Building a shed on a slope can be a bit more challenging than on a level surface, but with the right planning and execution, you can create a solid foundation that ensures your shed remains stable and level for years to come. This guide will walk you through the entire process of building a shed foundation on a slope, step-by-step.
Materials and Tools You’ll Need:
Materials:
- 4″x6″x12′ pressure-treated lumber (enough for the perimeter of your shed)
- Gravel or crushed stone for the base
- Rebar (typically 12” long pieces)
- 10″ structural screws
- Weed barrier fabric (optional, to prevent weeds)
Tools:
- Shovel
- Level
- Tape measure
- Circular saw or Miter Saw (to cut lumber, if necessary)
- Drill (for the structural screws)
- Hammer or mallet (for driving rebar)
- Gravel tamper (for compacting the stone base)
Step 1: Site Preparation
The first step is to assess the slope and determine the best location for your shed foundation. A slight slope (1-2 feet over the shed’s length) is manageable, but a steeper slope may require additional excavation, backfilling, and lumber.
For my shed foundation, there was a 25” slope over the 12’ length of the foundation – so I had my work cut out for me.
Steps to Prepare the Site:
Clear the Area: Begin by clearing the area of any debris, vegetation, rocks, or roots. This will give you a clean working space and prevent any future complications.
Measure and Mark: Use a tape measure to mark out the boundaries of your shed foundation.
I like to use batter boards to ensure that my foundation is perfectly square. For more information on layout, etc. check out my YouTube Video Linked above.
Make sure the corners form a square by measuring the diagonals—both should be equal to ensure your foundation is square.
Step 2: Building the Shed Foundation Frame.
Before you can start framing the shed base, you need to level the area under the back wall of the shed foundation.
Start by digging out any grass or vegetation and aim to have around 8”-12” wide strip of flat dirt under the back wall.
Compact the soil using a hand tamp and check for level.
Once the dirt is level, install 2”-4” of a stone base. A stable stone base helps with drainage and provides a solid foundation for the framing. Stone is also helpful for making any minor adjustment needed to make sure the area is perfectly level.
Once the foundation is level at the back of the shed, you can install the first piece of lumber.
I recommend 4”x6” lumber for shed foundation framing, but you can also use 4”x4” or 6”x6” lumber (just be sure that it is pressure treated).
Cut the lumber to the right length (12’ in my case) and place it on the stone.
To secure the board in place, pre-drill a 5/8” hole in the lumber every 2’-3’ and drive a piece of ½” rebar through the board and into the ground. Aim for around 2’-3’ or rebar depth into the ground.
Once the back board of the shed foundation is place, you want to place the first two boards on either side of the Shed Foundation. These boards will be at the same elevation as the back board of the shed, but they will only be a few feet long.
Because a 4”x6” piece of lumber is 5.5” tall, you want to “step-up” the shed foundation every time the slope has risen by 5.5” inches.
When building the sides of the shed, you are going to need to create as many steps as are needed to level the shed foundation to the grade elevation at the front of the shed.
Excavate these “steps” as needed and be sure to use a minimum of 2” of stone wherever the lumber contacts the ground.
Be sure to secure the boards in place using rebar that goes a minimum of 2’ into the soil. Additionally, for the boards that are towards the top of the foundation, secure them to each other using 10” Structural Screws (or similar).
For the front of the shed foundation, excavate, compact, and add stone as needed so that the final board will be level with the boards on either side. Once confirmed position the board in place, install the rebar, and secure the front board to the side boards using structural screws.
Step 3: Backfilling with Dirt and Installing Landscape Fabric:
Once the shed foundation has been framed, backfill the area within the foundation with dirt so that there is around 3”-6” gap between the top of the dirt and the top of the frame (we will fill that with stone in the next step).
If you have relatively flat area, you will not need too much dirt to backfill your foundation. In my case, due to the fact that the area was VERY sloped, I needed a ton of dirt to backfill my foundation. I recently picked up a Liberty 7K dump trailer which made this a lot easier – but backfilling is likely the most labor intensive part of this project.
Once you have filled the foundation with dirt, compact the soil using a hand tamp.
Finally, I recommend that you install landscape fabric over the soil. This will help to prevent weeds from growing up and will also prevent the stone that we apply in the next step from sinking and settling into the dirt.
Step 4: Backfilling with Stone
Once the dirt and landscape fabric is in place, you will want to fill the last few inches of the shed foundation frame with stone or grave (I recommend #57 Stone). I picked up the Stone in my Liberty 7K Dump Trailer, but you may find it easier to have the stone delivered to your project site.
Place the stone within the shed foundation frame and level it so that it is even with the lumber on each side.
Next, compact the stone using a hand tamp or a mechanical plate compactor. Once the stone is compacted, cut off any excess landscape fabric (if any).
Cost:
The cost of a project like this will depend on various factors like:
- Size of the Foundation (i.e. the amount of framing lumber needed)
- The amount of backfill dirt required
- The amount of backfill stone required
Below are the costs for my 12’x12′ shed foundation:
| Material | Cost | Quantity | Total |
| 4″x6″x12′ Pressure Treated Lumber | $30 | 12 Boards | $360 |
| Backfill Dirt | $120 | 6 Tons | $120 |
| Backfill Stone | $90 | 2 Tons | $90 |
| Landscape Fabric | $40 | 1 | $40 |
| Hardware (Screws and Rebar) | $80 | Various | $80 |
| TOTAL COST: | $690 |
Keep in mind that this was an extremely sloped area that required a lot of framing and backfill materials. For most projects, I would expect the cost to be close to $250.
Conclusion:
At this Point, you have successfully built a gravel shed foundation on a slope! Although this project is not very difficult from a skill perspective, this type of project is very labor intensive. Please keep that in mind as you decide if this is project you want to tackle or that you should hire out.
Thanks for reading and good luck Building!
If you want to learn how to Build a Cool Modern Shed on top of the shed foundation you just built, you can check out my “How to Build a Shed From Start to Finish Video” below:
DISCLAIMER: This is a reference guide only. Consult local code requirements. Links included in this article might be affiliate links. If you purchase a product or service with the links that I provide, I may receive a small commission. There is no additional charge to you! Thank you for supporting ATimprovements so I can continue to provide you with free content each week!