How to Build a Concrete Shed Base: A Step-by-Step Guide

When it comes to building a solid foundation for your shed, few materials can compete with concrete. A concrete shed base is strong, durable, and provides a perfectly level surface that resists shifting, settling, and moisture damage. Whether you're a house DIY enthusiast or embarking on a larger home improvement project, learning how to lay a concrete base is a valuable skill.

In this comprehensive guide, we’ll walk you through each step of laying concrete for a shed base. We’ll also cover essential concrete uses, practical tips, and answer common questions to ensure your project is a success.

Why Choose a Concrete Shed Base?

Concrete is a popular choice for shed bases because of its:

• Strength & Durability: It can support heavy loads and withstand harsh weather.

• Level Surface: Easier to achieve a flat, stable foundation.

• Resistance to Moisture & Shifting: Reduces the risk of structural damage over time.

While there are other base options like paving slabs or plastic grids, a concrete base is often considered the most reliable long-term solution for larger or heavier sheds.

Tools & Materials You’ll Need

Before you start, gather the following materials and tools:

• Builders square, brick line, wood stakes, line marker spray

• Shovel, rake, wheelbarrow

• Spirit level, tape measure

• MOT Type 1 sub-base material

• Timber for formwork (25mm thick)

• Damp-proof membrane (DPM)

• Cement, sand, aggregates (or ballast)

• Cement mixer (recommended) or mixing tools

• Wacker plate for compaction

• Tamping beam (a long straight piece of timber)

• Safety gear (ear defenders, boots, goggles)

• Plastic sheeting for curing

How to Build a Concrete Shed Base: Step-by-Step Instructions

1. Mark & Clear the Area

Start by marking out the exact dimensions of your concrete shed base. Use a builders square, brick line, and wood stakes to define the perimeter. A line marker spray can help highlight the area.

• Tip: Make the base slightly larger than the shed footprint for easier installation.

• Once marked, remove turf and dig down to a depth of 150mm (75mm for sub-base, 75mm for concrete).

2. Lay the Sub-Base

The sub-base is essential for preventing cracks and ensuring a stable foundation. We recommend using MOT Type 1 aggregate.

• Drive stakes into the ground so that the gap from the top of the stake to ground level is 75mm.

• Spread MOT Type 1 across the area.

• Compact it thoroughly using a wacker plate until level with your stakes.

Safety Note: Always wear ear defenders, safety goggles, and boots when operating machinery like a wacker plate.

3. Build the Formwork

Formwork holds the wet concrete in place until it sets.

• Use 25mm thick timber cut to the length and width of your base.

• Secure each board in place with stakes hammered into the ground at 1-metre intervals.

• Check levels and angles with a spirit level and builders square.

• Ensure the formwork is rigid and square; concrete can exert significant pressure when poured.

4. Add a Damp-Proof Membrane

A damp-proof membrane (DPM) prevents moisture from seeping up through the concrete into your shed.

• Cut the DPM to size and lay it inside the formwork.

• Overlap and tape any joints to form a complete barrier.

• The DPM should sit like a tray beneath the concrete.

5. Mix the Concrete

For a durable concrete shed base, a 1:2:3 mix ratio (1 part cement, 2 parts sand, 3 parts aggregates) is ideal. Alternatively, you can use 1 part cement to 4 parts ballast.

• For small bases, you can mix by hand in a wheelbarrow.

• For larger projects, a cement mixer is highly recommended.

• Mix thoroughly until you achieve a workable consistency — not too wet, not too dry.

6. Pour the Concrete

Begin laying concrete in one corner of your formwork.

• Use a wheelbarrow to transport the mix.

• Spread it evenly with a rake or shovel, aiming to fill sections about 1 to 1.5 square metres at a time.

• Slightly overfill by around 18mm above the formwork to allow for tamping.

• Push your shovel blade into the concrete repeatedly to eliminate air pockets.

7. Tamp & Level the Concrete

Tamping is crucial for compacting the concrete and achieving a level finish.

• With a helper, use a long straight piece of timber as a tamping beam.

• Starting at one end, use firm downward strokes across the base.

• Once compacted, slide the tamping beam in a sawing motion to level the surface.

• Fill any dips and repeat until you have a smooth, even surface.

• Use an edging trowel along the formwork to round off the edges, reducing the risk of chipping.

8. Cover & Cure the Concrete

To prevent the concrete from drying too quickly and cracking:

• Cover it with a plastic sheet, ensuring the sheet doesn’t touch the wet surface.

• Allow the concrete to cure for at least 24 to 48 hours before removing the formwork or stepping on it.

• Full strength will be achieved after 28 days, so avoid building your shed until then.

Common Questions About Laying a Concrete Shed Base

Can I Pour Concrete Directly Onto Soil?

Pouring concrete directly onto soil is not recommended. Without a proper sub-base, your concrete slab is at risk of sinking, shifting, and cracking. Moisture from the ground can also compromise the integrity of the concrete over time.

How Long Does It Take for Concrete to Fully Cure?

While concrete will harden enough to walk on after 24-48 hours, it typically takes up to 28 days to reach its full strength. Patience is key to ensuring a durable and long-lasting shed base.

What Are Some Other Concrete Uses in Home Improvement?

Concrete isn’t just for shed bases. It’s widely used in driveways, patios, garden paths, retaining walls, and foundations for various outdoor structures. Mastering laying concrete techniques opens the door to numerous house DIY projects.

Final Thoughts: Concrete Shed Base for a Lasting Foundation

Investing time and effort in building a solid concrete shed base is a smart move for any home improvement project. With careful planning, the right materials, and a methodical approach, you’ll have a strong, level base that stands the test of time. Whether you're new to house DIY or an experienced builder, this guide provides a reliable blueprint for success.