A raised bed greenhouse foundation is a low frame of rot-resistant timber, block or brick that lifts your greenhouse off the ground. The greenhouse bolts to the top, and the space inside can be filled with soil for planting. It is one of the easiest DIY bases you can build, as long as it is level, square and sized to your kit's manual.
✅ Quick answer
- A raised bed foundation is a perimeter frame that lifts the greenhouse a few inches to a couple of feet.
- Most backyard frames are one to three boards high. Taller frames need posts and cross ties so the soil doesn't push the walls out.
- Use naturally rot-resistant wood (cedar, redwood), lumber rated for ground contact, block or composite.
- Build to the exact base size in your manual, not the size in the product name.
- A taller base raises the door sill. Royal Victorian and Riga owners may need a drop door or door extension kit.
We are a small, woman-owned greenhouse business in Georgia. Foundations are one of the first things our customers ask about, often before they pick a model. This guide covers what we tell them on the phone: when a raised base makes sense, what to build it from and how to build it step by step.
What is a raised bed greenhouse foundation?
Think of a big raised garden bed with a greenhouse sitting on its rim. The frame runs around the outside edge. The greenhouse base rail is fixed to the top of it.
Inside, you choose the floor. Many growers fill the beds along the sides with good soil and leave a gravel or paver path down the middle. Others fill the whole frame with gravel and grow on benches.
- Perimeter frame: timber boards, landscape timbers, block or brick, one or more courses high.
- Bottom layer: a compacted gravel bed or shallow trench so the frame sits flat and drains well.
- Fixings: corner posts or brackets, plus anchors that tie the greenhouse to the frame and the frame to the ground.
- Fill: soil for in-ground beds, or gravel and pavers for a clean floor.
📝 How it differs from a stem wall: a stem wall is masonry built on a poured footing, often below the frost line. A raised bed foundation is lighter and simpler, and usually sits on gravel. For a heavy glass house or a cold, frost-heave area, read our guide on building a greenhouse on a stem wall first.
Why put a greenhouse on a raised bed foundation?
A raised base solves several small problems at once. Here is why our customers choose one:
- More headroom. Every inch of base adds an inch of height inside the greenhouse.
- Better soil. You fill the beds with fresh, loose soil instead of digging heavy clay.
- Drier frame. The base rail sits above puddles and splashing mud.
- Easier on your back. Raised beds bring plants closer to your hands.
- Works on gentle slopes. You can make one side taller to get a level top.
- Fewer weeds. A gravel layer and a clean frame edge slow weeds from creeping in.
Which material is best for a raised greenhouse base?
There is no single best material. It depends on your budget, the greenhouse weight and how long you want the base to last. Here is how the common choices compare.
| Material | Lifespan | DIY-friendly? | Good to know | Best for |
|---|---|---|---|---|
| Cedar or redwood | Long, if kept off wet soil | Yes | Naturally rot-resistant; no chemicals | Food beds, a natural look |
| Ground-contact treated lumber | Long | Yes | Today's residential lumber is arsenic-free | Budget builds |
| Composite / recycled plastic | Very long | Yes | Won't rot; can flex on long runs | Low upkeep |
| Concrete block or brick | Very long | Some skill | Heavy; needs a level bed or footing | Heavier kits, a built-in look |
| Maker's metal base frame | Matches the greenhouse | Yes | Designed for that exact kit | Riga and other matched kits |
Is pressure-treated lumber safe for a greenhouse base?
This is a common worry, and the old version of this article got it wrong. The EPA did not ban treated lumber. In 2002, it announced that wood treated with chromated copper arsenate (CCA) would be phased out of residential products, and makers stopped selling it for home use after 2003.
Today's residential treated lumber uses arsenic-free preservatives such as ACQ (alkaline copper quaternary) or copper azole. Purdue Extension lists these, along with cedar, redwood, composites, stone, block and brick, as the main options for garden structures.
- Choose boards labeled for ground contact if they will touch soil or gravel.
- If you want to avoid treated wood near food crops, use cedar, redwood, composite or block.
- Some gardeners line the inside face of treated boards with heavy plastic before filling with soil.
- Don't use old railroad ties or reclaimed lumber of unknown history.
How tall should a raised bed greenhouse foundation be?
Most raised greenhouse bases we see are one to three boards high. With 2x6 lumber, that is roughly 5.5 to 16.5 inches. A few growers go higher, up to around 3 feet, but tall frames act like retaining walls.
- One board (about 6 in): lifts the frame out of mud and splash. Fine for growing in the soil below.
- Two boards (about 11 in): a popular middle ground. Enough depth for most greens and herbs.
- Three boards (about 16.5 in) or more: deep beds for tomatoes and root crops, more headroom, but a bigger step at the door.
Soil pushes outward on the walls, and wet soil pushes harder. For anything over two boards, add posts at the corners and every few feet. Long sides may also need a cross tie in the middle.
💡 Our tip: decide what you will grow first. Lettuce and herbs are happy in shallow beds. Deep-rooted crops like tomatoes enjoy more depth, and you can always add containers on top.
How do you build a raised bed foundation for a greenhouse?
Here is the basic plan for a timber frame on gravel. Block and brick bases follow the same steps, with mortar or adhesive instead of screws.
- Get the exact base size. Use the outside base dimensions from your greenhouse manual. A "10x14" kit may really measure 9'8" by 14'.
- Check permits and call 811. Ask your town about setbacks. 811 marks buried lines for free.
- Pick a sunny, level spot. Lay out the outline with stakes and string. Measure both diagonals; they must match for a square frame.
- Clear and dig. Remove sod and topsoil about 4 to 6 inches deep along the frame line, or across the whole footprint.
- Add and tamp gravel. Fill with crushed gravel, then compact it flat. Check level in every direction.
- Build the first course. Cut boards to length, join the corners with screws or brackets and set the frame on the gravel. Level and square it again.
- Stack and tie the courses. Stagger the joints. Screw each course to corner posts on the inside.
- Anchor the frame. Use ground anchors or stakes so wind cannot lift the frame and greenhouse together.
- Fill and finish. Add soil to the bed areas and gravel or pavers to the path. Leave the top rail clean and flat.
- Fix the greenhouse. Bolt or screw the base rail to the frame at the points shown in your manual.
⚠️ Don't build from the brochure: base sizes are not the same as the size in a product name. Wait until you have the manual, and build to its numbers. Call us if any measurement is unclear.
How much lumber and soil will I need?
Two quick formulas will rough out your shopping list. They are also handy for checking a quote.
📐 Raised base formulas
Linear feet of boards = Perimeter × Number of courses × 1.1
Fill (cubic yards) = Bed length × Bed width × Depth in feet ÷ 27
Perimeter = 2 × (length + width) · 1.1 adds 10% for cuts and waste · Depth in feet = inches ÷ 12
🧮 Worked example: a two-board base sized for a Riga 4
- The Riga 4 measures 9 ft 8 in by 14 ft. Perimeter = 2 × (9.67 + 14) = about 47.3 feet.
- Two courses of 2x6: 47.3 × 2 × 1.1 = about 104 linear feet. In practice, that is four 14-foot and four 10-foot boards.
- Two 2x6 boards stack to about 11 inches (0.92 ft) of depth.
- Beds on both sides with a 3-foot path: bed width = 9.67 − 3 = 6.67 ft. Fill = 14 × 6.67 × 0.92 ÷ 27 = about 3.2 cubic yards of soil.
These are planning estimates only. Riga owners usually anchor the greenhouse with the matching Riga foundation frame, set in a 6-inch-deep trench. If you want to raise a Riga higher, call us first so we can check your plan and the door.
Will a raised base change my greenhouse door?
Yes, if the base is tall. The door sill sits on top of the base, so a 16-inch base means a 16-inch step. A door kit lowers the opening so you can walk straight in.
- Riga 3, 4 and 5: the Riga door extension kit covers up to 20 inches of extra height above the door. It uses the same 10mm twin-wall polycarbonate as the greenhouse.
- Riga 2s, 3s, 4s and Riga XL: there are matching kits for the Riga s models and the Riga XL.
- Royal Victorian: the Royal Victorian drop door kit drops the door 20 to 36 inches. Sliding-door models must upgrade to hinged doors first.
Here are approximate prices for the door and base parts mentioned above:
| Product | What it does | Approx. price* |
|---|---|---|
| Riga Foundation Frame | Anchors a Riga in a shallow trench | About $700 (Riga 2s) to $2,676 (Riga XL 9); about $1,274 for a Riga 4 |
| Riga Door Extension Kit (3, 4, 5) | Adds up to 20 in above the door | About $450 |
| Royal Victorian Drop Door Kit | Lowers the door 20 to 36 in | About $200 |
*Prices checked in October 2026. Prices can change and sales may apply, so click the model name to see today's price.
📌 About prices: manufacturers update their prices during the year, and we often run sales and promotions. The price on the product page is always the current one. Prices in this article are a guide from when we wrote it (October 2026), not a quote or guarantee. Found a lower price for the same model at an authorized retailer? We offer a Price Match Guarantee - please check the rules first. Call us and we will confirm today's price and any current deals.
Is a raised bed foundation strong enough for my greenhouse?
For many small and mid-size kits, a well-built, well-anchored frame works nicely. Heavier glass houses are a different story. Their snow and wind ratings often assume a concrete base.
- Read the manual first. Many makers state the base they expect. Follow it to protect your warranty.
- Glass greenhouses: the engineering letter for the Royal Victorian VI46, for example, bases its ratings on a solid concrete or grouted block wall. See our Royal Victorian foundation guide.
- Cold climates: a frame on gravel can move with frost heave. In the north, a footing below the frost line is safer.
- Windy sites: anchor both the frame and the greenhouse. Our guide on how to anchor a greenhouse to concrete covers the hardware.
Still deciding between bases? Our practical guide to a greenhouse foundation compares slabs, piers, timber, stem walls and maker frames side by side.
How can Mulberry Greenhouses help with your base?
We answer every question personally. Send us your model, a photo of your spot and the base height you have in mind. We will check the manual's base size, the door options and what to order together.
- Every greenhouse we sell ships free to the continental US.
- We can connect you with independent assembly crews once your base is ready.
- Planning beds inside? Browse our garden beds for raised beds and garden tables.
Frequently asked questions
Do I need a foundation for my greenhouse?
Yes. Every greenhouse needs a flat, level, anchored base. Without one, the frame can twist, doors stick and wind can move it. The base can be simple, like a timber frame on gravel, or a concrete slab.
Can I put a greenhouse directly on raised beds?
You can set a greenhouse on a sturdy raised frame built to its exact base size. Ordinary garden beds are usually too flimsy and not square enough. Build the frame as a foundation first, then fill it.
What wood is best for a raised greenhouse base?
Cedar and redwood are naturally rot-resistant. Lumber rated for ground contact also lasts well and costs less. Composite boards won't rot at all.
Is treated lumber safe near vegetables?
Arsenic-based CCA lumber was phased out of residential use after 2003. Today's residential lumber uses copper-based preservatives. If you'd rather avoid it, choose cedar, redwood, composite or block.
How deep should the gravel be under a greenhouse base?
Many builders use about 4 to 6 inches of compacted crushed gravel. Wet or soft ground may need more. Ask a local builder if your soil stays soggy.
Does a raised base keep a greenhouse warmer?
A little. Solid walls lose less heat than glazing, and soil holds warmth. For more ideas, see our guide on how to heat a greenhouse without electricity.
Planning a raised greenhouse base?
We are a small, woman-owned business in Georgia, and we answer every question personally. Tell us your model and the height you want, and we will help you plan the base, the door and your assembly crew.