Assembling a steel arch greenhouse: what the job really involves
Could you hold a 6 m sheet of polycarbonate over a steel arch while someone screws it down? That is the hardest moment in building a SteelRoot Plot 3.14, and it is why the product page asks for two adults, a dry weekend and ordinary tools. The frame bolts together, the sheets screw on, and only the two end walls are cut. Here is the whole job, from the kerb to the last strip of tape.
- Base first. On firm, level soil you need no concrete: strip the turf, then level and firm the ground before delivery day.
- Two adults, a dry weekend. The roof sheets are 2.1 × 6 m and go on whole, so one person cannot place them.
- One cutting job. Only the two end walls are cut, from one sheet, and the end frame itself is the template.
What comes on the pallet, and what do you supply?
One pallet holds the frame, the sheets, both doors fitted in their end frames, every bolt and screw, the ground anchors and the edge tapes. You supply the base, the tools and a second pair of hands.
The kit travels on one pallet, about 1.2 × 0.8 m and 2.3 m tall, weighing 65 to 120 kg depending on the length. A lorry leaves it at the kerb, and the driver does not carry it into the garden. From there it goes to the site by hand, a bundle at a time, so measure the gateway and the side passage before the day.
| Item | Where it comes from |
|---|---|
| Arches, crossbars and end frames in 40 × 20 mm closed steel profile, galvanised with a ZAM coating, pre-drilled | In the box |
| Twin-wall polycarbonate sheets, 2.1 × 6 m, 4 mm or 6 mm | In the box |
| Two stable doors, fitted in their end frames, with handles and catches | In the box |
| M6 bolts, nuts and wide washers, roofing screws and 4.2 mm screws, bagged and labelled by part | In the box |
| Ground anchors, counted for your length: 6 for a 4 m build at 1.00 m spacing | In the box |
| Solid tape for cut edges, breather tape for the bottom ends of the roof sheets | In the box |
| Roof vents with automatic openers, clear U-profile, steel arch straps | Optional, ordered with the greenhouse |
| U-brackets for a concrete, paved or timber base | Bought elsewhere |
| A firm, level base; a drill-driver and ordinary hand tools; a helper | Yours |
Check everything against the packing list within 14 days of delivery, even if you plan to build later. Anything missing or damaged is sent free, and parts are held for ten years.
What has to be ready before the first bolt?
A firm, level base marked out to the plan, the manual for your arch spacing read to the end, two adults and a dry forecast. The tools are a drill-driver and ordinary hand tools.
The base comes first, and it is the part most worth getting right. On firm, level soil you strip the turf, then level and firm the ground; the greenhouse stands straight on it and the ground anchors from the box hold it down. On concrete, paving or timber the legs sit in bolt-down U-brackets instead, which you buy from a builders’ merchant. Our guide to greenhouse bases compares the options.
The base plan, one page per length, shows where everything goes. A Plot 3.14 is 3.00 m wide on the ground and needs a base 3.2 m wide and your length plus 0.25 m. Keep 2 m clear of fences and buildings on every side, as the manual asks.
There are two manuals, one for arches at 1.00 m and one for the sturdier 0.67 m spacing, and our arch spacing guide explains the choice. Download yours, such as the manual for 1.00 m spacing, and read it to the end before delivery. Watch the forecast too: if a storm is due before you could finish, leave the polycarbonate in its pack until it has passed.
In what order does a steel arch greenhouse go together?
Mark out, bolt the arches, anchor as it goes up, cut the end walls, fit the end walls, then the roof sheets and their tapes.
- Mark out and level: peg the outer line, check that both diagonals match, and level the ground.
- Bolt the arches, crossbars and end frames together through the pre-drilled holes. Nothing is welded and the steel is never cut.
- Anchor the frame as it goes up: an anchor in each leg the manual marks, one at every corner and the rest along the length.
- Cut the two end walls from the last sheet, using the assembled end frame as the template.
- Screw the end walls on, tape their cut edges and free the doors.
- Lay the roof sheets whole over the arches, screw them down, then tape their ends.
Every step is drawn in the manual, and the product page carries a film of a whole build. Building a timber kit instead? Our wooden greenhouse assembly guide covers that job.
How do you cut the polycarbonate for the end walls?
With the end frame as the template: lay it on the last sheet, mark round it, screw the piece on, then trim to the arch with a knife. The roof sheets go on whole.
One sheet gives all four end-wall pieces: for each end, a large piece for the door and one side, and a small piece for the other side. You never measure a curve. Lay the last sheet flat, printed side up, set an assembled end wall on it and mark round the arch, cutting a little outside the line. Mark on the protective film and cut through it.
Fix the piece to the end frame with roofing screws while it lies flat on the ground, then run the knife along the arch and the spare comes away. The doors arrive fitted in their end frames and sit under the same piece, so cut along the gaps round them last; both halves of each stable door then open, with no door to measure.
If a cut goes wrong, any UK plastics supplier stocks the same clear twin-wall sheet, and we check your sizes before you order it.
How are the sheets fixed?
Self-drilling roofing screws with washers, placed by eye along each arch, with no marking out or pre-drilling. The roof sheets go on whole, so two people lift and hold each one.
The roof sheets are 2.1 × 6 m and run whole over the arch, with no joint across the slope, which is why two people are needed to lift and hold them. Each sheet is screwed to the steel at every edge with a self-drilling roofing screw and sealing washer, which cuts its own hole through the sheet into the steel; there are no clips to work loose. Space the screws by eye along each arch, drive them in straight and stop when the washer sits snug.
Peel the protective film as each sheet is screwed down. Solid tape closes the cut edges and breather tape goes on the bottom ends of the roof sheets. Optional roof vents, U-profile and arch straps go on as the manual shows.
Who builds it if you can’t?
A local handyman or fencing contractor can build it from the manual. We don’t fit greenhouses, so send them the manual and the base plan before they quote.
The build needs two people because of the 6 m roof sheets. If that is not you, a handyman or fencing contractor can work from the same manual. The product page puts the cost of fitting a steel greenhouse of this size at about £400 to £600, from fitting prices published by UK sellers in September 2026. Ask for a written quote that prices the base, carrying the kit from the kerb and the build separately, and says who buys any U-brackets and who takes the packaging away.
What should you check once it is up?
Anchors firm, screws snug, tapes sealed and both doors opening freely. Then walk round it once a year, as the warranty asks.
The warranty reads: 2-year manufacturing defect warranty on cellular polycarbonate panels + 10-year anti-corrosion warranty on the galvanised steel frame. An unanchored greenhouse is not covered for structural failure, so the anchors matter as much as the sheets. Keep staging free-standing and never drill the frame for it.
If you need a part later, email the part code, a photo and your order number; parts are held for ten years, and you pay the cost plus carriage, confirmed first. Single polycarbonate sheets are not posted as spares, because any UK plastics supplier cuts the same twin-wall and we check your sizes.
Quick answers about building a steel arch greenhouse
Not safely. The roof sheets are 2.1 × 6 m and have to be lifted over the arch and held while they are screwed, which takes two people.
Not on firm, level soil: strip the turf, level and firm it, and the ground anchors in the box hold the greenhouse down. Concrete, paving or timber need bolt-down U-brackets, bought elsewhere.
No. Both doors arrive fitted in their end frames. You cut round them once the end-wall piece is screwed on, and both halves of each door open.
Check the pallet against the packing list within 14 days. Anything missing or damaged is sent free.
What should be ready when the pallet arrives?
The base built to the plan, the manual read, a helper booked for a dry weekend, the route to the garden measured and the tools charged.
Seven things to have ready
Tick them off in the week before delivery. Your ticks stay on this device.
Read the manual first
The Plot 3.14 page carries the assembly manual, the base plan for every length and the film of a real build. An expert can answer anything the drawings leave open.
What we checked for this guide
Every figure about the Plot 3.14 comes from its product page and assembly manual. Checked 20 October 2026.
- 01WaldenhausPlot 3.14 base plan and foundation guide
- 02WaldenhausPlot 3.14 assembly manual, 1.00 m arch spacing
Related guides
Written by the Waldenhaus team. We sell outdoor saunas and greenhouses and write these guides from manufacturer manuals, official guidance and our own product pages. Where your equipment manual says something different, follow the manual.