Thinking about buying or building a backyard shed or storage building?
Before you start your project, you’re going to want to think about providing a solid and level foundation for your shed. Not planning properly for your shed site prep is likely to cost you time and money down the road. There are a variety of shed foundations available, but we typically recommend a gravel shed foundation as the best option.
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Planning for your gravel shed foundation
A gravel shed foundation is a job you may be able to tackle yourself. (If you do, be sure to read our step-by-step guide on how to install a gravel shed foundation.) It’s important to plan ahead and collect all the necessary equipment and supplies before you begin. In this article, we’ll think through the different components that will be part of your project.
Of course, if the project gets out of your depth, you can always hire a professional site preparation company to level and prepare the shed foundation for you!
Clearly, the most important part of the project is the gravel itself, and we will try to answer the question, “How much gravel do I need for a shed foundation?” There are a number of variables that we will examine further down, but for a simple answer, you will need approximately 3 to 4 cubic yards (about 5 tons) of gravel for the average shed foundation.
Supplies for building a gravel shed foundation
Before you rush out to buy gravel, there are a few different building materials required for a gravel shed foundation. They are:
- Treated lumber for the perimeter of your shed foundation.
- Rebar for fastening the corners of your lumber perimeter.
- Woven landscape fabric weed barrier
- Gravel! (Isn’t that a surprise?)
Since it’s sitting on the damp soil, you will want to choose a lumber that is treated to be in contact with a lot of moisture. Make sure the lumber you choose is rated for GC (Ground Contact). 4”x6” is generally a good size of lumber for your shed foundation perimeter.
It’s a good idea to use 1/2” rebar or a similar fastener to connect the corners of your lumber.
When it comes to the gravel, you will want to use a size that can be compacted to make a firm foundation, while also being permeable enough to let water soak through. The goal is to avoid both runoff and puddling. ¾” drainage stone is an excellent choice for your shed foundation. It’s also a good idea to add a fabric weed barrier beneath the gravel. This will give your gravel shed foundation more overall stability by separating the stones from the dirt below and help prevent pesky plants from sprouting around your building.
Calculating the size of your shed foundation.
Before you figure for how much gravel you will need to purchase, it’s important to calculate how large your shed foundation will be. As a rule of thumb, your gravel shed base should extend at least 12” further than each side of your shed.
For example, if you are building your shed foundation for a 10×12 structure, you should make it at least 12×14. This will allow water running down the side of your shed and dripping off the eaves to drain away easily. If the eaves of your shed are extra-long, you will want to extend the border of your shed foundation accordingly.
In addition to planning the width and length dimensions of your shed foundation, you will also want to calculate the necessary depth. In general, 4” to 6” is a good depth range for a gravel shed foundation.
How much gravel do I need for a shed foundation?
Now we get to the big question! How much gravel do I actually need for a shed foundation? The answer will of course depend on the size of the foundation that you calculated in the last steps. Gravel is usually sold by the ton or the cubic yard. A cubic yard of gravel is 27 cubic feet and weighs roughly 1.4 tons. For ease of calculation we will use cubic yards.
To determine the amount of gravel you will need for your shed foundation first calculate the square footage. In the example diagram shown, a 12×14 shed foundation, the square footage is 168. Next, multiply the square footage by the depth of gravel. If we use 6” of gravel, or 0.5 feet, our cubic footage is calculated as 168 x 0.5 = 84 cubic feet. Divide the number of cubic feet by 27 to find the number of cubic yards needed. In this case, 84/27 = 3.11 cubic yards of gravel or about 4.5 tons should do the trick.
Let’s look at several examples to get an idea of how much gravel each shed foundation will need. We’ll assume each shed foundation is 6” deep with gravel.
8×12 storage shed – this is a popular small prefab storage shed size.
- 10x14x0.5 = 70; 70/27 = 2.6 cubic yards or 3.6 tons of gravel
12×18 storage shed – this is a medium size backyard storage shed.
- 14x20x0.5 = 140; 140/27 = 5.2 cubic yards or 7.3 tons of gravel
24×24 modular garage – this would be a large foundation for the placement of a two-car modular garage.
- 26x26x.05 = 338; 338/27 = 12.5 cubic yards or 17.5 tons of gravel
As you can see there is a fairly wide range of possible sizes for a gravel shed foundation, each demanding a different amount of gravel. The amount of crushed stone needed for other types of gravel pads, such as a gravel base for a hot tub or a gravel fire pit patio area, will vary. With this guide, though, you should be able to make a good estimate.
Tips for Proper Installation of Your Shed Foundation
The actual installation of your shed foundation deserves an entire article of its own. However, here are a couple other tips to remember.
You should level the ground beneath your shed foundation before placing your gravel. Once you’ve marked out the corners of your future shed pad, you can use a 2×4 and a level to check the grade.
Your site should be perfectly level before you install the gravel. For a more detailed breakdown on the steps in this process, read our article about how to level ground for a shed or about how to build a shed base on uneven ground.
Once your gravel is in place, make sure you compact and level it properly. Compacting the gravel is an optional, but recommended step, because it helps to identify and eliminate soft spots in your foundation.
As you compact, make sure the gravel is level over the entire surface of your shed foundation. Higher gravel on the edges of your foundation can cause the floor of your shed to sag and bounce over the years.
While it’s not the most complex construction project you will ever see, getting your shed foundation right is an important first step in your backyard upgrade project. From figuring out how much gravel you need, to putting the final touches on your new building, it’s important not to cut corners. You want to get your money’s worth out of this investment! If you want to make sure your shed foundation is installed correctly, consider hiring a professional site preparation contractor!
All the best with your shed project!