A winter garden in Moscow is not Sochi: snow zone IV loads and frame design

Snow zones of Russia: where Moscow sits

Under SP 20.13330.2017 “Loads and Actions” (Russian building code), the territory of Russia is divided into 8 snow zones. Moscow and most districts of the Moscow Region fall into snow zone IV, with a characteristic snow load on a horizontal surface of S₀ = 240 kg/m² (2.4 kPa).

For comparison: Sochi is zone II (80 kg/m²), Krasnodar is zone III (120 kg/m²), St. Petersburg is zone III (120 kg/m²), and Vorkuta is zone VIII (560 kg/m²). Moscow is not a record-holder, but the load is three times higher than in Sochi.

Design snow load: formula and coefficients

The design load on the roof is determined by the formula from SP 20.13330.2017:

S = S₀ × µ × Ce × Ct

Where: µ is the roof shape coefficient (depending on the slope angle), Ce is the wind exposure coefficient for snow drift (0.85–1.0), and Ct is the thermal coefficient (for heated buildings, 0.8–1.0).

• Horizontal roof (µ = 1.0): S = 240 × 1.0 × 1.0 × 1.0 = 240 kg/m²
• Pitched roof at 30° (µ = 0.5): S = 240 × 0.5 = 120 kg/m²
• Pitched roof at 60° (µ = 0): snow does not accumulate

The practical takeaway: a pitched winter garden roof with an angle greater than 45° cuts the design load in half. At angles above 60°, snow slides off on its own and snow load is not considered. A steep roof is not only an aesthetic choice but also an engineering solution for reducing loads.

Wind load in Moscow

Moscow is located in wind zone III, with a characteristic wind pressure of W₀ = 0.38 kPa. For a glass facade, wind is the key horizontal load. Wind load calculation for a facade (building up to 10 m tall, terrain type B):

• Characteristic wind load: 0.38 × 0.65 = 0.247 kPa (25 kg/m²)
• Design value (with a safety factor of 1.4): 0.35 kPa (35 kg/m²)

For a glass facade 3 m tall with a 1.5 m span: the load per span is 0.35 × 3 × 1.5 = 1.58 kN. This is precisely the value that determines the minimum glass thickness and the spacing of the profile mullions.

Winter garden frame design: implications for cross-sections

For a typical glass roof with a 3.0 m span under a load of 240 kg/m², the total load on a beam will be:

• Snow: 240 × 3.0 = 720 kg/lin. m × 1.5 m (rafter spacing) = 1,080 kg
• Self-weight of the glass (44.2 + IGU): ~50 kg/m² × 3 × 1.5 = 225 kg
• Total: ~1,300 kg on a 3 m rafter beam

For such a load with an aluminum profile, you will need either a dedicated structural beam with a cross-section of at least 100×100 mm and a wall thickness of 5 mm, or a powder-coated steel rectangular tube. A standard 50 mm facade profile is not sufficient here — a frequent mistake when copying “European projects.”

How snow load affects the budget

Properly accounting for snow zone IV loads increases the cost of a winter garden roof structure by 25–40% compared with a “southern” solution:

• Reinforced profile (structural beams): +15–20% to the profile cost
• 55.2 laminated roof glass instead of 33.2: +8–12% to the glass cost
• Additional anchor points and perimeter beams: +5–8% to the installation cost

A project “saved” by understating loads may not survive its first Moscow winter. The fix is dismantling and replacing the entire roof structure.

Practical recommendations

• Design a roof slope of at least 20° — this reduces µ to 0.7–0.8 and helps snow slide off
• Use structural roof beams with a design deflection of no more than L/250
• Roof glass: 55.2 laminated glass for spans over 2 m, 44.2 laminated glass for spans under 2 m
• Provide a heating system for the roof drainage (heat-trace cable, 20–30 W/lin. m)
• During design, obtain calculations from a structural engineer certified to SP 20.13330