Greenhouse Snow Load Ratings: What the Number Actually Means

A snow load rating is the one specification on a greenhouse listing that decides whether the structure is still standing in March. It is also the number most often left out, most often misread, and most often compared against the wrong thing.

Part of the problem is that the figures sound generous. A rating of 15.4 pounds per square foot reads like a serious number, because fifteen pounds is heavy in the hand. Spread across a roof, it is not much snow at all. In most of the northern half of North America it is less snow than a single February storm delivers, and considerably less than the design load your local building code assumes for a garden shed.

This page covers what the rating measures, how to convert it into inches of real snow, how it compares against the ground snow load for your location, and why an unheated greenhouse has to carry more snow than a heated roof of exactly the same shape.

What a Snow Load Rating Measures

A snow load rating is expressed in pounds per square foot, usually written psf, and it states the maximum uniformly distributed weight the frame is claimed to support across its roof area. A greenhouse rated at 20 psf is stated to hold twenty pounds on every square foot of roof before the structure is at risk.

The word uniformly is doing a lot of quiet work in that sentence. Real snow does not arrive uniformly. It drifts against the high side of a roof, it slides and piles at the eave, and it loads one slope far more heavily than the other when the wind is steady from one direction. A frame carrying its rated load evenly across both slopes is in a much better position than the same frame carrying two thirds of that load concentrated on one.

The second thing worth knowing is where the number comes from. On hobby greenhouse kits it is almost always a manufacturer figure rather than an independently verified one. Very few kit makers publish test documentation, and almost none publish a stamped engineering drawing of the kind a custom greenhouse builder provides. The rating is a claim, and it usually arrives with conditions attached that appear in small type or not at all. Palram states its wind and snow figures on the basis that the separately sold Nature Series anchoring kit is installed. Riverstone states the MONT rating with the roof vents closed. Neither condition is obvious from a product listing photograph.

Why the Numbers Look Oddly Precise

The strangely specific ratings in this category are conversion artifacts. A rating of 15.4 psf is 75 kilograms per square metre. A rating of 20.5 psf is 100 kilograms per square metre. The precision is a unit conversion rather than a measurement, which is why the same greenhouse appears as 15 psf on one dealer page and 15.4 psf on another. Those are not two different models or two different production runs. They are the same underlying metric figure, rounded differently.

This is useful in two ways. It tells you the specification originated in a metric testing regime rather than an American structural one, which is a hint about how it relates to your local code figures. And it saves you from chasing a discrepancy that does not exist, which matters when you are trying to work out whether a dealer listing has been transcribed correctly.

What the Rating Means in Inches of Snow

Converting a psf rating into a depth requires knowing what the snow weighs, and snow weight varies more than almost anyone expects. Fresh, cold, dry powder runs roughly five to eight pounds per cubic foot. Snow that has sat for a few days and settled runs around twelve. Wet spring snow, or snow that has absorbed rain, reaches fifteen to twenty pounds per cubic foot and occasionally more.

That range means a single rating translates into wildly different depths depending on the storm.

Rating	Dry powder (6 lb/ft³)	Settled snow (12 lb/ft³)	Wet snow (20 lb/ft³)
15 psf	30 in	15 in	9 in
20.5 psf	41 in	20 in	12 in
24 psf	48 in	24 in	14 in
25 psf	50 in	25 in	15 in
32 psf	64 in	32 in	19 in

The right-hand column is the one to plan around. A structure rated at 15 psf reaches its stated limit under nine inches of wet snow, and nine inches of wet snow is an unremarkable event across most of the continent. The left-hand column is the one manufacturers are implicitly selling, and it only applies to cold, dry, early-season snow that has not been rained on and has not begun to settle.

What Your Location Actually Demands

In the United States, the reference figure is the ground snow load, written as pg, published in ASCE 7 and adopted through the International Residential Code. It is the design weight of snow on flat, open ground at your site, based on a fifty-year recurrence interval. Values run from zero in southern Florida through contour bands of roughly 10, 12, 14, 16, 19, 23, 27, 32, 38, 44, 52, 62, 73, 86, 101, 119 and 140 psf, with mountain regions requiring site-specific study. Canada uses ground snow load data published in the National Building Code, and the spread there is comparably wide, from the high teens on the coast of British Columbia to figures several times that in interior and northern regions.

Ground snow load is not the same as the load on a roof. The standard conversion for a flat roof is 0.7 multiplied by an exposure factor, a thermal factor and an importance factor. The 0.7 exists because wind, sunlight and heat escaping through the roof all mean a roof holds less snow than the open ground beside it.

The Unheated Greenhouse Penalty

This is where greenhouses diverge from every other structure in the backyard, and it is the part almost no product page mentions.

The thermal factor in that conversion rewards buildings that leak heat upward through the roof, because escaping heat melts the contact layer and helps the snow pack slide or settle. A heated house takes a thermal factor of 1.0. A structure kept below freezing, which describes an unheated greenhouse in January, takes 1.2. Running the numbers with typical residential assumptions, a heated roof sees roughly seven tenths of the ground snow load, while an unheated greenhouse sees something closer to eight tenths of it.

In practice that means the useful rule is to take your ground snow load and expect the greenhouse roof to face somewhere between two thirds and five sixths of it, then compare that against the kit rating rather than comparing the kit rating to the ground figure directly.

Work an example. A site with a 40 psf ground snow load, which covers a large stretch of Ontario, upstate New York and interior New England, produces a demand somewhere in the region of 27 to 34 psf on an unheated greenhouse roof. The strongest widely sold hobby kits publish 24 to 25 psf. The mid-tier publishes 15. Which is to say that across a meaningful share of the continent, no mass-market hobby greenhouse kit is rated for the winter it will be asked to survive.

That is not a reason to give up on a greenhouse in a snowy climate. It is a reason to plan for intervention, budget for a stronger structure, or accept the greenhouse as a three-season building and stop pretending otherwise.

Published Ratings Across the Market

The following are manufacturer-published figures for widely sold models. Specifications change, and dealer listings frequently lag the manufacturer, so treat these as a map of the tiers rather than a live specification.

Model	Snow load	Wind	Glazing
Palram Mythos	15.4 psf	56 mph	4 mm twin-wall
Palram Hybrid	15 psf	56 mph	4 mm twin-wall roof
Palram Glory	20.5 psf	56 mph	10 mm twin-wall
Riverstone MONT	24 psf	65 mph	8 mm twin-wall
Grandio Ascent	25 psf	76 mph	8 mm or 10 mm twin-wall
Custom-engineered glass	32 psf standard, 100+ psf on upgrade	85 to 140 mph	Tempered glass
Budget imports under $600	Not published	Not published or Beaufort	Film, or 4 mm panel

The pattern in that table is the useful part. Snow load tracks glazing thickness and frame mass closely, and the jump from the 4 mm tier to the 8 mm tier is where the rating roughly doubles. It also shows that the entire hobby kit market occupies a narrow band between 15 and 25 psf, and that stepping outside that band means leaving kits behind for engineered structures at several times the price.

When No Rating Is Published

The bottom of the market publishes nothing at all, and the absence is itself a specification. A manufacturer that has tested a structure and found a defensible number prints it, because it is a selling point. A manufacturer that prints nothing has either not tested or does not like the result.

Watch for the substitutes. Some budget listings advertise a wind rating expressed as a level rather than a speed, along the lines of a claim to be windproof to level four or five. That is the Beaufort scale, where level five is a fresh breeze of roughly 19 to 24 miles per hour, which is to say a windy afternoon. It reads like a rating and it is not one. Others describe a frame as heavy duty or reinforced without attaching a figure to either word.

Roof Geometry Matters as Much as the Rating

Two greenhouses rated identically can behave very differently, because the rating describes capacity and the roof shape decides how much load ever accumulates.

A steep pitch sheds. Snow slides off before it can build, particularly on smooth polycarbonate, and particularly if there is any warmth inside. A gothic arch or high-radius curve sheds better still, because there is no flat purchase anywhere on the surface, which is why that geometry dominates in serious snow country. A shallow pitch holds everything that lands on it. A lean-to attached to a house wall has the additional problem of drift, since snow sliding off the house roof lands on the greenhouse and concentrates there, which can put several times the ambient load on one strip of the structure.

Orientation matters for the same reason. A greenhouse with its ridge running across the prevailing wind loads unevenly, and the sheltered slope collects what the exposed slope loses.

How These Structures Actually Fail

Failure is rarely the dramatic total collapse that product warnings imply. On budget kits the glazing goes first, because thin panels held in channels deflect under load and pop out of the frame, at which point the structure has lost most of its bracing and the frame follows. On mid-tier aluminium kits the roof bars deflect and stay deflected, which is a permanent loss of capacity even after the snow melts, and the greenhouse enters the following winter weaker than it left the last one.

Ice is the case that catches people who have done everything else right. A gutter that fills and freezes puts a concentrated load along the eave, and ice at roughly 57 pounds per cubic foot is close to three times the weight of the heaviest wet snow. A greenhouse comfortably inside its rating on snow depth can be well outside it once the melt and refreeze cycle has run a few times.

Managing Load Instead of Buying Capacity

The practical approach in a marginal climate is to treat the rating as a threshold to stay below rather than a limit to run up against. Clear the roof when accumulation reaches somewhere around half to three quarters of the rated depth for the snow type you are looking at, and clear both slopes evenly rather than doing one side and leaving the other, since an unbalanced roof is a worse position than a uniformly loaded one.

Interior support posts under the ridge are the cheapest capacity upgrade available and are worth installing before the first storm rather than during it. Keeping a little heat in the structure helps more than the fuel cost suggests, because it melts the contact layer and lets the pack slide. And anchoring properly matters here too, since a frame that is not fixed to a solid base is already working under a load it was not rated to take.

How to Use the Number When You Buy

Start with your ground snow load rather than with a product page. Look it up for your specific address, take roughly two thirds to five sixths of it as the demand on an unheated roof, and use that figure as the floor when you shop. Then read the rating with its conditions attached, checking whether it assumes an anchoring kit you have not budgeted for and whether it assumes vents closed.

If the strongest kit you can find still falls short of your local demand, that is real information rather than a reason to buy the strongest kit anyway and hope. It means the honest options are a stronger engineered structure, a commitment to clearing the roof through the winter, or a greenhouse used from spring through autumn and left empty in January. All three are reasonable. Buying a 15 psf structure for a 40 psf site and finding out in February is not.