SteelRoot Wide 4.00 Galvanised Steel Arch Greenhouse, Twin-Wall Polycarbonate, Walk-In
Galvanised steel arch · twin-wall polycarbonate · walk-in
01Length
02Glazing
4 mm is the base specification; choose 6 mm for extra stiffness and warmth.
03Arch spacing
1.0 m is the standard spacing; choose 0.67 m for more exposed plots.
04Add-ons optional
SteelRoot Wide 4.00 Galvanised Steel Arch Greenhouse, Twin-Wall Polycarbonate, Walk-In
Galvanised steel arch · twin-wall polycarbonate · walk-in
Your build
A galvanised-steel arch greenhouse with 1.0 m standard arch spacing and a 0.67 m close-arch option.
Structure
Included · already in the price
Waldenhaus SteelRoot · kit
Open the kit before you buy it.
Catalogue I · Components
Twelve cards, every spec, no marketing puff. Click any card to open the file.
01
You choose the arch spacing at checkout. SteelRoot's base specification uses 1 m arch spacing. The optional 0.67 m close-arch layout adds more support ribs along the length for more exposed plots.
Shorter spans reduce polycarbonate panel deflection and reduce frame racking under lateral wind load. Stand inside a SteelRoot at 0.67 m and count the ribs over your head. That is the engineering you can see.
02
The 0.67 m close-arch option shortens unsupported spans. Supplier engineering rates the SteelRoot frame for 140 kg/m² snow load in that configuration.
Treat that as configuration-specific supplier data, not a blanket claim for every site. The 1 m standard arch spacing remains the lower-cost base specification; choose the close-arch option where exposure or snow loading makes shorter spans useful.
03
A steel arch greenhouse without ground anchors is exposed in UK wind. The anchoring kit is often a separate £400-£800 line item, and skipping it to save money leaves the structure exposed.
Every SteelRoot ships with the anchor count its length needs: 8 anchors on the 4 m starter, scaling to 24 on the 12 m flagship. Galvanised steel ground screws drive into typical UK soil with a hammer or wrench. No concrete pour. No skip hire. No setting time. No "foundation contractor" line on your invoice. The friction stops here.
04
Entry-tier aluminium greenhouses tend to lose panels in moderate UK wind because of their spring-clip glazing. Those clips can corrode, lose tension, and release.
SteelRoot panels are mechanically screw-fixed to the steel frame with stainless screws and EPDM washers. The optional 0.67 m close-arch spacing reduces the panel flex that loosens fixings over time. With no spring clips, there's no spring-clip failure mode.
05
40 × 20 mm closed rectangular hollow section, galvanised steel with ZAM (zinc-aluminium-magnesium) coating. The closed tube resists twist as well as bend, where an open profile flexes under lateral wind because it cannot resist torsion.
Every face is coated in a full-immersion bath, the tube interior included, and the coating self-heals at scratch and drill points. That carries our 10-year Waldenhaus anti-corrosion warranty on the frame.
06
You choose the thickness at checkout. 4 mm is the base specification: lowest cost and lightest, with enough light transmission for a full crop.
6 mm is the upgrade option: stiffer between supports, more impact-resistant and warmer. Both are UV-stabilised and screw-fixed at every edge with stainless screws and EPDM washers. Choose 6 mm when extra stiffness or thermal retention matters more than the lowest entry price.
07
Self-assembly · pre-drilled, bolt-assembled · common hand tools. Build time runs 1–2 days for the 4 m length, up to 3 days for the 12 m length.
What you need: a cordless drill, a spanner set, a spirit level and a step-ladder. No concrete, no concrete pour. A build-along video covers the full assembly sequence, with the link in your order pack.
08
Five lengths from 4 m (16 m²) to 12 m (48 m²). The 4.00 m width gives you two full growing rows plus a working path, equivalent to a small allotment plot in your own back garden.
2.30 m at the convex ridge, so you stand fully upright down the centre aisle. For three growing rows plus walkways, see the SteelRoot 4 m line (16.0–48.0 m²).
09
Wax-cylinder roof auto-vents are an optional add-on, not part of the base kit. Each opener pushes its vent open at +18 °C and closes it as the temperature drops, so the greenhouse ventilates itself while you are away. No power, no battery, no manual handle.
Add the number that suits your site at checkout, £99 each, up to four. The base SteelRoot already opens front and back: both gable doors have an opening upper section for cross-flow.
10
Every SteelRoot ships with an opening window in each gable door, two per kit. Both stable doors have an upper section that swings open on its own hinge.
That adds intake for the prevailing wind and gives cross-flow through the length of the structure, rather than only venting up and out at the apex.
11
Every part of the SteelRoot kit, from arch sections and panels to screws, anchors and hinges, is documented in the assembly manual with its own code.
Damaged a panel on install, or snapped a bracket at year three? Email us the code and a photo, and we source the matching replacement part for it. No discontinued-model dead-end.
12
First: the pallet arrives at kerbside. Roll out the parts, follow the illustrated guide, drive in the ground anchors, and raise the frame.
Next: mount the panels with stainless screws and EPDM washers, set the vents, hang the doors, and walk in. That covers the 4 m to 8 m lengths; 10 m and 12 m take a little longer, at your own pace.
In situ · real installs
1 / 8
Why we engineer it this way
Ten rows put SteelRoot next to two category alternatives: the entry-tier aluminium polycarbonate greenhouse and the polythene polytunnel, without naming brands. The aim here is to show why we made these specific engineering choices for British weather, rather than to rank one product above another.
Comparison points reflect typical category specs. Individual brands within each category may exceed these defaults, we encourage detailed comparison with any specific product you're considering.
Pick the length,
not the compromise.
FREE UK Mainland Delivery
FREE UK Mainland Delivery
FREE UK Mainland Delivery
FREE UK Mainland Delivery
FREE UK Mainland Delivery
FREE UK Mainland Delivery
Tighter than they have to be.
67 cm
Arch spacing · 1 m standard, 0.67 m close-arch option
Choose the 1 m standard arch spacing, or the 0.67 m close-arch option with 50% more support ribs along the length. The close-arch option shortens unsupported panel spans for more exposed plots.
1 m is the standard arch spacing. 0.67 m is the close-arch option.
Built for British weather
140 KG/M² SNOW
Supplier rating, 0.67 m close-arch build
Heavy UK winter snow: 30–50 kg/m². Scottish Highland exposure: ~100 kg/m².
Supplier-rated, in the 0.67 m close-arch build, at 140 kg/m². Correct anchoring per the included guide still applies.
Supplier-engineering rated. Assumes correct anchoring per the included guide.
The build
DETAIL · FULL-IMMERSION ZAM BATHWhere rust starts
Galvanised steel rusts from the inside: moisture trapped in a closed tube has nowhere to go. Spray-zinc reaches the outside surface; the inner wall stays exposed.
Every SteelRoot arch is hot-dipped in a full-immersion ZAM coating bath (zinc-aluminium-magnesium). Every face is coated, including the tube interior. ZAM self-heals at scratches and drill points where plain zinc cannot. That is what backs the 10-year anti-corrosion warranty.
ZAM coating, full-immersion bath · every face, inside + out
DETAIL · WAX-CYLINDER OPENERWhen it gets hot
Every roof vent is fitted with a sealed brass tube of paraffin wax. At +18 °C the wax expands and pushes the vent open. As the air cools, the wax contracts and the vent closes itself.
It runs on no power, no batteries and no phone app. It comes fitted at the factory, so there's no opener to assemble on the day.
Wax-cylinder auto-opener · opens +18 °C, closes as it cools · factory-fitted
DETAIL · PRE-DRILLED ARCH × BOLTWhere install starts
On-site drilling is where amateur kits go wrong. You mark, you drill, the gap doesn't match, the panel won't seat, and a Sunday afternoon turns frustrating.
Every SteelRoot frame is pre-drilled in a fixed jig, with tolerances to a millimetre. You bolt, you don't drill. Panels actually fit on first assembly.
Factory pre-drilled · jig-controlled tolerance · bolt-only assembly
Promise, in one number
10 yr
10-year Waldenhaus anti-corrosion warranty on the galvanised steel frame
If the steel frame rusts through within 10 years, we ship a replacement.
There is no supplier hand-off. Full warranty terms, exclusions and claim process are published in the FAQ below before purchase.
Coverage starts day one. Full warranty terms in the FAQ below.
Delivery · from order to your door
1–4 weeks
Free UK mainland delivery · tail-lift, kerbside drop
Made to order rather than pulled from a shelf where it has sat for a year. Your delivery date is confirmed within 1 to 4 weeks, so you can plan the install, line up a second pair of hands, and be ready when the pallet arrives.
Highland / Outer Hebrides / Northern Isles, please contact us before ordering for delivery quote
Direct despatch
PRE-DRILLEDBuilt
Each SteelRoot frame arrives pre-drilled and hot-dip galvanised. Every bracket, screw, washer and ground anchor for the build ships together in one labelled box, ready to assemble.
40 × 20 mm RHS galvanised steel · pre-drilled · hand-packed
PALLET · 65 KG (4 M) / 105 KG (12 M)Crated
Every part flat-packed onto a single europallet. Strapped down, kraft-wrapped and weatherproofed for the journey from the EU to the UK and onwards to your kerb. Stencil-marked so the despatch team knows which crate is yours before it leaves the floor.
Single europallet · stencil-marked · kerbside delivery
ON YOUR KERB · TAIL-LIFT DROPDelivered
Tail-lift truck drops the pallet at your kerbside, no forklift needed. Unwrap the kraft, check the labelled parts box against the included parts list: every bolt, bracket and ground anchor accounted for before you start the build. Self-assembly can carry it round to the build spot.
Tail-lift kerbside drop · labelled parts box · 2-adult lift
Not disposable
Every SteelRoot component is documented, coded and replaceable. Email us with a part code or a photo · we'll source the matching replacement part for it.
Long-term parts continuity · documented SKUs, not a discontinued batch
Three questions
~ 60 seconds
Not sure which length?
Three quick questions about how you'll use it, and we'll match you to the right SteelRoot length with the spec rationale and total cost.
SteelRoot's frame and glazing are engineered for typical UK wind and snow exposure when correctly anchored. The closed RHS galvanised steel section, 1.0 m standard arch spacing, optional 0.67 m close-arch spacing, screw-fixed twin-wall polycarbonate and included ground anchors are the mechanisms doing the work. We don't claim it is "stormproof".
Exceptional weather beyond the engineered envelope is excluded, and the exact terms are published before checkout, with no post-purchase small print. Below that envelope, panel and frame defects are covered. See our warranty page for the full terms. If your plot is on the Pennine spine, the Outer Hebrides or a Shetland coastline, email us before ordering. We'd rather discuss your exposure than have you discover the limit the hard way.
1.0 m is the standard arch spacing. The 0.67 m close-arch option adds more frame-to-panel intersections along the length, so unsupported polycarbonate spans are shorter and panel flutter is reduced.
At 0.67 m spacing, supplier data rates SteelRoot at approximately 140 kg/m² snow load. Treat that as configuration-specific data for the close-arch option. Choose 1.0 m for the base specification, or 0.67 m when your site is more exposed or you want the shorter-span build.
Self-assembly with common hand tools. The frame is pre-drilled and bolt-assembled, with components arriving numbered and grouped by build stage. Realistic build times working at a comfortable pace:
4 m 1–2 days · 6 m 2 days · 8 m 2–3 days · 10 m 3 days · 12 m 3 days.
What you need: a cordless drill, a spanner set, a spirit level, a step-ladder. That covers it. No concrete pour, no concrete pour. A QR-code-linked build-along video covers every step. Build-support line open weekday and Saturday mornings during install season.
Ground anchors are included as standard: one bent-L galvanised steel anchor per 1 m of length, so 4 anchors on a 4 m model, 6 on a 6 m, scaling up to 12 on a 12 m. They drive into prepared ground and pin the arch base. Correct anchoring is required for the 10-year frame anti-corrosion warranty to remain in force.
Most UK competitors sell ground anchors separately at £60–£110 per screw. At a 6 m length that is a £360–£660 add-on after the headline price. We made the decision to include them because a greenhouse without anchors is not a finished product, it is a sail.
The frame is hot-dip galvanised 40 × 20 mm rectangular hollow section steel with a ZAM coating (zinc-aluminium-magnesium). ZAM gives self-healing edge protection at scratches and drill points: the coating chemistry passivates exposed steel surfaces as the structure ages. The 10-year anti-corrosion warranty covers through-corrosion penetration. Surface oxidation and light edge spotting are cosmetic and normal, so they sit outside the cover.
Practical care: avoid grinding or drilling new holes after the build, since every hole is pre-drilled at the factory for this reason. If a bolt-head gets scuffed during assembly, a touch of cold-galvanising spray from any DIY store closes the gap. No annual treatment, no oiling, no painting required.
Both are offered, and you pick at checkout. 4 mm is the base specification: lowest cost, lightest, and it grows a full crop. 6 mm is stiffer between supports, holds about 33% better thermal retention than 4 mm, and steps up impact resistance against hailstones and falling debris. The cellular flutes are larger and it flexes less in strong wind.
Choose 4 mm if you want the lowest entry price. Choose 6 mm when extra stiffness, impact resistance or thermal retention matters more than the lowest price.
Cross-ventilation is standard on every SteelRoot: both gable doors are two-part stable doors, and the upper section opens independently as a cross-flow door window, giving two opening points on every model. Wax-cylinder roof auto-vents are an optional add-on (£99 each) that open around 18°C and close around 12°C on their own.
For Cornish or Mediterranean-style heat exposure, side-louvre or gable-vent kits are a planned accessory. If your site is south-facing and bakes from June to September, a roll-down shading kit (sold separately) covers the gap.
SteelRoot 4.00 m wide footprints, in real growing language:
4 m / 16 m²: two full-size raised beds plus a working path; quarter-allotment scale.
6 m / 24 m²: three raised beds, year-round salad and tomato programme.
8 m / 32 m²: small-allotment scale; tomatoes, beans, cucumbers and brassicas under one roof. The most-ordered SteelRoot length.
10 m / 40 m²: serious-grower volume with overwintering capacity.
12 m / 48 m²: commercial-domestic scale; cut-flower microbusiness viable.
If you're between two lengths, take the longer one. The £150–£200 step is small money compared with rebuilding 18 months in.
Usually no. The 4.00 m line has a ridge height of 2.30 m, comfortably under the 2.5 m UK Permitted Development cap for outbuildings on most domestic plots. Check with your local council if any of these apply: conservation area, listed building, plot under an Article 4 Direction, structure closer than 2 m to a boundary, total outbuildings already covering 50% of your garden.
SteelRoot is classified as a greenhouse rather than a polytunnel, which helps where allotment councils ban polytunnel covers on aesthetic grounds. The plain-language Permitted Development checklist ships in your order pack so you can verify before delivery.
1–4 weeks from order for UK mainland, included in the price. Every SteelRoot is made to order. Production typically takes 2–3 weeks, plus 5 working days UK pallet transit. You receive an "In production" email within 5 working days of order, then an "On its way" email with tracking when the pallet is despatched.
The pallet arrives on a tail-lift truck and is dropped kerbside. SteelRoot pallets are 1.2 × 0.8–1.0 × 2.1 m and weigh 65–105 kg depending on length. The 2.1 m height is worth checking against any access constraints (low garage doors, narrow side gates) before ordering. You'll receive an SMS the day before with a 4-hour delivery window. Scottish Highlands, the islands and Northern Ireland have surcharges or limited service, calculated at checkout or quoted by email. Full carrier terms on our shipping policy.
You have 14 days from delivery to start a return, unused, in original packaging. We'll arrange the collection through our carrier. SteelRoot pallets are large, so you pay the carrier collection cost, typically £75–£175 depending on length and your region. We'll quote the exact figure on request before you commit. Full terms on our refund policy.
If the structure has been built and you're stuck on something specific, say a wrong size, a delivery damage that surfaced during the build, or a missing part, talk to us first. Most problems sort out short of a full return.
SteelRoot uses standardised component SKUs produced to a fixed spec. Panels are 4 or 6 mm twin-wall polycarbonate, fasteners and brackets are commodity stainless or zinc-plated hardware, and we keep them documented and coded so we can source replacements rather than re-engineering each time.
Replacement parts go through our customer-service team at customer-service rate. SteelRoot is a sub-brand of Waldenhaus, a multi-product UK operator that maintains a living parts catalogue. If you bought a SteelRoot in year 1, you can still buy parts for it in year 10.
If provider-backed finance is available for your order, it will be shown at checkout by the payment provider before you commit.
We do not publish manual monthly payment estimates on the PDP because the final offer depends on provider approval and the selected build.
Polytunnels cost less upfront, typically £400–£1,000 for an equivalent footprint. Over a decade the cost picture rebalances: the polythene cover needs replacement every 5–7 years at £200–£400 plus a day of re-trenching, and a torn cover in a winter storm is a documented pattern rather than an edge case. Tensioned-film systems are also banned at many allotment sites and in most conservation areas on aesthetic grounds.
SteelRoot is one capital purchase, with screw-fixed twin-wall polycarbonate that doesn't perish, an architectural arch silhouette that councils accept, and better thermal retention than a single-skin film. If you love your polytunnel for the growing but the film cycle has worn thin, this is the next step. If you want the lowest £/m² for a dedicated growing-only space, polytunnels remain a sensible answer.
This is a different category, rather than the same product made cheaper. Entry-tier aluminium polycarbonate kits (£170–£800) use open C-section extrusion that racks under moderate UK wind, spring W-clip glazing that pops out before the first winter, and warranty terms that often narrow weather cover just when a British winter tests the structure.
SteelRoot uses closed RHS galvanised steel, screw-fixed twin-wall polycarbonate, included ground anchors and a clear, published warranty position. The customer who has already lost a cheap-tier greenhouse to wind damage is the customer SteelRoot is built for. You pay roughly 2–3× the entry-tier price once, instead of replacing every two or three winters.
Deeper reading
Real engineering numbers, UK planning rules, and honest comparison guides · pick any topic that helps your decision.
The SteelRoot Wide 4.00 is the widest Waldenhaus galvanised steel arch greenhouse for growers who want a larger, practical structure without a film-cover replacement cycle. It is 4.00 m across and 2.30 m to the ridge, with a frame of 40 × 20 mm RHS galvanised steel with ZAM coating (zinc-aluminium-magnesium), screw-fixed twin-wall polycarbonate (4 mm standard, 6 mm option) and a modular arched form for serious growing space.
Use this product page for the Wide 4.00 specification and ordering details. Use the SteelRoot greenhouse range page to compare the line, lengths and configuration choices, or see the narrower SteelRoot Plot 3.14 side by side. If you are arriving from a polytunnel search, start with the comparison logic in our polytunnel vs polycarbonate greenhouse guide.
