Greenhouse & Polycarbonate Roof Snow Load Calculator
A continuously heated greenhouse melts snow from below, so ASCE 7 allows a reduced thermal factor Ct = 0.85, but only if heat is maintained. Unheated hoop houses and cold frames do not get the reduction.
The worksheet
ASCE 7-22At 25° the §7.3.4 minimum does not apply, so the balanced load of 12.36 psf governs.
≈ 18,537 lb total on 1,500 sq ft of roof plan
Large gable roof (W > 20 ft): windward side at 0.3·Ps, leeward side at Ps plus a drift surcharge from the ridge.
surcharge 22.74 psf over 7.26 ft · hd 1.86 ft · γ 17.9 pcf
Flat-roof load Pf = 0.7 × Ce(1) × Ct(0.85) × Is(1) × Pg(30) = 17.85 psf. Sloped-roof balanced load Ps = Cs(0.69) × Pf = 12.36 psf.
| Ground snow load, Pg | 30 psf | site input |
| Exposure factor, Ce | 1 | Table 7.3-1 |
| Thermal factor, Ct | 0.85 | Table 7.3-2 |
| Importance factor, Is | 1 | Table 1.5-2 |
| Slope factor, Cs | 0.69 | Fig. 7.4-1 |
| Snow density, γ | 17.9 pcf | Eq. 7.7-1 |
| Settled depth at design load | 8 in | design ÷ density |
- 01Estimated snow density ≈ 17.9 pcf, so 12.36 psf is roughly 8 in of settled snow.
- 02The §7.6.1 unbalanced (leeward-drift) case is computed above for this gable/hip roof; at roof steps, parapets and walls also check drift loads (§7.7) with the drift calculator, plus sliding snow (§7.9) onto anything below.
- 03Ground snow load Pg is set by your local building department / the ASCE 7 Hazard Tool. Always confirm the value for your exact site before you build or submit.
Greenhouses rated for snow
Your design roof snow load is 12.36 psf. Polycarbonate kits state a snow-load rating. A continuously heated greenhouse gets the Ct = 0.85 reduction; an unheated hoop house does not, so it needs a higher rating.
Palram / Canopia and similar rigid-panel greenhouses, with a stated roof snow-load rating.
Shop greenhouse kitsInternal post and bow-bracing kits that raise a hoop house or kit greenhouse's snow capacity.
Shop bracing kitsSome links are affiliate links: if you buy through them we may earn a commission at no extra cost to you. These are starting points, not engineering endorsements. Always confirm a product's rated snow load meets the design load above.
Notes for greenhouse / polycarbonate roofs
- Ct = 0.85 only applies to a greenhouse kept continuously warm with low-R glazing. If the heat can fail, design it as a normal heated (Ct = 1.0) structure.
- Polycarbonate and glass are slippery surfaces, so the slope factor sheds snow sooner: a steeper greenhouse roof carries much less.
Notes & questions
01How much snow load can a greenhouse take?+
Compare the kit's rated load to the ASCE 7 design load from this calculator. A continuously heated greenhouse uses Ct = 0.85; an unheated one does not, so it carries more snow.
02Do I get the heated-greenhouse snow reduction?+
Only if the greenhouse is continuously heated with low-R glazing. If heating can be interrupted, ASCE 7 says to design it as an ordinary heated building (Ct = 1.0) instead of 0.85.