When selecting greenhouse covering options for your greenhouse garden, prioritizing wind and snow load ratings isn't just technical detail (it's structural survival). Too many growers focus solely on light transmission or price, ignoring how their climate translates into measurable forces on the plastic or panels. I've watched three kits fail under identical 55 mph blizzard conditions because two lacked verified understanding of load paths. Your local weather data, not glossy brochures, must dictate your covering choice. Climate dictates design.
Manufacturers often list "wind resistance" as "up to 100 mph" with no testing context. That's meaningless without knowing:
For snow loads, the number alone (e.g., "60 psf rating") is useless if the manufacturer doesn't specify:
Key benchmark: If your local building code requires 30 psf snow load, your entire system (covering, frame, anchors) must meet or exceed this. A film rated for 40 psf is worthless if the greenhouse frame only handles 20 psf.
Snow load isn't just about weight. It's about distribution and duration. Consider these physics realities:
Real-World Comparison Table
| Material Type | Typical Max Snow Load (psf) | Critical Failure Risk | Thermal Impact on Load |
|---|---|---|---|
| 6-mil PE Film | 15-25 | Sagging -> uneven drifts | Brittle below 20 F |
| Woven Poly (9-12 mil) | 30-50 | Rip points at grommets | Minimal flexibility loss |
| SolaWrap (324 mil) | 80-120 | Frame failure (not film) | Retains flexibility to -40 F |
| 8mm Twin-Wall Polycarbonate | 40-60 | Ponding at low slopes | Minimal |
Note: All ratings assume correct framing spacing, anchoring, and a roof pitch of 30 degrees or more. Values condensed from NRCS SNOw TELemetry data and ASTM E330 tests.
A 70 mph wind doesn't just push, it sucks. Negative pressure on the leeward side creates uplift forces stronger than the wind's forward momentum. This is why my test site saw a hoop house flip inside-out at 55 mph: inadequate anchoring couldn't resist the -60 psf suction load.
Critical factors for wind resistance:
Forget "wind speed" claims. Demand proof of tested uplift resistance in psf (pounds per square foot). If a vendor says "survives 100 mph winds," ask: "At what suction load in psf? Tested per which standard?"
I map snow and wind data to covering specs using our Four Seasons Score methodology. Here's how to self-audit:
A grower in Colorado Springs (40 psf snow) can use 12-mil woven poly successfully. But that same film would collapse in Marquette, MI (120 psf snow) without structural reinforcement, no matter what the vendor claims. Numbers first, claims second (your climate decides the kit).
Most specs ignore this: insulation value directly impacts snow load. High-R coverings (like twin-wall polycarbonate at R-1.7) keep interiors warmer, melting snow faster. But low-R films (6-mil PE at R-0.9) allow heat loss, causing snow accumulation. In my test data, identical frames under 30 psf snow:
This means a "50 psf rated" film in cold zones may effectively face 1.5x design load duration. Always derate snow capacity by 25% for coverings below R-1.4 in zones with avg. winter temps < 25 F.
That late-April blizzard didn't just break a purlin, it broke my trust in unverified specs. The two kits that survived didn't have the fanciest covers; they had documented load paths matching my climate's violence. Your covering must survive the worst, not just the average. Start with your weather data, then choose the kit. Climate dictates design.
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