the two of us picked up the shed and moved it out of the way.

We then removed a depth of 4 inches of ground covering; put in 4 inches of gravel, leveled and packed it down.

Then we used 24”x24”x 2” patio stones to cover the gravel. We then picked up his shed and placed it on the patio stones with a 2” sill gasket between wall and patio stone.

A couple of tapcon screws pre-drilled in the bottom wall flashing helped the walls would stay put. Took us less than a day to do the job…although it took him a week to empty it!

Concrete Slab or Foundation Wall

You’ve raised and secured your shed, now what? You need to excavate for a floating slab, a footing and foundation wall, or a small pad to rest deck blocks or piers on. For most sheds with attached floors, a foundation wall or deck blocks are adequate. A shed with no floor is more likely to need a slab to act as a floor.

I helped another buddy with a floorless shed a couple of years ago. We were able to raise and level his 12’x24’ shed using bottle jacks. We set the jacks on concrete blocks lifting against 4“x4”s bolted to the walls. The 4”x4”s extended about a foot beyond the shed’s corners. Once raised, we put blocks under the 4”x4”s so we could release the bottle jacks and stabilize the structure.

With the shed lifted, we were able to dig out under the walls, and then use a bobcat to excavate the rest. We formed up the perimeter, tamped gravel in, put plastic down as a barrier, then put in a rebar grid. With that done, we had ready mix concrete delivered and poured the floor. We smoothed the concrete, left his daughter’s footprints in the corner, and waited 48 hours for it to cure. Once cured, we rolled out sill gasket, put the bottle jacks in place, and lowered the shed onto its new foundation. Almost as easy as it sounds…and we had the permits.

Here are two pro tips to assist you with planning your slab foundation:

If you’re building a foundation wall for your shed, the process is similar to the slab. You have to raise the shed to create a level base. Here are a couple more pro tip videos to assist you:

I’ve helped do this several times, most recently a 20’x30’ century-old cabin built on stone pillars. We used hydraulic jacks to lift and stabilize the structure. Using spades and picks, we dug out for the footing and block foundation walls. It took us a couple of months, but we were successful! And no, it wasn’t easy!

Conclusion

Regardless of your shed size, it is possible to put a foundation under it. Remember the basic steps; lift, level, stabilize. Once that’s done, you can put a foundation under it.

Choose the foundation that fits the shed, its location, and your budget. Small sheds can go on concrete slabs or blocks. Larger sheds can also use slabs or blocks as a foundation to raise them off the ground. Concrete pads and foundation walls are usually used for larger sheds. Larger sheds are often more structurally complex.

Lastly, check and adhere to local codes and ordinances and have all the necessary permits. You don’t want the foundation pulled out from under your shed.

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