Sourcing Modular Greenhouses and Sheds From China: Steel Wall Thickness, Film Grade and the Wind Load Nobody Calculates

Kit greenhouses and modular garden sheds are one of the most commoditised categories on the market, which means the factories compete almost entirely on price and cut exactly where you cannot see it: the steel. I have watched a buyer order a container of what was described as a heavy-duty polytunnel, only to find the frame was made from tube so thin that a modest storm folded the whole structure. The photos on the listing looked identical to the good version.

This is a category where you cannot trust the render, and you cannot trust the word "heavy-duty." You can, however, force the numbers onto paper and then verify them with a micrometer and a scale before the goods ship.

Steel wall thickness and tube profile

The frame is the product. Everything else is consumable. Chinese suppliers describe greenhouse and shed frames by tube diameter and a nominal wall thickness, but nominal and actual are frequently different worlds. A tube sold as 1.0 mm wall is often rolled from slightly thinner strip stock, and cheap frames use thicknesses that would surprise you.

Two things to check:

Actual wall thickness measured with a proper gauge at several points on the tube, not at the end where the factory may have finished it nicely. Take your own calipers to the sample.
Coating weight. Pre-galvanised tube and hot-dip galvanised tube behave very differently in a wet climate. Ask for the zinc coating weight in grams per square metre, not just the word "galvanised." Thin pre-galvanised tube will rust at the cut ends and the connector joints within a couple of seasons.

Ask the factory to state the grade of steel as well. Mild steel is the norm; some suppliers push thinner high-strength tube as a weight-saving measure, which can work if the engineering supports it, but you need to know which you are buying, because the two do not behave the same under bending loads.

Film and covering grade

For polytunnels and film greenhouses, the covering is a genuine performance item. The cheap film degrades and goes brittle within a single season of UV exposure. The good film lasts several years and keeps its light transmission, which is the whole reason you bought a greenhouse in the first place.

What to ask for:

Thickness in microns. A heavier film resists tearing and lasts longer, but it also costs more and weighs more for freight.
UV stabiliser content and expected lifespan. Get the supplier's stated years and be sceptical of anything promising a decade from a standard film.
Light transmission percentage and whether it is a diffused film. Diffused film spreads light more evenly, which reduces leaf scorch and helps in strong-sun markets.
Layers and structure. Multi-layer co-extruded films generally outperform single-layer films for the same thickness, because the outer layers carry the UV package.

For sheds with polycarbonate or acrylic glazing, the equivalent numbers are the sheet thickness and the UV-protected surface. Cheap polycarbonate without a proper UV coating yellows and cracks, and that is a warranty claim.

Wind and snow load — the calculation nobody does

This is the single biggest cause of flattened structures and angry customers. A greenhouse that stands happily in a sheltered valley will be destroyed on an exposed coastal plot, and the difference is wind load. The supplier's promotional material rarely states the design wind speed the frame can survive.

Before you buy, work out the realistic exposure for your target market. Coastal areas, hilltop sites and regions with named storm seasons need far more bracing and heavier steel than the factory's base model assumes. Then demand the frame's design wind speed, the base anchoring method, and the number and position of the diagonal braces. Many cheap kits rely on the covering itself for stiffness, which is exactly why they fail — the film stretches or tears and the frame has nothing left to hold it square.

Snow load is the mirror problem for northern markets. A flat-roofed shed or a wide-span greenhouse will collect snow, and thin steel will simply fold under the weight. Ask for the design snow load and the roof pitch. A steeper pitch sheds snow far better, and it is worth specifying for snowy markets even at a small cost premium.

Foundation, anchoring and assembly reality

None of the steel matters if the structure is not anchored. Ask what base fixing the kit includes and whether it suits your market's ground conditions — a kit designed for a poured concrete slab will not work for a customer who wants to bolt it to a timber deck or drive ground anchors into soil. Mismatches here generate a flood of customer-support requests and returns.

Also, get the assembly instructions translated properly into your market's language. A kit that is beautifully made but ships with a poorly translated one-page diagram will generate returns and bad reviews regardless of the steel quality.

Pre-shipment inspection checklist

Actual tube wall thickness measured with calipers at multiple points.
Zinc coating weight in g/m², and quality of the galvanising at cut ends and joints.
Film thickness in microns, UV stabiliser claim, expected lifespan and light transmission.
Design wind speed and number/position of diagonal braces.
Design snow load and roof pitch for cold markets.
Base anchoring hardware included, and ground-condition compatibility.
Market-language assembly documentation, checked by a native speaker.

Put the measured values into the inspection report, not just a pass/fail. A frame that passes at 0.9 mm when you ordered 1.0 mm is a negotiation, not a pass. And confirm the supplier's actual specifications and certifications directly against their own website before you commit to volume — the renders all look the same online.