A greenhouse has to resist two quite different loads, and the design features that help with one often hurt the other. Wind pushes and lifts, and it is worst on a tall structure with a large flat side. Snow presses straight down, and it is worst on a shallow roof with a long unsupported span. A structure optimised purely for one is compromised for the other, which is why greenhouse ratings are always quoted as a pair.
The important practical point is that both ratings assume the structure is assembled exactly as designed, every fixing is in place, and the house is anchored. A rating is a statement about a correctly built structure, not a promise about the one in your garden.
What snow actually weighs
Snow density varies by nearly a factor of three depending on its water content, which is why the same depth of snow can be harmless one week and dangerous the next.
| Snow type | Density | 6 in on the roof | 12 in on the roof | 18 in on the roof |
|---|---|---|---|---|
| Fresh dry powder | About 7 lb per cu ft | 3.5 lb per sq ft | 7 lb per sq ft | 10.5 lb per sq ft |
| Settled dry snow | About 12 lb per cu ft | 6 lb per sq ft | 12 lb per sq ft | 18 lb per sq ft |
| Wet heavy snow | About 20 lb per cu ft | 10 lb per sq ft | 20 lb per sq ft | 30 lb per sq ft |
| Old compacted snow with ice | About 30 lb per cu ft | 15 lb per sq ft | 30 lb per sq ft | 45 lb per sq ft |
Set that against typical hobby ratings of 15 to 30 pounds per square foot and the picture is clear. A foot of powder is a non-event. A foot of wet snow is at or over the rating of a light kit. And snow that partly melts and refreezes gains density without gaining depth, which is the situation that catches people out because the roof does not look any worse than it did the day before.
Pitch, span and why kit roofs collapse
Two geometric factors decide whether snow stays on a roof. The first is pitch. Below roughly 20 degrees, snow simply sits. Above roughly 30 degrees on a smooth surface, most of it slides off on its own, and polycarbonate and glass are both slippery enough for this to work once the surface warms slightly. Most hobby gable greenhouses sit somewhere in between, which is why they hold snow.
The second is span. A roof panel supported only at its two ends deflects in the middle under load, and deflection grows very rapidly with span. This is why a wide house is disproportionately more vulnerable than a narrow one at the same rating, and why adding a single ridge purlin or a temporary internal prop under the ridge is such an effective emergency measure.
A curved tunnel roof behaves differently again. The curve sheds well near the top and holds snow near the eaves where the surface is shallow, so a tunnel tends to accumulate a wedge of snow along both sides. On a light tunnel this can push the hoops out of shape permanently even without a collapse.
The measures that actually keep a house standing
- Clear the roof rather than trusting the rating. Every inch removed before it consolidates is worth several after. A soft-headed roof rake used from the ground, working eave to ridge in strips, is the whole technique.
- Heat the house, even slightly, during a heavy fall. A greenhouse held a few degrees above freezing melts the contact layer and lets snow slide, which is why heated houses survive falls that flatten unheated ones. This is the single most effective measure available and it costs one night of heater running.
- Prop the ridge. A single timber post from the ground to the underside of the ridge, cut to a firm fit, transforms the load path on a small house. Two props on a longer one. Keep a pair cut to length in the greenhouse through winter.
- Take the film off a tunnel for winter if you are not using it. A bare hoop frame carries no snow at all, and re-skinning in spring is cheaper than replacing a folded structure.
- Keep the eave line clear. Snow sliding off a roof piles against the side of the house and then acts as a buttress that stops the next lot sliding. Clearing the base is as important as clearing the roof.
- Check the pitch you are buying. Where snow is a routine problem, a steeper gable or a house explicitly rated for high snow load is worth more than any other specification, and it is not something that can be retrofitted.
Reading a manufacturer rating honestly
Ratings are not comparable between makers unless you read the conditions. Some quote a distributed load on the roof only, some quote a figure for the whole structure, some assume the house is fixed to a concrete base and some assume a timber kerb. A structure such as the Palram Canopia Hybrid 6 x 10 Greenhouse ($713.99) from an established maker publishes its assumptions and its parts list, which is worth real money at the moment you need a replacement panel or a rating you can rely on. Unbranded kits frequently publish neither.
Where the published rating is marginal for your area, the honest options are a smaller house, a steeper roof, a house with more frame in it, or the discipline of clearing the roof after every fall. Those are better answers than hoping. The published ratings for common structure types are collected in the snow and wind load rating chart.
Winter structural insurance
Structural load is the half of winter that threatens the building. The other half, holding a temperature the crop can live at, is covered in heating a greenhouse in winter and insulating a greenhouse.