In mid-winter check every building monthly for snow load, ice dams, condensation, blocked vents and blocked exits, and judge the snow by its weight and not by depth alone: 60 cm of fresh snow loads a roof with 0.47-1.41 kPa, but 60 cm of wet spring snow loads it with 2.35-3.77 kPa, above the 1.92 kPa that tops the moderate-snow design range. Weigh a bucketful of snow, work out the load, and clear the roof from the ground before the load reaches the roof’s design figure.
Part of Building Maintenance Schedules
Mid-winter emergency checks are the quick monthly inspections, and the extra checks after storms, that catch overloaded roofs, ice damage, condensation and blocked vents and exits while the weather prevents repair.
The Guide’s 60 cm Depth Rule Is Too Late For Wet Snow
The maintenance guide asks for quick monthly winter checks: ice dams (ice at the eaves that forces meltwater under the roofing), snow cleared if accumulation exceeds 60 cm, especially on flat or low-slope roofs, condensation inside (a sign of ventilation trouble), ventilation openings not blocked by snow or ice, and fire exits working and clear. Of these, the 60 cm rule hides the most error, because a roof carries weight and snow of the same depth varies a great deal in weight.
Worked Example: What 60 Cm Of Snow Weighs
The live-loads guide (Live Loads) gives snow densities of 5-15 lb per cubic foot for fresh snow, 15-25 for old compacted snow and 25-40 for wet spring snow, and typical design loads on the horizontal plan of 10-20 lb per square foot (light zone), 20-40 (moderate) and 40-70 (heavy). Conversions: 1 lb/ft³ = 16.0 kg/m³; 1 lb/ft² = 47.9 Pa. The load is q = density × g × depth, with g = 9.81 m/s².
| Snow | Density | Load at 0.60 m | Depth that equals 1.92 kPa |
|---|---|---|---|
| Fresh, light | 80 kg/m³ | 80 × 9.81 × 0.60 = 471 Pa = 0.47 kPa | 1,920 ÷ (80 × 9.81) = 2.4 m |
| Fresh, dense or old, low | 240 kg/m³ | 1.41 kPa | 1,920 ÷ (240 × 9.81) = 0.82 m |
| Old, high | 400 kg/m³ | 2.35 kPa | 0.49 m |
| Wet spring | 640 kg/m³ | 3.77 kPa | 0.31 m |
The design loads in kPa: light 0.48-0.96; moderate 0.96-1.92; heavy 1.92-3.35. So 60 cm of old high-density snow (2.35 kPa) already exceeds the whole moderate range, and 60 cm of wet snow (3.77 kPa) exceeds even the top of the heavy range. For the same roof, the depth at which the top-of-moderate load is reached falls from 2.4 m of fresh snow to 0.31 m of wet snow.
Three things make this worse. A drift against a taller wall can load the roof beside it at 2 to 3 times the normal figure (live-loads guide). Rain falling on snow adds water: every millimetre of rain is 1 kg/m², or 9.8 Pa. And an earth roof already carries 8.8-11.8 kPa of soil at 0.6 m (Earth Load Calculations), to which wet snow adds up to 3.77 ÷ 8.83 = 43%. A steep roof sheds snow; the guide reduces the design load by half over 30 degrees, so the depth rule matters most on flat and low roofs.
The design loads are the loads a roof is meant to carry, with a margin above them whose size is not given here. Reaching the design figure leaves no allowance for a drift, for rain or for the weight of the person clearing.
What You Need
- A depth stick and a bucket of known volume, such as 10 L, with a spring balance or beam scale
- The roof’s design load, or the snow zone it was built for
- A roof rake or long pole, and a shovel
- A thermometer or sense of the weather, to catch thaw and rain on snow
- A log for the monthly checks
Method: Weigh, Compute, Decide
Step 1: Take a core. Push the bucket down through the snow, empty it into itself or fill it from the full depth, and weigh the contents. For a 10 L (0.010 m³) sample weighing 4 kg the density is 4 ÷ 0.010 = 400 kg/m³.
Step 2: Measure the depth on the roof, and on the worst drift.
Step 3: Compute the load: density × 9.81 × depth. At 400 kg/m³ and 0.49 m: 400 × 9.81 × 0.49 = 1,923 Pa, 1.92 kPa.
Step 4: Compare with the design load. If the figure is not known, use the lowest design figure for snow country as a cautious trigger, which is my judgement and not a corpus figure.
Step 5: Clear from the ground, evenly from both sides and from the eaves upward, with a rake.
Step 6: Check the openings: chimney caps and vents free of snow and ice, exits free of drifts, ice dams chipped from the eaves from below, condensation traced to its cause.
Common Mistakes
- Trusting depth. The same 60 cm is light powder or three times the weight in wet snow.
- Clearing one side only. An unbalanced load is a new load.
- Blocking an outward-opening door with a drift. A door that opens by pushing opens outward into the snow; keep a path clear each morning and plan a second exit on the lee side.
- Sealing every vent against the cold. Condensation and carbon monoxide build up.
Safety
Do not stand on the roof you are trying to save
A person adds weight to a roof that is already near its limit, and a rotted rafter under the snow will not hold. Clear from the ground. Watch for signs of overload: doors and windows sticking, creaking, new cracks, a sagging ceiling; if they appear, empty the building and stay out. Keep clear of the eaves, because the snow slides off in a sheet. Check that chimneys and vents are clear before lighting a stove (Carbon Monoxide Risks).
What’s Next
- Learn next: Leak Detection & Emergency Patching — what to do when winter finds a weakness
- Combine with: Roof Maintenance — preventing the problem in summer