How to Build a Shed on an Existing Concrete Slab

If you’re fortunate enough to have a pre-existing concrete slab in your yard, then you are already halfway to building extremely solid storage shed.

Before you get started throwing up a shed on your concrete slab, you’ll need to give it a thorough inspection. You don’t want to start your shed only to find that it isn’t level or is below grade.

So how to build a shed on an existing concrete slab? Below we’ll take a look at the challenges of building on a pre-existing concrete slab and detail how to build on top of any type of slab.

How to Build a Shed on an Existing Concrete Slab

Why Would You Build a Shed on Existing Concrete?

Building on a concrete slab is a no-brainer. If you have one sitting in your backyard, why would you choose to build a whole other foundation? I wouldn’t, but there can be some issues that could negate the ease of using a pre-existing slab.

Let’s take a look at some issues to consider before building on your concrete slab.

Pros

  • Shaves time off your shed construction
  • Provides a nice, flat, even surface
  • Sturdy and resistant to weather
  • Negates the need for a wood floor

Cons

  • Condition of concrete is unknown – may chip or shift
  • The slab may not be square – or level
  • May be below grade

Before You Start: Check Local Building Codes and Regulations

Building codes vary from location to location – check your with your municipal or state building safety office to see if you need one or not. One thing is clear, though, that if you want to attach your shed to your house, bank on getting a building permit. Building code treats that as an add-on to your living space – not as a shed.

A quick search on the internet is helpful in showing how codes vary. For instance, if you live in New Hampshire, you need a permit for any shed. On the other hand, if you live in Seattle, then you only need one if your shed is more than 120 square feet. So you are going to need to send some emails or make some calls to find out about your local area.

Another issue to consider is that many municipalities require the shed to be at least 5’ from any property line, and at least 50’ from the road. Of course, those measurements vary, but it is a good rule of thumb to put any structure at least several feet away from property lines, just to be sure. Again, check your local building code for exact measurements.

How to Build a Shed on Pre Existing Concrete

Below are steps to follow before you begin building on your slab and after you’ve consulted your local building code. Again, the most important part of your shed is the foundation. If it falters, you can’t just move your shed onto another foundation. Here’s how to avoid any mishaps:

Step 1: Check Condition of the Slab

Is the slab structurally sound? You’ll need to do a thorough inspection of it before you build.

Start by measuring the slab to know its exact dimensions. Make sure it is square.

If your slab is flush with the ground, then you have a problem. Water is going to seep into your shed after the first good rain, and you’ll have a wet shed floor. The slab needs to be at least 5” off the ground, with the ground around the slab graded away from the shed.

Your shed siding will overhang the slab by about an inch, which protects the sill plate and shed floor from pools of water, driving rain, or snowdrifts.

building a shed on concrete slabCheck the condition of the concrete itself. Is it chipped or cracking? If so, there are some concrete repair products that will allow you to repair chips of pretty much any size, if used in conjunction with fine gravel. If there are cracks, there are several options that allow you to squeeze out compound into the cracks and let them set.

Was gravel or sand used underneath the slab? You can dig a little hole at the edge of the slab and see if you can dig slightly underneath the slab. Ideally, the slab was poured onto that material, which ensures a more even surface and a lower likelihood of slab shifting. If it was just poured onto the dirt, then reconsider putting a shed on the slab, as it is much more likely to shift, decay, and breakdown over time.

The same goes for missing rebar or mesh. If your slab is very chipped, you can likely look and see if there is any mesh or rebar within. Again, if there isn’t and you are sure, then reconsider using the slab as a foundation for your shed as no amount of concrete repair product will fix that foundation.

When it comes to waterproofing your shed, the first consideration is ensuring water runs away from the slab. If it doesn’t, then that should be your first task. If it is a raised slab but still below grade, then you’ll want to waterproof the sides of the slab. Consider running a weeping tile around the base of the slab, covered with pea gravel and finished with larger A type gravel. Run the tile either to a french drain or towards a lower, drained area.

Lastly, your slab may not be level and is sloped. If this is the case, you must level the slab before building. Shimming an entire structure is not advisable, ever, so you’ll have to either find a new foundation for your shed or pour new concrete on top to achieve a level surface. While this is lots more work, it still beats, making an entirely new foundation for a shed.

Step 2: Plan Your Build

Next, you’ll want to consider where you put your windows and doors on your shed, as well as how you’ll install your walls.

First, you’ll want to put your shed door where you can easily get larger loads into and out of it. Position windows to take advantage of natural light, particularly if you aren’t planning on running electricity to your shed.

There are a couple of ways to go about attaching walls to your shed. One is the double bottom method. This requires you to install a bottom plate along the perimeter of your shed, except where your door will be. Your shed framing will go on top of the bottom plate. It is called double bottom because there will be two flat bottom studs at the bottom of the frame once the walls are in place.

Pros

  • Provides an easy outline for putting up walls
  • Allows you to properly line up L bolts before installing walls

Cons

  • Requires more lumber
  • Takes more time
  • Longer anchor bolts required
  • 8’ siding pieces may not fit over the double bottom

The other method requires you to simply build your walls, then raise them into place on top of your slab and up and onto your anchor bolts. While this is the more traditional method, some issues might arise.

Pros

  • Requires less time
  • Less lumber needed
  • Ensures siding pieces will properly overlap wall frame and slab

Cons

  • Interior wall material, such as drywall, is closer to the ground
  • Slightly more difficult to install with one person

Step 3: Build Shed Walls

Time to build the shed walls. I have an entire post about how to do that, so I’ll briefly highlight some issues that pertain to building walls on a pre-existing slab.

It is critical that your bottom plate is pressure-treated lumber. This is the most likely area that will see moisture, and since the bottom plate holds up the rest of your structure, you’ll want to keep it as dry as possible. Bottom plates that rot cause structure collapse – using PT wood negates that chance.

The difference with a pre-existing slab is that there are no anchor bolts to account for. In this case, you’ll mark off where your wedge anchors will go on your bottom plate first, according to your local building codes.

Here’s how to build walls for your shed:

  • Use the slab to construct your walls
  • Lay out all the pieces first – studs, bottom, and top plates
  • Measure and nail the bottom plate into the vertical studs
  • Nail the top plates to studs
  • Ensure door and window openings have an appropriately sized header above openings

Step 4: Install Sill Gaskets

how to build a shed on a slabNext, you’ll want to install your sill gasket onto the underside of your bottom plate.

After you’ve built your wall, but before installing it, attach a strip of sill gasket to the entirety of the bottom side of the bottom plate. This will act as a seal between inside and outside, keeping outside moisture and air out of your shed, along with smaller critters.

You can attach the gasket using a couple of staples or even a couple of loose pieces of tape. Once the wall is up, the pressure keeps the sill gasket firmly in place. Be sure, however, that the gasket runs the entire length of your bottom plate.

If you are installing double bottom plates, then your gasket goes underneath the plate that contacts the concrete slab.

Step 5: Install Walls

Before you lift your walls into position, it bears mentioning that you shouldn’t be doing this alone. Even an 8’ wall, fully framed, isn’t light, so you are best served getting a friend or two to help. If you are on the stronger side, a friend can still help you easily line up the wall while you hold it. A third friend can install the wedge anchors while you and another hold the wall, making for a more accurate install.

Start with the back wall and work your way to the front. You’ll need a 2×4 bracing the first wall since it is standing all alone. Nail a 2×4 to a center stud of the back wall you just erected. Connect the other end to a 2×4 staked into the ground several feet away. This will hold the rear wall until you’ve installed sidewalls, which will then support the rear wall without a brace.

When attaching the rear wall to the slab, see this article here. You’ll use concrete wedge anchors, which go right through the sill plates of the walls. Be sure to install these before putting up your side walls, and the front.

Once you’ve installed your rear wall, install the side wall. Do this in the same manner as your rear wall. You’ll nail your side wall to the end studs of your rear wall. Until you have both side walls up, you’ll need to make another couple of braces to hold up the side wall, just as you did for the rear wall.

Once you’ve erected all the walls, consider a masonry silicone sealant to put down between your sill plate and concrete slab.

Conclusion

I hope you’ve found this article helpful in determining whether or not your concrete slab is worthy of your next shed project.

Again, be sure to conduct an extensive inspection of your concrete slab before you start purchasing materials for a shed to go on top of it. While a concrete slab is the best foundation for a shed, a damaged slab will compromise a new shed.

Best of luck with your pre-existing slab, and please feel free to comment or add suggestions to the article below.

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