3 August 1998
COMMENTARY ON SNOW LOADS
1. PURPOSE AND SCOPE. This document provides guidance for designing roofs subjected
to snow loads. The primary discipline addressed in structural, but this guidance also applies to
architectural, mechanical, and electrical issues.
2. APPLICABILITY. These instructions are applicable to all USACE elements involved with
the design of buildings and other structures, including repairs and modifications as well as new
a. Use the current edition of American Society of Civil Engineers (ASCE) Manual 7,
"Minimum Design Loads for Buildings and Other Structures." Copies are available from ASCE,
1015 15th Street, N. W., Suite 600, Washington,D.C. 20005-2605. ASCE's phone number is
(202) 789-2200. Users of this document should not use the mandatory provisions of the
Standard itself without becoming familiar with the Commentary on Snow Loads appended to the
Standard. The Commentary explains the rationale behind the provisions and contains examples
that illustrate their use.
b. Site-specific ground snow loads for military installations and other places of interest to
DOD are tabulated in TI 809-XX, "Load Assumptions for Buildings." That information is based on
a detailed snow load case study at each place. Occasionally, the case study answer differs from
the value on the national snow load map in ASCE Manual 7. When a difference exists, the
tabulated value in TI 809-XX should be used. A copy of each case study is maintained at CEMP-
ET. Snow loads for foreign locations are also tabulated in TI 809-XX. Caution is urged when
using these foreign values since each is based on local experience only, not an extreme-value
statistical analysis of recent meteorological data. Wherever possible, host country expertise
should be sought and host country snow loads compared to those tabulated in TI 809-XX.
c. In some areas of the United States extreme local variations in snow lads preclude
mapping on a national scale. In such areas the national snow load map in ASCE Manual 7 does
not present a ground snow load, but indicates that a snow load case study is needed. The data
and methodology used to conduct snow load case studies are presented in Cold Regions
Research and Engineering Laboratory (CRREL) report "Snow Loads for the United States."
Additional snow load case studies are available through CEMP-ET.
4. BUILDING CONFIGURAITON. The snow load provisions of ASCE Manual 7 indicate how
dramatically the geometry of a building influences the snow loads on its roof. Problems can be
avoided and more economical designs developed when snow load issues are considered by the
design team as the show of the building evolves. Snow will drift into areas of "aerodynamic
shade" (see figure 1). Figure 2 illustrates such places on the kinds of problems that are
5. UNBALANCED LOADS. Figure 3 shows a "saw-tooth roof" on which wind has moved snow
from its upper portions into its valleys creating unbalanced snow loads. Such unbalanced snow
loads are covered in ASCE Manual 7 for roofs with a slope of 1 or more. ASCE Manual 7 does
not require consideration of unbalanced loads for lower slopes, but its Commentary warns that