How to Build the Perfect Concrete Shed Base

Building a durable and level shed base is one of the most important steps before installing a shed, workshop, or outbuilding. A properly prepared base ensures stability, prevents shifting, and increases the longevity of the structure.

1. Preparing the Subbase

The first step in building the perfect shed base is to establish a solid subbase. This is typically made using Type 1 MOT hardcore material, which provides a strong and stable foundation.

• Depth: You’ll need around 100mm to 150mm of compacted Type 1 MOT depending on soil type and shed size.

• Goal: Achieve a flat, compact surface that supports the concrete slab evenly.

Leveling the Subbase

Bring in the Type 1 material and level it across the base area. There are two main leveling methods:

• Laser Level Method: For high precision, use a laser level and receiver to guide your height levels.

• Manual Level Method: If you don’t have access to laser tools, use a long level (like a 6-foot spirit level) and your eye to get as close to level as possible.

Although subbase leveling is important, perfection here isn’t critical. The next step — building the shuttering — will define the final level and square shape.

2. Compacting the Subbase

Once the Type 1 MOT is evenly spread, it’s time to compact it. A compacting plate (also known as a Wacker plate) is used for this step.

Steps to Compact:

1. Turn the main switch on, ensure fuel is on, and engage the choke (typically marked by a propeller icon).

2. Pull-start the machine — it should start easily if well-maintained.

3. Turn off the choke once the engine runs smoothly.

4. Adjust the speed control to your desired level.

When compacting:

• Move the machine up and down the length of the base, then across the width.

• Overlap slightly with each pass to ensure complete coverage.

• Allow the machine’s weight to do the work — avoid forcing it.

Pro Tip: On hot, dry days, lightly hose the area before compacting. This helps reduce dust and allows better compaction.

3. Building the Shuttering Frame

With the subbase compacted, the next step is to build the shuttering frame — a wooden frame that holds the concrete in place and determines the final dimensions of the base.

Materials:

• Use 6x2-inch timber for strength and ease of handling.

• Cut the timber to your required shed size — for example, a 3.4m x 4.5m base.

Cutting the Timber:

1. Measure and mark the width and length accurately.

2. Use a square (roofing square or speed square) to mark straight cuts.

3. If one side overlaps another, add twice the timber thickness to your length measurement.

Once the four sides are cut:

• Fix them together using 100mm wood screws at each corner.

• Ensure tops are flush, but don’t worry about squareness yet — that’s checked next.

4. Checking for Square

Accurate alignment is crucial to avoid uneven shed walls or concrete edges.

To check for square:

1. Measure diagonally from one corner to the opposite.

2. Measure the other diagonal the same way.

3. If there’s a difference (e.g., 10mm), adjust the frame by half the difference in the right direction.

For instance:

• One diagonal = 5765mm

• Other diagonal = 5775mm

• Move one side by 5mm to balance both at 5770mm.

Once both measurements match, your frame is square.

5. Fixing and Leveling the Frame

Next, secure the shuttering frame in position using stakes made from 2x2 timber.

Steps to Stake and Level:

1. Cut the end of each stake into a sharp diagonal point for easy driving.

2. Drive the first stake at the highest point of the base using a club or sledgehammer.

3. Fix the frame to the stake using screws.

4. Use a laser receiver or spirit level to determine level points at each corner.

5. Adjust the frame height up or down until all corners are perfectly level.

If using a laser level:

• Keep the laser and receiver in a fixed position.

• Move the receiver to each corner and raise/lower the frame until the tone indicates level.

Continue around all corners, ensuring the frame is even and square.

6. Reinforcing the Frame

With the frame level, reinforce it by adding extra stakes every 1 meter around the perimeter.

• These stakes prevent the frame from bowing outward under concrete pressure.

• Cut the top of each stake at a slight angle so it doesn’t interfere with leveling tools.

If the ground behind the frame is soft or uneven, add bracing to hold the frame steady.

7. Final Compaction and Sand Layer

Once the frame is secure, check the surface again. Add or remove Type 1 to bring it up to the bottom edge of the shuttering. Re-compact the surface thoroughly.

Next, spread a thin layer of sharp sand over the compacted subbase.This layer:

• Protects the DPM (plastic membrane) from being punctured by sharp stones.

• Provides a smooth surface for the DPM to rest on.

Use the back of a rake (not the teeth) to spread and level the sand.

8. Laying the Damp Proof Membrane (DPM)

The DPM acts as a moisture barrier, preventing water from seeping up through the concrete base.

How to Lay the DPM:

1. Cut the black plastic sheet to fit the base size, leaving some extra length to run up and over the sides of the frame.

2. Lay it flat, covering the entire area.

3. Overlap joints if using multiple sheets and tape them securely.

Tip: It’s better to have excess DPM folded up the sides than to fall short.

9. Adding Reinforcement Mesh

Next, add steel reinforcement mesh sheets on top of the DPM.

Purpose:

• Reinforces the concrete.

• Reduces cracking over time.

• Increases load-bearing strength.

Cut sheets to fit the area, overlapping slightly where needed. Be careful not to pierce the DPM underneath.

10. Using Mesh Spacers (Meshmen)

The reinforcement mesh must sit slightly above the DPM — not flat on it — to be effective.

Steps:

• Use meshmen, small concrete spacers, to raise the mesh off the ground.

• Place one spacer at each corner of every square (about every 30–40 cm).

This ensures the mesh sits within the middle of the poured concrete, where it provides maximum structural support.

11. Calculating Concrete Quantity

Before ordering, calculate the exact volume of concrete needed.

Formula:

Volume=Length×Width×DepthVolume = Length × Width × DepthVolume=Length×Width×Depth

For example:

• Length: 4.5m

• Width: 3.4m

• Depth: 0.14m (140mm)

4.5×3.4×0.14=2.1 cubic meters of concrete4.5 × 3.4 × 0.14 = 2.1 \text{ cubic meters of concrete}4.5×3.4×0.14=2.1 cubic meters of concrete. Order a little extra to account for spillage and unevenness.

12. Pouring the Concrete

You can pour concrete directly using wheelbarrows, but for large bases, using a concrete pump is faster and cleaner.

Once the concrete arrives:

1. Begin pouring evenly across the base.

2. Use a straight timber beam or tamping bar to spread and level it.

3. Work from one side to the other, slightly overfilling and scraping back excess concrete.

Tamping also helps remove air pockets and ensures a denser, more uniform slab.

13. Leveling and Finishing the Surface

After pouring, it’s time to level and smooth the surface.

Leveling Steps:

• Lay a straight edge across the shuttering and drag it (tamping motion) to flatten the surface.

• Work in sections about 2–3 feet at a time.

• Once the first pass is done, repeat in the opposite direction for complete flatness.

If you want a more professional finish, use the following tools:

A. Edging Trowel

Run along the edges of the slab to create a smooth, rounded corner.This prevents chipping and adds strength to the edges.

B. Bull Float

Use a bull float — a large flat float with a long handle — to smooth and level the top surface.This produces a clean, even finish and helps bring up fine cement paste for a polished look.

14. Setting and Curing

Allow the concrete to set and cure properly before placing any shed or structure on top.

• Initial Set Time: 24–48 hours (no walking or loading)

• Full Cure: Around 28 days for maximum strength

During hot weather:

• Cover with plastic or damp burlap to prevent rapid drying and cracking.

15. Final Checks

Once cured:

• Remove the shuttering frame carefully.

• Trim any excess DPM from the sides.

• Inspect for surface cracks or dips and patch if needed.

The result should be a solid, level, and waterproof concrete base ready to support your shed, workshop, or any outdoor structure.

Conclusion

Building the perfect shed base isn’t complicated — it’s about precision, preparation, and patience.From compacting the subbase to screeding the final concrete surface, each step contributes to a strong and reliable foundation.