Roof Snow Load Calculator: How Heavy Is the Snow Now?

The calculation uses current roof depth, not accumulated snowfall or an assumed ground-to-roof factor.

Density. Condition/age brackets combine published physical observations: fresh dry snow 50–100 kg/m³ (SLF); approximately 200 after days and 300 after about a month (NRC CBD-37, attached s7 p. 3); older sintered snow up to 400 (SLF); wet seasonal pack about 250–300 in January and 400–450 in April (Wyoming Climate Atlas §5.3). The displayed brackets are screening groupings, not universal limits. Their midpoint is the central estimate. Measured mode uses $\rho\pm\Delta\rho$ with an allowance chosen by the user.

Optional seasonal proxy. Sturm et al. (2010), Eq. 6 and Table 4 (s1 pp. 8–10):

The original units are g/cm³ for density and cm for vertical depth. $D$ runs from −92 on October 1 through −1 on December 31, then 1 on January 1 to 181 on June 30; it is not deposit age. Model parameters are scientific data, not design-load tables. A ±130 kg/m³ sensitivity span is based on the paper's test-data density-error spread (s1 p. 11), clipped to 1–1000 kg/m³. It is not a roof confidence interval. One representative density is also applied to the deeper patch; the model is not fitted again to a local drift.

Mass and plan load. Perpendicular snow thickness is converted to vertical depth with $h_v=h_n/\cos\theta$. For each density scenario:

Ice thickness is vertical and added-water thickness is plan-equivalent. Constants: $g=9.80665$ m/s², $\rho_w=1000$ kg/m³ and $\rho_i=917$ kg/m³. USACE EM 1110-2-1612 §2-2 gives an engineering ice-density approximation of 915–917 kg/m³.

Deeper accumulation. $h_{peak}=h_v+\Delta h_v$. The extra depth is a uniform block over its entered plan area; it is measured accumulation, not a code drift coefficient. Total mass is $M=m_{base}A+\rho\Delta h_vA_d$, and average load is $Mg/A$. Peak local load is not spread across the whole roof.

Pitch. Actual surface area is $A/\cos\theta$. Gravity traction per surface area is $q\cos^2\theta$ normal to the roof and $q\cos\theta\sin\theta$ downslope. These are force components, not a sliding threshold or snow-guard design.

Comparison and chart. The local high-density scenario is compared with user-entered, applicable roof design and action loads; zero means unknown. Distress overrides comparisons. The chart changes representative depth while holding density, extra depth, ice and retained water fixed; it is not a forecast of settlement or weather.

• Assess one roof section with a common pitch and snow-density model. Enter horizontal projected areas.
• Both snow-depth inputs exclude separately entered solid ice and use the selected measurement direction.
• Added ice and retained water are uniform across the assessed area. Do not double count liquid already included in bulk snow density or water produced by melting existing snow.
• A deeper patch uses the same density as the representative pack. Dense wind slabs, valleys, eave ice dams and separate lower roofs require separate assessments; run each critical area using its own measured bulk density.
• No snow leaves because the roof is pitched. Deposited slide snow can be entered as extra depth; impact while it lands is outside this static model.
• Low/high scenarios describe density sensitivity only. Measurement error, unknown layers, footprint uncertainty and structural deterioration are not automatically included.
• Design and action values default to unknown. Enter comparable, unfactored roof snow-and-associated-water values verified by a qualified structural engineer for the actual loading pattern.

Use for static gravity-load screening from present roof observations. Snow type and age provide approximate scenarios, not guaranteed density bounds. The wet-pack bracket is a regional seasonal example; rain-soaked slush, very dense drifts and ice-rich layers can be heavier. Use measured density when uncertain.

Sturm's model is a seasonal Northern Hemisphere ground-snow model (October–June), not a roof-deposit aging model. Its predictive performance is poor at small depths, particularly below 30 cm; uncertainty remains substantial even for deeper packs. The ±130 kg/m³ span is not validated for roofs. A zero measurement allowance does not imply zero uncertainty.

FEMA P-957 (January 2013), §§4.2 and 5.3, informs the response guidance. Structural warning signs or collapse concern require stopping removal, evacuation and urgent professional help regardless of calculated ratios. Clearing should be undertaken by experienced, insured professionals with fall protection, exclusion zones below, protection around skylights and a sequence that avoids unbalanced loads. Do not stockpile removed snow on the roof.

The entered design load is not a measured failure capacity. Uniform-load comparisons cannot establish member adequacy under local drifts. This tool does not implement a jurisdictional design code or determine ground snow, load factors, combinations, resistance, sliding friction, impact, ponding or snow-guard capacity. Check against the standard in force and local requirements. Installation or modification of roof safety systems belongs to qualified persons.