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Greenhouse snow and wind load ratings

The short answer

A hobby greenhouse rated to 20 pounds per square foot of snow load will carry roughly 20 to 24 inches of light dry snow or only 5 to 7 inches of wet snow, because wet snow weighs about three times as much per inch. Wind ratings are worth even less than snow ratings because they assume the structure is correctly anchored, and the pegs supplied in the box almost never achieve that.

Typical hobby rating
15 to 25 psf
Light dry snow
about 0.9 psf per inch
Wet snow
about 3 psf per inch
What voids a rating
Poor anchoring

Researched from published specifications and verified owner reviews · Updated 2026-08-16

Snow load is quoted in pounds per square foot of roof area, and it is the most misunderstood number on a greenhouse specification sheet, because snow depth and snow weight are only loosely related. Fresh light powder weighs roughly 0.9 pounds per square foot per inch of depth. Settled snow weighs about 1.5. Wet snow or partly thawed snow weighs about 3, and ice weighs about 5.

That range is why the same 24 inches of snow either sits harmlessly on a roof or collapses it. A structure rated to 20 pounds per square foot carries about 22 inches of powder, 13 inches of settled snow, or 7 inches of wet snow. The dangerous condition is not a blizzard, it is 6 inches of snow followed by rain.

Figure 1. Snow depth a rating actually supports · 7 rows
Figure 1. Snow depth a rating actually supports
RatingLight dry powderSettled snowWet snowSnow then rain
10 psf 11 in 7 in 3 in Under 3 in
15 psf 17 in 10 in 5 in 4 in
20 psf 22 in 13 in 7 in 5 in
25 psf 28 in 17 in 8 in 6 in
30 psf 33 in 20 in 10 in 7 in
40 psf 44 in 27 in 13 in 10 in
50 psf 56 in 33 in 17 in 12 in
Assumes 0.9 psf per inch for light dry powder, 1.5 for settled snow, 3.0 for wet snow and 4.0 for snow saturated by rain. These are commonly published densities and real snow varies. Ratings assume uniform loading; drifting concentrates load on one side and can exceed the rating locally at half the average depth.

What a load rating does not promise

Every published rating carries assumptions, and hobby greenhouse ratings carry more than most. Four of them matter.

  1. It assumes the structure is fully and correctly assembled. Every bolt tightened, every brace fitted, every panel in its channel. A frame missing its diagonal braces because they seemed optional during assembly has a fraction of its rated capacity.
  2. It assumes correct anchoring to a rigid base. A greenhouse resting on soft ground with the supplied pegs is not anchored, and a frame that can move cannot develop its rated strength.
  3. It assumes uniform loading. Snow drifts. A lee slope beside a fence or a house wall can carry twice the average depth, so a roof at half its rated average can be locally overloaded.
  4. It does not usually include the two loads acting together. Wind pressure on a snow-laden roof is the combination that actually fails structures, and hobby ratings are generally quoted separately for each.

Watch out

Clear snow from the outside, and never from inside

Pushing up on a loaded polycarbonate panel from inside concentrates the entire local load onto the point you are pushing, and the panel or its clip fails there. Clear from outside with a soft broom or a roof rake, working from the eave upward, and stop if the panels are flexing. If the house is not safe to reach, it is safer to lose the greenhouse than to be inside it.

Figure 2. Typical ratings by structure type · 8 rows
Figure 2. Typical ratings by structure type
StructureTypical snow ratingTypical wind ratingPractical weak point
Fabric-covered low tunnel Not rated, collapses under any accumulation 20 to 30 mph The cover tears at the pin points before the hoops fail.
Fiberglass hoop tunnel over a bed Not rated 25 to 35 mph Hoops flatten and stay flattened. Treat as a frost cover only.
Steel-frame film tunnel, walk-in 5 to 15 psf, if braced 35 to 50 mph The film cover at the door frame, and the anchoring.
Budget polycarbonate kit, light frame 10 to 20 psf 35 to 45 mph Panels walking out of their channels once the frame racks.
Quality polycarbonate kit, heavier frame 20 to 30 psf 50 to 65 mph Door hardware, then the anchoring.
Timber-framed greenhouse 25 to 40 psf 55 to 70 mph Rot at the sill plate where it meets the ground.
Aluminum-framed glass greenhouse 15 to 25 psf 50 to 65 mph Glass panes lifting out of their clips in uplift.
Cold frame with polycarbonate lid Effectively unlimited at that span Depends entirely on ballast The lid blowing open and tearing its hinges off.
Ranges span the products in each class rather than describing any single model. Always read the specific manufacturer figure, and treat the absence of a stated rating as meaningful information: a maker with a rating worth quoting quotes it.

Roof pitch does more for snow than the rating does

A steeper roof sheds snow rather than accumulating it, and the effect is large. Below about a 20 degree pitch, snow sits. Between 25 and 30 degrees, it sheds partially once the surface warms. Above roughly 35 degrees, it sheds readily. Most hobby greenhouse roofs sit between 25 and 35 degrees, which is why they cope with more snow than their rating suggests in a climate where daytime temperatures rise above freezing, and cope with much less where they do not.

The material matters too. Snow slides off glass and off smooth polycarbonate far more readily than off textured or dirty surfaces, and a roof that has been shaded by a tree all winter holds snow that a sunlit roof would have shed. This is one more reason siting matters, covered in siting and orientation.

Wind: uplift is the failure, not pressure

A greenhouse rarely fails by being pushed over. It fails by being lifted. Wind flowing over a curved or pitched roof generates negative pressure above it, exactly as it does over an aircraft wing, and the resulting uplift acts on the whole roof area at once. On a 6 by 8 house that is roughly 60 square feet of roof, and at 50 miles per hour the uplift can exceed several hundred pounds.

Which means anchoring is the whole game, and it must resist upward force rather than sideways force. Screw-in earth anchors such as Ciyivak 18 in Screw-In Ground Anchor Kit with Straps work because they develop resistance against the soil above the helix. Pegs pushed into soft ground pull straight out. The other essential is to keep the door shut in wind: an open door pressurizes the interior and adds internal pressure to external uplift, which is why so many failures happen on the day someone left it propped open.

Standing up to load

Anchoring against uplift, door hardware that latches properly, spare glazing for when something does break, and the staples that keep a fabric cover pinned.

The full anchoring method is in anchoring a greenhouse against wind, the load detail in snow and wind load, and the frame quality question in best polycarbonate greenhouses.

Related on this site

Common questions

6 answers

+ How much snow can a greenhouse hold?

A structure rated to 20 pounds per square foot carries roughly 22 inches of light dry powder, 13 inches of settled snow, or only 7 inches of wet snow, because wet snow weighs about three times as much per inch. The most dangerous condition is a few inches of snow followed by rain, which can exceed a 20 psf rating at 5 inches of depth.

+ What snow load rating do I need for a greenhouse?

Match it to your local design ground snow load, which your building department publishes, and add margin for drifting. In much of the southern United States 10 to 15 psf is adequate. In the northern tier and at elevation, 30 to 50 psf is realistic. A hobby kit rated 20 psf in a 40 psf climate will need clearing after every significant fall, which is a maintenance commitment rather than a one-time purchase.

+ Should I clear snow off my greenhouse?

Yes, from the outside, with a soft broom or roof rake, working upward from the eave. Never push up from inside: that concentrates the whole local load onto the point you are pushing and fails the panel or its clip there. If the fall is heavy and continuing, clearing part way through is safer than waiting, because a partly cleared roof is stronger than a fully loaded one.

+ What wind speed can a hobby greenhouse take?

Quality polycarbonate kits are commonly rated 50 to 65 miles per hour and budget kits 35 to 45, but every one of those figures assumes correct assembly and correct anchoring to a rigid base. The supplied pegs do not achieve that on soft ground. With proper screw-in earth anchors most kits meet their rating; without them, failures occur well below it.

+ Why do greenhouses blow away rather than blow over?

Because the dominant wind force on a pitched or curved roof is uplift, not sideways pressure. Air flowing over the roof creates negative pressure above it, and that acts on the entire roof area at once. On a 6 by 8 house with 60 square feet of roof, uplift at 50 miles per hour can exceed several hundred pounds. Anchoring must therefore resist upward force specifically.

+ Does roof pitch affect snow load capacity?

It affects whether the load ever accumulates, which matters more than capacity. Snow sits on a roof below about 20 degrees of pitch, sheds partially between 25 and 30 degrees once the surface warms, and sheds readily above about 35. A steep, smooth, sunlit roof in a climate that thaws during the day may never test its rating; a shallow, shaded one in a continuously freezing climate will.

Working out the figures for your own house and season? The Greenhouse Build & Growing Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.