How to Build a Concrete Shed Base: Complete DIY Guide 2025

Building your own concrete shed base might seem daunting, but I'm here to tell you it's absolutely achievable – even if you've never mixed concrete before! With the right approach, you'll have a rock-solid foundation that'll outlast your building by decades.

Whether you're planning a shed for garden tools, creating a garden room office, building a log cabin retreat, or constructing a greenhouse for year-round growing, getting the foundation right is crucial.

Complete overview of concrete shed base construction process

Why Choose Concrete?

• It lasts forever: I've worked on concrete bases from the 1970s that are still perfect. Compare that to timber bases needing replacement every 10-15 years.
• No more wonky doors: Your building stays square and true for decades. No more sticking doors from settling foundations.
• Moisture protection: Ground moisture rots timber and causes havoc. Concrete with proper damp-proofing keeps everything dry.
• Pest resistance: Mice and insects love timber bases. Concrete? Not so much.
• Heavy-duty capability: Perfect for ride-on mowers, workshop machinery, or wooden greenhouses with growing equipment.

Essential Tools & Materials

Tools You'll Need

• Measuring: 5m tape measure, builder's square, spirit level (1m minimum)
• Digging: Spade, rake, wheelbarrow
• Concrete work: Mixer (hire), tamping beam, float/trowel
• Safety: Protective clothing, rubber boots, safety goggles

Materials Shopping List

• Concrete: Ready-mix bags OR separate cement/sand/aggregate
• Sub-base: MOT Type 1 hardcore (75-100mm depth)
• Other: Sharp sand, 25mm formwork timber, wooden stakes, heavy-duty DPM, duct tape

Step-by-Step Construction

Step 1: Planning & Site Selection

Choose wisely: Well-drained, level ground, 1m from boundaries. For greenhouses, also consider sun exposure and wind protection. Check out proper ground preparation for optimal results.

Size correctly: Make base 100-150mm larger than your building on all sides.

Step 2: Mark Out & Excavate

Use pegs and string, check corners are square by measuring diagonals (must be identical).

Dig deep: 150-200mm total depth

• 75-100mm for sub-base
• 75-100mm for concrete

Step 3: Install Sub-Base

Spread MOT Type 1 hardcore evenly (75-100mm depth).

Compact thoroughly: Use wacker plate (hire essential). You'll know it's ready when you can walk on it without footprints.

Add thin sand layer to protect damp-proof membrane.

Step 4: Build Formwork

Use 25mm thick timber, pre-drill holes, check corners with builder's square.

Secure with stakes every 600mm, ensure perfectly level.

Step 5: Install Damp-Proof Membrane

Create plastic "tray" inside formwork:

• Use 1000-gauge DPM minimum
• Overlap joints 150mm, seal with duct tape
• Avoid walking on once installed

Step 6: Calculate Concrete Needs

Simple calculation: Length (m) × Width (m) × Depth (m) = cubic metres needed

Example: 3m × 2m × 0.1m = 0.6 cubic metres

Always order 10-15% extra for spillage and variations.

Mix ratios:

• Standard: 1 cement : 2 sand : 3 aggregate
• Alternative: 1 cement : 4 ballast
• Add water gradually until "thick porridge" consistency

Step 7: Mixing & Pouring

Concrete mixing ratios and techniques

The 2-hour rule: Once water hits cement, you have 2 hours to complete everything.

Mixing sequence:

1. Dry ingredients first
2. Add water slowly while mixing
3. Stop when properly combined

Start at one corner, work systematically. Push spade vertically to release air bubbles.

Step 8: Levelling & Finishing

Professional tamping and finishing techniques

Tamping technique:

1. Firm blows to compact
2. Sawing motion to level
3. Fill low spots and repeat

Step 9: Curing & Protection

Cover with damp hessian or raised plastic sheeting.

Timeline:

• 24-48 hours: Walkable with care
• 7 days: Remove formwork
• 28 days: Full strength, ready for building

Step 10: Prepare for Installation

Floor bearer installation preparation

Install pressure-treated timber bearers (75mm × 50mm) spaced 400-600mm apart.

Garden Rooms, Log Cabins & Greenhouses

The same process applies to all garden buildings, but with key differences:

Weight Considerations

• Sheds: 200-500kg loaded
• Garden rooms/log cabins/greenhouses: 2-5+ tonnes

Depth Requirements by Size

Building Size	Concrete Depth	Reinforcement
Small (up to 15m²)	100-125mm	Optional
Medium (15-25m²)	125-150mm	Recommended
Large (25m²+)	150-200mm	Essential

Special considerations:

• Garden rooms: Precision-fitted doors/windows demand perfect levelling
• Log cabins: May need engineering calculations for heavy timber construction
• Greenhouses: Need perfect drainage, small greenhouse or lean-to greenhouse options available

For specific requirements, consult our Greenhouse Buyers Guide.

When to Call Professionals

• Buildings over 30m²
• Sloping sites
• Heavy log cabins
• Commercial greenhouses
• Year-round heated buildings

Alternative Base Options

• Paving slabs: Quicker installation but can settle over time. Good for smaller buildings.
• Timber frame: Cheaper initially but needs replacing every 10-15 years. Not ideal for summerhouses or regular-use buildings.
• Plastic grids: Eco-friendly, quick to install, but limited load capacity.

Bottom line: For most garden sheds built to last, concrete remains the gold standard.

Common Mistakes to Avoid

• ❌ Skipping sub-base – Leads to cracking after first winter
• ❌ Rushing excavation – Uneven digging = wonky building
• ❌ Ignoring weather – Don't pour concrete before storms
• ❌ Weak formwork – Use proper 25mm timber, stake well
• ❌ Going solo – Concrete work needs at least two people
• ❌ Impatience with curing – Wait the full 28 days

Costs & Timeline

Budget Estimates (UK, 2025)

• Small base (2m×3m): £200-350 total
• Medium base (3m×4m): £325-550 total
• Large base (4m×6m): £500-850 total

Realistic Timeline

• Day 1: Planning and marking
• Day 2: Excavation and sub-base
• Day 3: Formwork and membrane
• Day 4: Concrete day!
• Days 5-28: Curing patience

Maintenance & Longevity

Annual check: Look for cracks, check drainage, ensure nothing's settled.

Crack repair: Small cracks = concrete filler. Large cracks = professional advice.

Expected lifespan: 50+ years with proper construction and minimal maintenance.

Conclusion

You've just learned to build a foundation that could outlast your building by decades. Yes, it's more work upfront, but the payoff is enormous – a perfectly level, bone-dry building that stays solid for generations.

Take your time with each step, don't cut corners, and wait the full 28 days for curing. Whether you're working in your garden room office or growing prize vegetables in your greenhouse, you'll appreciate that solid foundation every single day.

Now stop reading and start digging – that perfect base won't build itself!

Now read: How to Build a Paving Slab Base for a Greenhouse

• How much will this actually cost me?

  Budget £200-350 for a typical garden shed base (3m×2m) including tool hire. Larger bases cost £500-850. Expensive upfront but cheaper than replacing failed foundations every few years.

• Do I really need to wait 28 days?

  Yes – 28 days for full strength. You can remove formwork after a week, but don't build until fully cured. Rushing this ruins everything.

• Can I do this alone?

  You need help for concrete pouring and tamping – it's definitely a two-person job. The rest you can tackle solo if reasonably fit.

• What if it rains?

  Light rain during prep is fine, but don't pour concrete in heavy rain. Cover fresh concrete with raised plastic if rain threatens after pouring.

• My garden slopes – still possible?

  Gentle slopes are manageable, but steep slopes need retaining walls or stepped foundations. Get professional advice for slopes over 1:10.

• Buy or hire concrete mixer?

  Hire unless you're planning multiple projects. Mixers cost £300+ to buy, £30-50/day to hire, plus no storage/maintenance hassles.

• Most common mistake?

  Rushing preparation stages. Skipping sub-base, poor compaction, or inadequate excavation always causes problems later. Take time with groundwork.

• How do I know concrete mix is right?

  Should hold together when squeezed but not be sloppy. Think thick porridge consistency. Add water gradually – you can't take it back out!

• Can I use this for greenhouse foundations?

  Absolutely! Same method but ensure perfect drainage around perimeter for watering runoff. Garden offices and greenhouses need precision levelling – any movement causes expensive door/window problems.

• Different depths for different buildings?

  Yes – sheds need 75-100mm, garden rooms need 100-150mm, large buildings need 150-200mm with reinforcement. The heavier the building, the thicker the base.