Also in: Disaster Response and Recovery
Flat-Roof Snow Load
The design snow load on a flat roof, from the ground snow load for the site.
Example
You enter
- Ground snow load Pg (psf) 30
- Exposure factor Ce 1
- Thermal factor Ct 1
You get
- Flat-roof snow load Pf 21 psf
- Pg (psf) 30
Checked against published worked examples from: ASCE.
Details, formula, and sources
ScopePf = 0.7 * Ce * Ct * Pg per ASCE 7-22, with Pg the ground snow load for the risk category (7-22 dropped the importance factor Is).
Formula or tableFlat-roof snow load Pf = 0.7 × Ce × Ct × Pg (psf), ASCE 7-22 Eq. 7.3-1; Pg is the ground snow load for the risk category (7-22 removed the importance factor Is). Optional leeward drift hd = 1.5 sqrt(pg^0.74 lu^0.70 W2^1.7 / gamma), Eq. 7.6-1.
Source and editionASCE 7-22 (Minimum Design Loads for Buildings and Other Structures). Section 7.3.
Free accessASCE 7 licensed; principles free in published structural texts. ground snow loads by risk category free from the ASCE Hazard Tool and ATC Hazards by Location (hazards.atcouncil.org).
What governsEstimate. AHJ and licensed structural engineer govern.
API field namesPg_psf, Ce, Ct, Pf_psf
- Default Ce / Ct 1.0 / 1.0 (partially exposed, heated) unless user supplies; no Is, since the risk-category ground snow load carries itASCE 7-22 §7.3
- Default W2 0.65, the top of the lower-48 winter wind parameter range (about 0.25 to 0.65), which overstates the drift where winter wind is calmer; enter the Figure 7.6-1 valueASCE 7-22 Figure 7.6-1; STRUCTURE magazine on the 7-22 snow changes
- Pg source entered by the user: the ASCE 7-22 ground snow load for the building's risk category, from the ASCE Hazard Tool (ascehazardtool.org); a 7-16 map value is not interchangeableASCE 7-22 §7.2