By Ryan, Planet Greenhouse
The foundation is the single question that stalls more greenhouse purchases than any other. People fall in love with a Royal Victorian, get their heart set on year-round growing, and then freeze at one line: what do I set it on?
Here's the honest answer, the same one we give on the phone every week. Most people who call us have no idea what foundation they need. That's normal. It's not a sign you're in over your head — it's the part nobody explains until you ask.
What foundation does a greenhouse need?
For every Janssens greenhouse, we recommend an 8-inch concrete strip foundation with corner anchoring that runs down to your local frost line. A full concrete slab works too. Either way, it has to be 100% level and flat, and sized to the greenhouse's outer dimensions.
That's the short version. The reasoning matters, though, because your climate and your site change the details.
A greenhouse is a heavy, permanent aluminum-and-glass structure — not a shed you can shim. It needs to sit on something load-bearing that reaches solid, frost-free ground. If the footing stops short of the frost line, freeze-and-thaw cycles heave the ground under it, and over a few winters that racks the frame and stresses the glass. The concrete is what keeps everything square for decades.
One detail people miss: aluminum structure dimensions are always the outer dimensions. When you pour, you're matching the outside footprint, not guessing at an interior. Keep the foundation close to those exterior dimensions so rainwater can't seep under the frame.
Not sure what your frost line is or how your yard drains? Call us and we'll walk your site with you before you order a single bag of concrete.
Strip foundation or full slab — which is better?
Both are legitimate. A strip foundation — concrete poured in a continuous footing around the perimeter, on the outer dimensions — is what we recommend most often. It's less concrete, it's cheaper, and it gives the frame a solid, level bed exactly where the weight lands.
A full slab — a complete concrete pad across the whole footprint — also works well and gives you a finished floor to work on. It costs more and uses more material, but some buyers prefer the clean, sealed interior, especially if they're rolling carts or setting up heavy benches.
| Strip foundation | Full slab | |
|---|---|---|
| Concrete used | Less — perimeter only | More — entire footprint |
| Cost | Lower | Higher |
| Floor | Your choice inside (gravel, pavers, etc.) | Finished concrete floor |
| Best for | Most installs; drainage-friendly | Heavy interior use, wheelchair/cart access |
Whichever you choose, the non-negotiable is level. A greenhouse frame that's out of level fights you at every panel, and tempered glass has no patience for a twisted frame.
When can you skip the concrete and anchor at the corners?
Sometimes — for small rectangular greenhouses — you can anchor at the four corners only instead of pouring a full perimeter foundation. It's a real option, and for a compact model on stable ground it can be enough.
But there's a catch worth knowing before you count on it: if you go the four-corner route, do not add a lowered threshold. The Low Threshold Kit cuts into the base frame, and without a continuous foundation under it, that cut section won't be properly supported. Corner anchoring and a low threshold don't mix.
For anything larger, or anywhere with real frost, real wind, or soft ground, come back to the strip foundation. It's the difference between a structure that shrugs off winter and one you're re-leveling in spring.
Do Cathedral and larger models have special foundation requirements?
Yes — and this one isn't optional. On Cathedral models, the portal frames and the PRO210 corner anchors must be embedded in concrete. The Cathedral uses a heavy-duty portal-frame construction, and those frames rely on being locked into the pour to do their job. Tell your contractor this upfront, before they form anything, because it changes how the footing gets laid out.
If you're looking at a large Janssens greenhouse — a big Cathedral, or a custom Gigant — the foundation stops being a weekend DIY pour and becomes a real conversation with a masonry contractor who's seen your site. That's a good thing. At that size and price, you want it engineered right.
How much does a greenhouse foundation cost?
This is the number we won't invent, and you should be wary of anyone who quotes it blind. Foundation cost swings enormously by region, and the drivers are real: frost depth (a footing in Georgia and one in Minnesota are different pours), site conditions (slope, drainage, rock, and tree roots all add cost), access for a concrete truck, and local masonry rates, which vary widely between markets.
For a real figure, you need a trusted local masonry contractor who has walked your site. If you'd rather not source one yourself, we can put you in touch with an installer in our network who does qualified prep work. For the full cost picture — greenhouse, freight, foundation, and labor — see our companion guide on what a glass greenhouse actually costs.
Getting your site ready to pour
A few things to have squared away before the concrete truck shows up:
- Level, firm, obstacle-free ground. Stable soil, no buried roots or debris.
- Room to work. Leave about 30 inches of clearance around the greenhouse for ladders and glazing during assembly.
- A sheltered spot. Build where you're protected from the worst wind and storms.
- Anchoring hardware isn't included. Plugs, wedge bolts, and chemical anchors vary by region and contractor — your installer supplies what suits your ground.
- Check local building rules. Some townships want a permit the moment you pour a permanent foundation. Handle that first
And one timing rule from the manuals: assemble only in dry, windless weather. Glass panels and gusty days don't belong together.
The bottom line
For a Janssens greenhouse, plan on an 8-inch concrete strip foundation, corner-anchored down to your frost line, dead level, on the outer dimensions. A slab is a fine alternative. Small models can sometimes ride on four corner anchors — just skip the low threshold if they do. Cathedral portal frames must be set in concrete. And always get local professional advice, because frost depth and code are local by nature.
Every Janssens greenhouse is backed by a 15-year warranty and built in Lier, Belgium — a structure meant to last decades. The foundation is what lets it. Get this part right and everything above it takes care of itself.
Still not sure what your site needs? Call us or send your zip code and we'll talk through your frost line, your drainage, and whether a strip, a slab, or four corners is right for you. No obligation — this is exactly the kind of thing we're here for.