Why Structure Comes Before Equipment on Every Serious Project
Most buyers start a hydroponic project with channels, tanks and controls. The structure arrives last, usually as a line item in the greenhouse supplier’s quote that nobody reads in detail. That order is backwards, and it is expensive to discover in year two.
A greenhouse carrying hung gutters, a 40 kg/m² cucumber crop, thermal screens, shade curtains and a full irrigation manifold is not the same building as a seasonal vegetable tunnel. Hydroponic greenhouses carry higher permanent loads, are usually heated year-round, and are expected to survive a 15–20 year design life without a production stop. This guide sets out how to specify the load cases, what the numbers actually mean on a drawing, and which foundation type fits which ground — plus the clauses that belong in your supply contract before steel is cut.
Step 1: Name the Load Cases, Not Just “Wind and Snow”
Structural failure is almost never caused by one load. It comes from a combination the designer never priced. Ask your supplier to state, in writing, which of the following are included in the calculation and which are excluded.
| Load case | What it covers | Typical design value | Why hydroponics changes it |
|---|---|---|---|
| Dead load (D) | Steel, glazing, screens, fixed equipment | 15–30 kg/m² | Twin screens and heavy glass push this up quickly |
| Crop load (Qc) | Plants, gutters, slabs, fruit weight | 10–40 kg/m² | Tomato and cucumber on wire trellising sit at the top of the range |
| Suspended equipment | Pipework, lighting, ducts, manifolds | 5–15 kg/m² | LED top lighting and recirculation mains are rarely budgeted for |
| Snow (S) | Ground snow load converted to roof load | Region-specific, 0.5–2.5 kPa | Heated houses shed snow; unheated tunnels accumulate it |
| Wind (W) | Positive pressure, suction, uplift | 0.4–1.2 kPa basic wind pressure | Uplift governs anchorage more than downward pressure does |
| Seismic (E) | Horizontal acceleration | Region-specific | Tall Venlo gutters and tank farms need explicit checks |
The three combinations that matter. In most codes the governing cases are D + S (downward, roof-loaded), D + W (uplift and suction on the windward roof and sidewall), and D + Qc + W (full crop plus a wind event). If your supplier only quotes a single “wind resistance” figure in km/h without referencing a code and a return period, that number is marketing, not engineering.
Step 2: Ground Snow vs Roof Snow — The Conversion People Get Wrong
Meteorological services publish ground snow load. The roof sees a different number, and the difference between the two is where most undersizing happens.
- Shape coefficient. Flat and shallow-pitch roofs hold more snow than steep ones; valleys between adjacent spans hold more than ridges.
- Thermal coefficient. A heated greenhouse melts and sheds snow, which reduces the load — but only if the heating is reliable and the roof surface is warm. A boiler failure in a snow week creates the worst-case combination.
- Sliding and drift. Snow sliding off a high gutter onto a lower span, or drifting against a gable end, produces local loads far above the average.
- Exposure. Open sites and coastal sites increase both wind and drift; sheltered sites inside a forest belt behave differently again.
Rule of thumb we use for budgeting: a 1.0 kPa ground snow load on a multi-span heated Venlo with 22° roof pitch typically resolves to a roof snow load in the 0.4–0.7 kPa range, while a cold single-span tunnel on the same site can see 0.8–1.0 kPa because nothing melts. Ask for the coefficients used, not just the result.
Step 3: Wind Is an Uplift Problem, Not a Push Problem
Buyers visualise wind as a horizontal shove. On light steel greenhouses, the governing case is usually suction: negative pressure on the leeward roof and gable trying to peel the structure up and out of its foundations.
| Check | What fails if ignored | What to require |
| Basic wind pressure | Global frame capacity | Code reference + return period (commonly 50-year) + terrain category |
| Uplift at the base | Columns pulling out of footing | Anchor bolt pull-out capacity with a stated safety factor |
| Gutter and ridge connections | Local failure long before frame failure | Connection drawings, not just member sizes |
| Cladding retention | Film or polycarbonate peeling in a storm | Fixing spacing reduced at corners and eaves |
| Door and vent openings | Internal pressure spike | Wind-sensitive vent interlock and auto-close |
Corner zones and eaves carry pressure roughly 1.5–2 times the field of the roof. If glazing fixings are spaced uniformly across the whole roof, the perimeter is the part that goes first — and it takes the internal climate with it.
Step 4: Crop and Equipment Loads Are a Design Input You Own
The greenhouse supplier cannot guess your suspended weight. You have to hand it over, and you have to hand it over early enough that it appears in the frame calculation rather than in a site modification.
- Crop load. Leafy greens on NFT: 10–15 kg/m². Dutch bucket tomato on high wire: 25–40 kg/m² at full fruit load, plus the weight of the slab and the gutter water.
- Trellising. Wire and hook systems transfer point loads into the gutter or into a dedicated wire beam — check which one your structure was designed for.
- Top lighting. LED fixtures plus cable tray and busbars: 5–10 kg/m², and a maintenance walkway load if your design includes one.
- Irrigation mains. A full 110 mm PVC main carries roughly 12 kg per linear metre when charged. Route it on supports, not on the gutter.
- Tanks. A 20 m³ reservoir is 20 tonnes. It does not belong on the greenhouse floor slab unless that slab was designed for it.
Step 5: Pick the Foundation From the Soil Report, Not From the Last Project
| Foundation type | Suits | Watch out for |
|---|---|---|
| Independent pad footings | Firm ground, standard multi-span Venlo | Differential settlement; needs a continuous beam or tie at column head |
| Strip / continuous footing | Soft or variable ground, heavy crop loads | Higher concrete volume; drainage detailing at the perimeter |
| Driven or screw piles | Reclaimed land, high groundwater, fast-track builds | Corrosion allowance and pull-out testing on site |
| Raft / slab-on-grade | Container and indoor farms, tank farms, packing areas | Must be designed for point loads and for washdown drainage falls |
Two requirements are non-negotiable regardless of type: a geotechnical report with at least one borehole per 1,000 m² (and a bearing capacity figure), and a frost depth check. A footing founded above frost line in a continental winter will heave, and heave shows up first as gutters losing slope — which the crop reads as a drainage problem, not a foundation problem.
Step 6: Tolerances — The Clause Nobody Negotiates
- Column base tolerance of ±5 mm in plan and ±3 mm in level, verified before the frame is released
- Gutter slope tolerance for NFT: the as-built slope must stay within the design range over the full run; specify a survey, not a visual check
- Foundation curing time before frame erection; early loading is a common cause of cracked pedestals
- Hold-down testing: pull-out test on a percentage of anchors, documented in the handover pack
Ask for the CAD drawing set to include foundation general arrangement, anchor layout, and the reaction table. A supplier who can produce the reaction table can usually produce a defensible structure; one who cannot is asking you to accept risk you cannot see.
Six Mistakes We See on Site
- Accepting “80 km/h wind resistance” as a specification. Without a code, return period and terrain category, that figure is unverifiable.
- Adding screens and lighting after the frame is calculated. Retrofitting load capacity into finished steel costs more than specifying it up front.
- Hanging irrigation mains off the gutter. The gutter is a structural member and a hydraulic surface, not a pipe rack.
- Founding above frost depth in cold climates, then chasing slope problems for years.
- No soil report. Foundations sized on assumed bearing capacity are a gamble on the worst possible line item to get wrong.
- Ignoring the tank. Reservoirs and silos are frequently placed on slabs that were never designed for concentrated load.
FAQ
What wind speed should a commercial hydroponic greenhouse be designed for?
Specify a basic wind pressure tied to a recognised code and a return period (commonly 50 years), with terrain category and building height stated. A single km/h figure is not a design basis and cannot be checked by your engineer.
Do heated greenhouses need less snow load capacity?
Often yes, because a thermal coefficient credits the melting effect — but only if the heating system has redundancy. If a boiler failure during a snow event is possible, the unheated case governs and should be checked separately.
How much crop load should I declare for Dutch bucket tomato or cucumber?
Budget 25–40 kg/m² including fruit, foliage, slab and gutter water at peak load, and pass that figure to the frame designer before the structure is calculated. Leafy greens on NFT sit far lower at 10–15 kg/m².
Can I put the reservoir tank inside the greenhouse?
Yes, but the tank needs its own foundation and often its own thermal isolation. A 20 m³ tank is 20 tonnes and will not sit safely on a standard floor slab without a designed base.
What documents should I require at handover?
Design load summary with code references, foundation general arrangement drawing, anchor layout and specifications, pull-out test records, steel mill certificates, and an as-built gutter level survey.
Does the structure affect my construction schedule?
Directly. Foundation curing plus a survey hold point typically adds two to four weeks before frame erection. It is the cheapest insurance in the whole build and should be in the programme from day one.
Get the Load Case Confirmed Before Steel Is Cut
Send your site location, greenhouse dimensions, crop plan and any suspended equipment list through the quote form and our engineering team will return a load case summary, a foundation recommendation and the reaction table for your local code basis — including the suspended loads that hydroponic systems add and conventional greenhouse quotes leave out.