Greenhouse Base Preparation UK: Anchors, Slabs and Concrete — A Walkthrough

The base under a greenhouse is the single most under-rated decision in the buying process. Spend three weekends on the perfect timber kit and twenty minutes on the base, and you'll be re-shimming the door for years. Spend an extra weekend doing the base properly and the structure should sit happily for the lifetime of the timber. This is the operational walkthrough — what to choose, why, and how to actually build it.

Our greenhouse buying guide covers the decision between base types at a high level. This piece goes into the build detail. If you haven't yet chosen the structure itself, the wooden greenhouses range overview is the right starting point.

Written by Alex Goldgewicht, founder of Waldenhaus. Reviewed by the Waldenhaus product team. Last updated 7 May 2026.

What the base actually has to do

Three jobs, ranked by importance:

1. Stay level. Without a level base, the door reveal racks, glazing panels load unevenly (cracking polycarbonate at compression points), and the structure flexes in wind in ways it isn't designed to. Aim for under 5 mm fall across the long dimension.
2. Anchor the structure. A 110 kg empty greenhouse will move in a Force 8 gale unless mechanically tied to the ground. Doesn't matter how good the timber is.
3. Drain reliably. Standing water under the base wicks into timber, accelerates rot at the sole plates, and rusts steel hardware. Soil under the base should drain at the same rate as the surrounding garden.

That's it. Aesthetic tidiness, slight thermal mass, ease of cleaning — those are nice extras, not the brief.

Three base types that work in the UK

For 95% of UK gardens, your decision is between three options. Each is genuinely good for different situations.

Before you dig or drive anchors, do a 5-minute services check. Domestic gardens commonly have buried electrical cables (lighting, garden sockets), water supply or irrigation lines, and occasionally drainage pipes within the top 600 mm of soil. If you have utility plans or you laid services yourself, refer to those. If anything is unknown, dig the first 200 mm by hand with a fork (no spade or driven post) to feel for resistance, and stop immediately if you contact pipe or cable. Wear gloves and safety boots; no hammer-driving anchors blind into uncertain ground.

Option A — Storm-Proof Anchor system (no concrete)

What it is: galvanised steel ground anchors, typically 600 mm long, driven into firm soil at each corner of the greenhouse and at mid-span on longer SKUs. The structure's sole plate bolts directly to the anchor heads. The greenhouse sits on whatever the existing surface is — compacted soil, gravel, paving, or established lawn — and the anchors carry both the weight and the wind load.

Best for:

• Allotments where the site committee prohibits concrete (most do)
• Rural plots with firm clay or stony loam soil
• Anyone who might want to relocate the structure in 5–10 years
• Anyone in a hurry — install time is 30 minutes

Worst for:

• Pure sandy soil (anchors don't get enough grip)
• Waterlogged clay that softens to mud in winter
• Sites with mature tree roots in the corner positions

Build sequence:

1. Mark out the four corners with stakes and string. Check diagonals match within 5 mm.
2. Compact the soil under each corner — heel-stomp ten times per corner.
3. Drive each anchor with a club hammer or fence-post driver. The anchor goes in vertically; tap, don't lever. Most UK soils take 2–3 minutes per anchor.
4. The anchor head sits 25 mm proud of the soil. Sole plate bolts through this.
5. Repeat for mid-span anchors on NORDIC-L upwards.

Cost (DIY): £0–£50 if anchors are included with the kit (NORDIC includes them as standard). £80–£140 for an aftermarket set if your kit doesn't include them.

Labour: 30–45 minutes for two adults, no specialist tools.

Option B — Paving slabs on hardcore (semi-permanent)

What it is: a frame-shaped perimeter foundation made of concrete paving slabs laid on a compacted hardcore sub-base, with the greenhouse sole plate sitting directly on the slabs. The interior of the greenhouse remains soil — important for ground beds, where most growers want roots in actual earth.

Best for:

• Sites where a clean visual finish matters (visible from the house)
• Ground that drains poorly without intervention (slabs raise the structure 50 mm)
• Plots where you want to plant into the ground inside the greenhouse
• Long-term commitment (10+ years before you'd consider moving the greenhouse)

Worst for:

• Rented allotments (not movable, may breach tenancy if site requires "removable" structures)
• Sloped sites where the slab depth varies dramatically end-to-end
• Anyone in a hurry — half a weekend minimum

Build sequence:

1. Mark out the footprint, plus 100 mm clearance on all four sides for the perimeter slab.
2. Excavate to 150 mm depth across the marked area. For a NORDIC-M footprint plus perimeter clearance (~9 m²), that's roughly 1.3 m³ of soil to barrow off — around 30 wheelbarrow loads.
3. Lay 100 mm of MOT Type 1 hardcore. Compact in 50 mm layers with a wacker plate (hire one — £50/day, transforms the result).
4. Lay 25 mm of sharp sand on top, screeded flat to ±3 mm.
5. Bed each slab on a 5–10 mm bed of mortar (or sand for non-permanent fix). Check level and diagonal as you go. Slabs should sit ±2 mm of each other at all joints.
6. Allow 24 hours before loading.

Cost: £100–£200 for materials (60×60 cm slabs, hardcore, sand, mortar). Tool hire £50/day for a wacker plate.

Labour: 8–12 hours for two adults, spread over a weekend. Genuinely physical work.

Option C — Full concrete pad (permanent)

What it is: a continuous concrete slab covering the full footprint of the greenhouse, typically 100 mm thick over a 100 mm hardcore sub-base, with the greenhouse sole plate bolted to the cured concrete.

Best for:

• Permanent commercial-style installations (market gardens, demonstration plots)
• Locations with extreme exposure where the additional dead weight matters
• Greenhouses that will never contain ground beds (the entire interior is staging or paving)

Worst for:

• Anyone who wants to grow root crops in the ground — concrete kills that option entirely
• Allotments — almost universally prohibited
• Sites with awkward access for a concrete delivery (mixer truck or barrow runs)
• Anyone who might change their mind about location in the next 30 years

Build sequence:

1. Mark out and excavate to 200 mm depth across the full footprint.
2. Lay 100 mm of compacted hardcore.
3. Place edge formwork and a vapour barrier (DPM membrane).
4. Pour 100 mm of C25 concrete over the membrane. Tamp and float to a level finish.
5. Set anchor bolts into the wet concrete at the corner positions per the kit's spec.
6. Cure for at least 7 days before loading; 28 days before drilling for fixings.

Cost: £200–£400 in materials for an 8×10 ft footprint, plus tool hire. £600–£900 if you hire a contractor.

Labour: 1–2 days of skilled work, plus cure time. Realistically a job for a contractor unless you've poured concrete before.

Decision matrix: which base for your situation

Drainage: the detail most guides skip

If your existing garden floods, your greenhouse base will too — and capillary action will pull moisture into the timber sole plates within a year.

Three drainage interventions, in increasing order of intervention:

1. French drain perimeter trench. Dig a 200 mm wide × 300 mm deep trench around the greenhouse, fill with 20 mm gravel wrapped in geotextile, with a 100 mm perforated pipe at the bottom drained to a soakaway or rain garden 3 m away. Adds 4–6 hours to the base prep but solves chronic waterlogging.
2. Raised crushed-stone pad. If you're on Storm-Proof Anchors but the ground sometimes stands in water, lay 75 mm of 20 mm clean stone over a geotextile membrane across the full footprint before driving anchors. Raises the structure above standing water without going to slab finish.
3. Soakaway crate, mains-connected. For sites that flood seasonally, a buried soakaway crate (typical capacity 100 L) with a perforated inlet from the greenhouse gutter downpipe handles roof runoff that would otherwise pool around the base. Materials £80, half-day install.

For most UK domestic gardens, none of this is necessary. For allotment sites near rivers, estate-house greenhouses on heavy clay, and any site where you've ever had standing water visible — at least intervention #2 is worth the 30 minutes.

The detail that really matters: ventilation gap under the sole plate

Wooden sole plates rot from underneath, not from above. The cause is trapped moisture between the timber and the surface beneath — even pressure-treated timber rots if held damp continuously.

Two cheap interventions that extend timber life by 5+ years:

• 3 mm air gap. Sit the sole plate on small DPC strips (damp-proof course plastic) at every fixing point, leaving a deliberate 3 mm gap between timber and base material. Air can circulate; moisture evaporates.
• Capillary break. Where the sole plate meets a hard surface (slab, concrete), lay a strip of butyl tape or DPC membrane between them. Stops moisture wicking up into the wood.

Both are 10-minute additions during base prep. Skip them and timber sole plates rot earlier than they need to — and rot at the sole plate is the most common warranty edge case.

What about the interior surface?

Inside the greenhouse, you have three options too:

• Bare soil. Best for ground beds — direct planting into earth. The default for most allotment growers. Cover paths with weed-suppressant matting + bark chip or pea gravel for cleanliness.
• Paving slabs throughout. Best for staging-only setups (no ground crops). Easy to clean, no weeds, slight thermal mass benefit (stones absorb day heat, release at night).
• Mixed — perimeter slabs for staging, central soil bed for cropping. The pragmatic compromise. Most NORDIC owners end up here within two years of installation.

Whatever you choose, install before the first crop. Disturbing an interior surface mid-season damages root systems and bruises crops.

Common base prep mistakes (from three years of customer reports)

1. Levelling on uncompacted soil. Soil settles 10–20 mm in the first six months, especially after the first rain cycle. The structure ends up with a corner-low. Solution: heel-stomp the corners thoroughly before anchoring; better, leave the marked-out site for a week with light foot traffic before final levelling.
2. Slabs laid on sand alone (no hardcore sub-base). Sand washes out under the slab over winter; slabs sink unevenly. Always use compacted hardcore as a sub-base.
3. Forgetting the vapour barrier under a concrete pad. Without DPM, ground moisture wicks up through the concrete and the timber sole plate sits damp. Cheap mistake to make once.
4. Putting the greenhouse on the lawn "just for the summer." Six months later, the grass is dead and matted under the structure, the soil has compacted unevenly, the door is racked. Either commit to a base or commit to moving it monthly.
5. Underestimating the perimeter clearance needed. Most greenhouses need 0.5 m clear access on the door side and 0.4 m on the back wall. Plan the base footprint to include this access strip; don't fight neighbouring planting later.

What we install on Waldenhaus customer sites

When asked, our default recommendation is:

• Allotment install: Storm-Proof Anchors on compacted soil, with a French drain perimeter if the site floods. £0 extra in materials beyond what's included with NORDIC. 1 hour of work.
• Suburban back garden: Storm-Proof Anchors if the lawn is firm; paving slab perimeter if the customer wants the visual finish. Roughly 50/50 split in customer choices.
• Permanent rural install: Storm-Proof Anchors plus a perimeter drain. Concrete pad is rare — we've only recommended it for two installs in three years, both market-grower commercial setups.

The Storm-Proof Anchor system is included as standard with every NORDIC; we don't sell paving or concrete materials. The point of this guide is to help you make the right call independently.

Frequently asked questions

Do I really not need concrete?

For most domestic and allotment installs, no — Storm-Proof Anchors carry the wind load and the structural weight without concrete. Concrete is genuinely required only for: (a) commercial installations expecting 30+ year permanence, (b) sites with chronically unstable soil (peat, made-ground, etc), or (c) where local building rules specifically demand it.

What about a polystyrene foundation kit?

We don't recommend them. Polystyrene foundation systems work in dry continental climates but tend to degrade faster in UK conditions — freeze-thaw cycles can crack the foam over a few seasons, water gets in, the structure begins to subside unevenly. Stick with anchors, slabs, or concrete.

Can I lay paving slabs directly on grass?

Technically yes, but you'll regret it within 18 months. Grass dies under the slabs (releasing moisture and creating voids), worms tunnel under and between slabs (wobbly finish), and weeds grow up between every joint. Always excavate and lay a hardcore sub-base.

How long do Storm-Proof Anchors last in the ground?

Galvanised steel ground anchors are designed for long in-soil service in typical UK conditions. The galvanising layer is sacrificial — even when it eventually depletes, the underlying steel remains structurally sound for many years afterwards. We have not had a Waldenhaus customer report an anchor failure due to corrosion in the time the product has been in market.

Does the base affect the warranty?

The 10-year anti-rot frame warranty (conditional on re-treating timber with an approved wood preservative every 2–3 years) + 5-year polycarbonate warranty applies regardless of base type, provided the base is genuinely level and provides adequate drainage. We don't dictate base method; we do reserve the right to inspect (via photos) if a warranty claim relates to timber rot at the sole plates. In three years we've never had to do this.

What if my plot slopes?

For falls under 100 mm across the long dimension, Storm-Proof Anchors with shimmed sole plates handle it cleanly. For 100–300 mm fall, paving slabs on stepped hardcore is the right call (excavate the high end, build up the low end with hardcore). For 300+ mm fall, you're into terraced retaining wall territory — get a quote from a local groundworks contractor before ordering the greenhouse.

Choose your NORDIC wooden greenhouse

By size

• 8×6 ft — NORDIC-S £1,499 · patio / starter / small allotment
• 8×10 ft — NORDIC-M from £1,599 · family veg garden sweet spot
• 8×13 ft — NORDIC-L from £1,699 · serious year-round growing
• 8×16 ft — NORDIC-XL from £1,799 · multi-crop with two ground beds
• 8×20 ft — NORDIC-XXL from £1,899 · walk-in / market grower

By configuration

• Lean-to wooden greenhouse · wall-mount, space-efficient
• Polycarbonate greenhouses UK · 4 mm CrystalLight™ glazing detail

Decide before you buy

• Greenhouse Buying Guide UK 2026 · full step-by-step framework
• Wooden Greenhouses overview · the NORDIC range explained
• Shop the NORDIC range — from £1,499

Before you start the base: confirm your plot is Permitted Development compliant — Conservation Areas and Listed Buildings need permission first.

Before the base — choose the spot: Where to Place a Greenhouse UK — the 5 decisions (orientation, sun, frost pockets, drainage, access). Get placement right before laying slabs.