What it checks
The module designs roof snow loads for structural members by calculating balanced flat roof snow load, sloped roof snow load, and unbalanced surcharges. It applies the strength design method using ASCE 7-22 Chapter 7 provisions to determine the governing snow load intensity. The calculation sequence begins with the flat roof snow load, which incorporates exposure, thermal, and importance factors. The module then applies a slope reduction factor to derive the sloped roof design snow load. It checks this value against the minimum roof snow load requirement. Finally, the module evaluates unbalanced snow conditions for gable and hip roofs, including drift height and surcharge extents.Inputs
- Snow Load - Ground snow load pg (psf) from ASCE 7 Figure 7.2-1.
- Site Exposure - Site exposure category ("A", "B", "C", "D") and roof exposure ("fully", "partially", "sheltered").
- Thermal Properties - Thermal condition ("heated", "unheated_or_vented", "freezer", "greenhouse") and roof thermal resistance R (h*ft2*F/Btu).
- Roof Geometry - Roof slope (degrees), slippery surface flag, and eave-to-ridge distance W (ft).
- Importance - Importance factor Is or risk category ("I" through "IV").
- Unbalanced Parameters - Winter wind parameter W2 and simply supported prismatic flag for the rafter system case.
- Overrides - Minimum roof snow load override (psf), where 0.0 skips the minimum check.
What it reports
- Flat Roof Load - Balanced flat roof snow load pf (psf), calculated as 0.7 times Ce, Ct, Is, and pg.
- Slope Factor - Slope reduction factor Cs, a value between 0.0 and 1.0 based on roof slope and thermal conditions.
- Sloped Roof Load - Sloped roof design snow load ps (psf), which is the governing load and includes the minimum check if it controlled.
- Minimum Check - Boolean flag indicating whether the minimum roof snow load governed the final ps value.
- Unbalanced Case - Structured case identifier ("not_required", "rafter_w20", "general", or "legacy_simplified") for the unbalanced load scenario.
- Surcharge Load - Rectangular surcharge intensity (psf) stacked on the leeward ps for unbalanced conditions.
- Surcharge Extent - Horizontal extent of the surcharge from the ridge (ft), calculated as (8/3) times hd times the square root of S.
- Drift Height - Drift height hd (ft) and snow density gamma (pcf) used in the unbalanced calculations.
- Factor Basis - Textual description of where each exposure and thermal factor came from, including table and row/column references.
Code references
The engine cites ASCE 7-22 Chapter 7, specifically §7.3 for flat roof snow load, §7.3.4 for minimum roof snow load, §7.4 for sloped roof snow load, §7.6.1 for unbalanced snow loads, and §7.7 for drifting reminders. It utilizes Table 7.3-1 for exposure factors, Table 7.3-2 and Table 7.3-3 for thermal factors, and Figure 7.6-2 for unbalanced load distributions.Limits
The module returns drifting information as a note rather than a computed load. Unbalanced snow loads are informational for gable roofs. The legacy path remains unchanged when new fields are empty, falling back to old maps. Results are engineering aids; the engineer of record is responsible for the design.Try it free in your browser
The Snow Load calculator runs this module's engine with no install and no signup.