How to Build a Concrete Base for a Greenhouse: A Simple DIY Guide
Key Takeaways
- ✅ A concrete base lasts 20+ years and provides the most stable foundation for any greenhouse.
- 📏 You’ll need 100–150 mm depth for most greenhouses (deeper for larger structures).
- 💷 Total cost £200–£800 depending on size and whether you DIY or hire help.
- ⏱️ Allow 2–3 days: one day for prep and pouring, 3–7 days for curing.
- 📐 Proper levelling is crucial – even a 5 mm difference can cause glazing problems.
Why Bother With a Concrete Base?
Look, I get it. You’ve just spent good money on a shiny new greenhouse and now someone’s telling you to pour concrete? Can’t you just plonk it on the grass?
Well, you could. But here’s what I’ve seen happen: the frame twists, doors won’t close, glass cracks, and within a year you’re dealing with a wonky mess. A proper concrete base stops all that nonsense.
Concrete does three brilliant things:
- Keeps everything level – glass doesn’t like movement, and concrete doesn’t move.
- Blocks weeds and pests – no more rats setting up shop under your tomatoes.
- Lasts forever – OK, maybe not forever, but easily 25+ years.
I built my first greenhouse base about eight years ago. Made every mistake in the book (more on that later). The second one? Textbook perfect. That’s what I’m sharing here.
What You’ll Need
Materials
- Concrete mix – either ready-mix bags or separate cement, sand, and gravel (6:1 ratio).
- Hardcore/MOT Type 1 – about 75–100 mm depth worth.
- Timber for formwork – 150 mm wide boards work well.
- DPC membrane – damp proof course sheeting.
- Wooden stakes – to hold your formwork in place.
Tools
- Spade and shovel
- Spirit level (get a long one, 1200 mm minimum)
- Wheelbarrow
- Rake
- Wooden float or steel trowel
- Tape measure
- String line
- Hammer
- Vibrating plate compactor (you can hire these for about £40/day)
💷 Rough costs: For a standard 8×6 ft greenhouse, you’re looking at £200–£400 if you do it yourself, or £500–£800 with labour.
Planning Your Base (Don’t Skip This Bit)
⚠️ Don’t eyeball it. Your concrete base needs to be at least 2-5cm larger than your greenhouse base plinth on all sides. The base plinth will sit on top, and you want a bit of extra room to make it easier to fit.
Measure twice. Then measure again. I once built a base that was 50 mm too narrow. Had to chip away concrete and add more. Absolute nightmare.
📞 Check with your council: Most domestic greenhouses don’t need planning permission, but if it’s going near a boundary or you’re in a conservation area, give them a ring. Five-minute call now beats a five-figure enforcement notice later.
| Greenhouse Size | Recommended Base Depth | Concrete Volume Needed | Approx. DIY Cost |
|---|---|---|---|
| 6×4 ft | 100 mm | 0.22 m³ | £150–£250 |
| 8×6 ft | 100 mm | 0.45 m³ | £250–£400 |
| 10×8 ft | 125 mm | 1.0 m³ | £400–£600 |
| 12×10 ft | 150 mm | 1.8 m³ | £600–£800 |
Step‑by‑Step: Building Your Concrete Greenhouse Base
Step 1: Mark Out and Dig
String lines are your best friend here. Mark your corners with pegs, run string between them, and check your diagonals are equal. If they’re not, you’ve got a wonky rectangle.
Dig out to about 200–250 mm deep total. That’s roughly a spade depth plus a bit. You want 75–100 mm for hardcore, then 100–150 mm for concrete.
Keep the sides fairly straight. You’re not digging the Channel Tunnel, but ragged edges make the formwork annoying to fit.
Step 2: Compact Your Hardcore
Tip in your MOT Type 1 or hardcore. Spread it level with a rake. This is your sub-base – it stops the concrete sinking into soft ground.
✅ Compact it properly. I once skipped the compacting step. Six months later, one corner had dropped 20 mm. Hire that plate compactor – your back will thank you, and your greenhouse won’t look drunk.
Go over it three or four times until you can walk on it without leaving footprints.
Step 3: Build Your Formwork
Nail together a rectangle from your timber boards. The inside measurements should match your base size exactly. Use stakes every 600–800 mm to hold it in place, driven into the ground outside the boards.
Get your spirit level out. Check every side. Check the diagonals. Check it all again. This is where “level” happens – once the concrete’s in, you’re stuck with what you’ve got.
💡 Pro tip: Brush a little old engine oil on the inside faces of your formwork boards – it makes them much easier to remove later.
Step 4: Lay Your DPC Membrane
Roll out your damp proof membrane over the hardcore. Overlap joins by 150 mm minimum. Bring it up the inside edges of your formwork.
This stops moisture creeping up through the concrete into your greenhouse frame. Wood and damp are not friends.
Step 5: Mix and Pour Your Concrete
Right, this is the big bit. If you’re doing a small base (under 1 m³), ready-mix bags are easier. Bigger than that? Get a concrete truck to deliver – it’s not much more expensive and saves your entire weekend.
The mix: If you’re mixing yourself, go with 1 part cement : 2 parts sand : 3 parts gravel (or 1:5 ballast). Add water gradually until it’s like thick porridge. Too wet and it’ll be weak; too dry and you can’t work it.
Pour it in, starting from one corner. Work it into the edges with your shovel. You want it roughly 20 mm higher than your final level – it’ll settle as you work it.
Step 6: Level and Finish
Here’s where that long spirit level earns its keep. Drag a straight timber board (a “screed board”) across the top of your formwork, sawing back and forth. This strikes off the excess and leaves you with a flat surface.
Fill any low spots and screed again. Check with your spirit level across different directions.
Once it’s flat, smooth the surface with your wooden float. You don’t want it mirror-smooth (that’s slippery when wet), just reasonably even. A slightly textured finish is perfect.
Step 7: Curing Time
⚠️ This is important: Concrete doesn’t “dry” – it cures, and it needs moisture to cure properly.
Cover your base with plastic sheeting. Leave it alone for at least 3 days. A week is better. Yes, I know you’re excited to build your greenhouse. Wait anyway.
In hot weather, lift the plastic and spray it with water once a day. Sounds weird, but concrete that dries too fast cracks.
After 7 days, you can build on it. Full strength takes 28 days, but seven is fine for a greenhouse.
Common Mistakes (I’ve Made Them So You Don’t Have To)
- Skipping the hardcore – concrete on soft ground will crack and sink. Just don’t.
- Not checking level – 5 mm out feels like nothing, but across 3 m your door won’t close.
- Pouring in hot sun – concrete cures too fast and cracks. Pour early morning or late afternoon in summer.
- Wrong depth – too thin (under 75 mm) will crack under weight; too thick wastes money and time.
- Forgetting expansion gaps – for bases over 3 m, put in an expansion joint. Concrete expands and contracts with temperature.
What About Other Base Options?
Paving slabs: Cheaper and faster, but they shift over time. Good for small greenhouses under 6×4 ft. If you’re considering this option, check out our complete guide to building a paving slab base which covers the pros and cons in detail.
Gravel: Rubbish. Weeds come through, ground shifts, frame goes wonky.
Timber base: Works for wooden greenhouses but rots after 5–10 years. You’ll be rebuilding it.
For lean‑to greenhouses, concrete is pretty much essential because they’re attached to your house wall and need to be spot‑on level.
Is Concrete Worth the Effort?
Short answer: yes.
Long answer: If you’re spending £500–£2000 on a greenhouse, spending another ~£300 and a weekend to make sure it lasts 25 years is a no‑brainer. I’ve seen too many expensive greenhouses destroyed because someone tried to save £200 on the base.
Plus, there’s something satisfying about building something permanent. That base will outlast you, your greenhouse, and probably your grandkids’ greenhouse.
Useful Resources
- For more detailed planning, check out the Greenhouse Buyers Guide which covers base requirements for different greenhouse types.
- The Royal Horticultural Society has excellent guidance on greenhouse foundations and drainage requirements.
- For building regulations specific to your area, visit your local council’s planning portal – most have searchable databases for permitted development rules.
Frequently Asked Questions
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How deep should a concrete base be for a greenhouse?
For most garden greenhouses, 100 mm (4 inches) is spot on. Go deeper – 125–150 mm – if you’re building something larger than 10×8 ft or if your soil is particularly soft clay. The hardcore underneath adds another 75–100 mm, so you’re digging 200–250 mm total.
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Can you put a greenhouse directly on soil?
You can, but you shouldn’t. Soil shifts with moisture and temperature changes. Within months, your greenhouse will be wonky, doors won’t close properly, and glass panels might crack from the frame twisting. Plus, you’ll get weeds growing up through the floor and potentially pests burrowing underneath. Save yourself the headache and build a proper base.
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What is the cheapest way to build a foundation for a greenhouse?
Paving slabs on compacted sand are the cheapest proper option – around £100–£150 for a 6×4 ft base. But “cheapest” often means “most problems later.” They shift over time and need re‑levelling every few years. For ~£100 more, a concrete base lasts decades with zero maintenance. The maths is pretty simple really.
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Is a concrete base best for a greenhouse?
For anything you plan to keep more than five years, yes. Concrete is level, permanent, blocks weeds and pests, and doesn’t rot or shift. It’s more work upfront but you’ll never have to think about it again. Paving slabs are OK for temporary setups or very small greenhouses, but concrete wins for durability and long‑term stability.
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How long before I can build on my concrete base?
You can walk on it carefully after 24–48 hours, but wait at least 3 days before building your greenhouse on it. A full week is better. The concrete reaches about 70% of its final strength in seven days, which is plenty for a greenhouse. Full cure takes 28 days, but you don’t need to wait that long.
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Do I need steel reinforcement in my greenhouse base?
For domestic greenhouses under 12×10 ft, no. The base thickness and concrete mix provide enough strength. You’d only need mesh reinforcement for very large greenhouses (over 150 sq ft) or if you’re building on particularly dodgy ground. Overkill for most garden setups.
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What if my ground isn’t level to start with?
That’s actually normal – most gardens slope at least a bit. You’ll just dig deeper on the high side to create a level base. Use your string lines and spirit level to work out the levels before you start digging. The formwork holds everything level while the concrete cures, so a sloping site isn’t a problem – just a bit more digging.
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Can I do this in winter?
You can, but it’s trickier. Don’t pour if temperatures are below 3 °C or if frost is forecast within a week. Cold concrete takes longer to cure and can be damaged by freezing. Spring through autumn is ideal. If you must do it in winter, use a winter‑grade concrete mix and cover the base with insulating blankets while it cures.
