How to Build a 12x20 Concrete Shed Pad
- Staff Desk
- 3 hours ago
- 7 min read
Creating a solid concrete foundation is one of the most important parts of any shed or small-structure build. A proper shed pad ensures stability, prevents moisture damage, and extends the life of the structure. This blog explains every step involved in building a 12x20 concrete shed pad, from excavation and forming to finishing and curing.
1. Planning the Project
Before construction begins, planning is critical. Start by determining the location, size, and elevation of the pad. In this example, the slab measures 12 feet by 20 feet—a standard size for most large storage sheds or small workshops.
Tools and Materials
Power buggy or wheelbarrow for moving soil or concrete
Shovel and rake for grading
Form boards (2x6 lumber)
Wood stakes and kickers
Plastic sheeting (15-mil vapor barrier)
Play sand or fine base material
Rebar or fiberglass reinforcement
Concrete mix (3000 psi or higher)
Trowels, bull float, fresno, and edging tools
Saw for control joints
Water source for curing
Proper planning includes confirming access to the work area. If space is tight, consider using smaller equipment, such as an electric power buggy, which can navigate narrow paths and minimize surface disruption.
2. Site Preparation
A. Clearing the Area
Begin by removing all sod, roots, and organic material from the pad area. Organic matter can decompose and cause voids beneath the slab, leading to cracks later. Remove about two to three inches of topsoil until the surface is clean and firm.
B. Determining Elevation
For drainage, the slab should sit slightly higher than the surrounding ground.
On the high side of the lawn, keep the slab roughly 2 inches above grade.
On the low side, it may rise 5–6 inches, depending on the slope.
This elevation difference prevents water from collecting near the foundation and helps keep the shed interior dry.
C. Compacting the Base
Use a hand tamper or plate compactor to compress the soil. Proper compaction minimizes future settling and strengthens the sub-base. Compact the entire area uniformly until it feels solid underfoot.
3. Setting Up Formwork
Form boards define the shape and level of the concrete slab. In this build, 2x6 lumber was used, providing a slab depth of about 4 inches in the center and up to 10–12 inches around the perimeter.
A. Establishing Layout
Start by setting one long form parallel to an existing wall or reference line.
Use a 4-foot level to make sure the form is perfectly level.
Square the layout using the 3-4-5 or 6-8-10 triangle method:
Measure 6 feet along one side, 8 feet along the adjacent side, and the diagonal between them should be 10 feet.
Adjust until the diagonal measurement is exact.
After squaring the first two sides, measure and align the remaining forms from these reference points.
B. Checking for Accuracy
Once all forms are in place, measure diagonally from corner to corner. The difference between the two measurements should be minimal—within ⅜ inch is acceptable for this span. Re-adjust the forms if necessary.
C. Adding a Thickened Edge
To support the shed walls and allow for anchor bolts or expansion fasteners, include a thickened edge around the perimeter. A thicker border (up to 10–12 inches) gives the structure extra bearing strength and prevents cracking when fasteners are installed later.
4. Installing a Vapor Barrier and Base Layer
A vapor barrier prevents moisture from rising through the slab, protecting both the shed flooring and the contents stored inside.
Roll out 15-mil plastic sheeting across the compacted soil.
Overlap seams by at least 6 inches and tape them securely.
Add a thin layer of play sand on top to protect the plastic during the pour and to help level the surface.
If the ground underneath is particularly uneven, additional sand may be used to fill low spots. Smooth the surface with a rake and ensure the base is uniform.
5. Reinforcing the Slab
Reinforcement prevents cracking and adds tensile strength to concrete. Two common methods are steel rebar and fiberglass rebar.
A. Using Fiberglass Rebar
In this project, Owens Corning PinkBar fiberglass rebar was used. It’s lightweight, non-corrosive, and easy to cut, making it ideal for residential use.
Install the rebar diagonally across the slab in a grid pattern.
Tie intersections together using plastic or zip ties.
Keep the reinforcement raised slightly above the vapor barrier using small rebar chairs or supports.
Since fiberglass rebar does not rust, it remains unaffected even if later saw cuts expose it.
6. Preparing for the Concrete Pour
Before the truck arrives, double-check every detail:
Forms are level and secure.
Reinforcement is positioned correctly.
The area is clean and accessible.
A clear path exists for moving concrete from the truck to the slab location.
If using a power buggy or wheelbarrow, ensure the access path is firm and free of debris.
7. Mixing and Placing Concrete
A 3000-psi pea-gravel concrete mix works well for shed pads. Adding fiber mesh reinforcement helps prevent shrinkage cracks and increases durability.
A. Adding Fibers
Fibers are mixed directly into the truck before pouring. Always verify slump consistency when adding fibers—around 4 ½ inches is a good balance between workability and strength.
B. Placing Concrete
Begin pouring at the farthest point from the truck or access route.
Spread the concrete evenly using shovels or rakes.
Work in manageable sections to prevent cold joints.
Make sure the mix flows under the form edges and around reinforcement without leaving air pockets.
8. Screeding and Leveling
Use a straight 2x4 screed board to strike off the excess concrete.
Rest the screed on the form edges and pull it backward in a sawing motion.
Fill any low spots immediately.
Continue screeding until the entire surface is level with the top of the forms.
Because this slab is level across its surface, water will naturally sit in the middle unless a slight crown is added. For future builds, a 3⁄8-inch crown at the center helps water run off and prevents standing puddles.
9. Floating and Finishing
Once screeded, the surface should be consolidated and smoothed.
A. Bull Floating
Use a 3-foot wood bull float to smooth the surface and push larger aggregate down. This step brings cement paste to the top, preparing the surface for later finishing.
B. Edging
Apply a ½-inch radius edging tool around the perimeter. While optional, this rounded edge makes stripping the forms easier and reduces chipping.
C. Troweling
After the bleed water evaporates:
Use a fresno trowel for intermediate smoothing.
Follow with a funny trowel or steel finish trowel for a refined surface.
Keep strokes consistent and avoid overworking the concrete, which can cause surface dusting later.
10. Removing Stakes and Forms
Once the concrete sets enough to hold its shape, remove all vertical stakes. Leaving only the kickers in place allows easy stripping later.
After about 24 hours, gently pry away the form boards. Cleaning the edges immediately helps preserve the forms for reuse and prevents spalling.
11. Curing the Concrete
Curing ensures maximum strength and prevents cracking. Concrete must retain moisture for several days after placement.
Recommended Curing Schedule
Begin watering one hour after finishing.
Lightly spray the surface every two hours during daylight for at least three days.
Avoid direct high-pressure water streams that can erode the surface.
Keeping the slab moist promotes even hydration and reduces the risk of surface dusting or shrinkage cracks.
12. Cutting Control Joints
Control joints allow the slab to expand and contract naturally without random cracking.
Wait until the concrete is hard enough to walk on without leaving marks.
Mark joint lines approximately every 10 to 12 feet—in this case, splitting the 12x20 pad into two equal 12x10 sections.
Use a concrete saw to cut joints at least ¼ of the slab thickness deep.
Because this slab contains fiberglass rebar, saw cuts can intersect reinforcement without causing corrosion issues.
13. Final Inspection
Once the concrete is fully cured and joints are cut, inspect for:
Smooth, even surface
Clean edges
Proper drainage and no large puddles
Any light surface water is normal for a flat slab, especially before a roof or structure is added.
14. Why a Thickened Edge Matters
The thickened perimeter serves two purposes:
Structural strength – It provides a stable base for shed walls or columns.
Anchor support – It allows secure installation of expansion bolts, Tapcons, or other fasteners without risking concrete fracture.
Without a thickened edge, drilling into a standard 4-inch slab can lead to cracking or spalling.
15. Maintenance and Long-Term Care
A. Water Control
Ensure proper site grading so water runs away from the pad. Avoid directing downspouts toward the slab.
B. Regular Cleaning
Occasional washing prevents debris buildup and maintains appearance. If needed, apply a mild concrete cleaner and rinse thoroughly.
C. Sealant (Optional)
Applying a penetrating sealer can further protect the surface from stains, moisture, and freeze-thaw damage, especially in colder climates.
D. Avoid Heavy Loads
A 3000-psi slab with fiber reinforcement is suitable for typical shed use, but avoid concentrated loads from heavy machinery unless the slab was designed for that purpose.
16. Common Mistakes to Avoid
Skipping the vapor barrier: Allows ground moisture to rise into the slab.
Incorrect leveling: Causes poor drainage and uneven flooring.
Insufficient compaction: Leads to settlement and cracking.
Over-troweling: Weakens the surface and creates dusting.
Neglecting curing: Results in brittle, weak concrete.
Following correct procedures ensures a durable pad that will support a structure for decades.
17. Example Cost Breakdown (Approximate)
Actual prices vary by location, supplier, and project size.
18. Benefits of a Concrete Shed Pad
Durability: Withstands weather and weight better than gravel or wood bases.
Low maintenance: No settling, rot, or pest damage.
Clean appearance: Smooth, uniform surface ideal for framing.
Versatility: Can later serve as a patio or small workspace if not used for a shed.
19. Technical Specifications Summary
20. Completion and Use
After full curing—typically seven days for light use or 28 days for maximum strength—the pad is ready for framing and anchoring. The concrete surface provides a stable, level base for shed walls, ensuring accurate framing alignment and long-term durability. Although this example was designed as a shed pad, the same process applies to small garages, workshops, or outdoor utility buildings. A correctly built slab not only supports the structure above but also enhances property value and usability.