ISO 4355:2013

Bases for design of structures — Determination of snow loads on roofs

ISO 4355:2013 specifies methods for the determination of snow load on roofs. It can serve as a basis for the development of national codes for the determination of snow load on roofs. National codes should supply statistical data of the snow load on ground in the form of zone maps, tables, or formulae. The shape coefficients presented in ISO 4355:2013 are prepared for design application, and can thus be directly adopted for use in national codes, unless justification for other values is available. For determining the snow loads on roofs of unusual shapes or shapes not covered by ISO 4355:2013 or in national standards, it is advised that special studies be undertaken. These can include testing of scale models in a wind tunnel or water flume, especially equipped for reproducing accumulation phenomena, and should include methods of accounting for the local meteorological statistics. Examples of numerical methods, scale model studies, and accompanying statistical analysis methods are described in ISO 4355:2013 (Annex G). The annexes of ISO 4355:2013 describing methods for determining the characteristic snow load on the ground, exposure coefficient, thermal coefficient, and loads on snow fences are for information only as a consequence of the limited amount of documentation and available scientific results. In some regions, single winters with unusual weather conditions can cause severe load conditions not taken into account by ISO 4355:2013. Specification of standard procedures and instrumentation for measurements is not dealt with in ISo 4355:2013.

Bases du calcul des constructions — Détermination de la charge de neige sur les toitures

General Information

• Status: Published
• Publication Date: 02-Dec-2013
• ICS: 91.080.01 - Structures of buildings in general
• Technical Committee: ISO/TC 98/SC 3 - Loads, forces and other actions
• Drafting Committee: ISO/TC 98/SC 3 - Loads, forces and other actions
• Current Stage: 9093 - International Standard confirmed
• Completion Date: 04-Jan-2022

Relations

• Revises: ISO 4355:1998 - Bases for design of structures — Determination of snow loads on roofs
• Effective Date: 20-Feb-2010

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 4355
Third edition
2013-12-01
Bases for design of structures —
Determination of snow loads on roofs
Bases du calcul des constructions — Détermination de la charge de
neige sur les toitures
Reference number
ISO 4355:2013(E)
©
ISO 2013

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ISO 4355:2013(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2013
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
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Published in Switzerland
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ISO 4355:2013(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Snow loads on roofs . 3
4.1 General function describing intensity and distribution of the snow load on roofs . 3
4.2 Approximate formats for the determination of the snow load on roofs . 3
4.3 Partial loading due to melting, sliding, snow redistribution, and snow removal . 4
4.4 Ponding instability . 4
5 Characteristic snow load on the ground . 4
6 Snow load coefficients . 4
6.1 Exposure coefficient . 4
6.2 Thermal coefficient . 6
6.3 Surface material coefficient . 6
6.4 Shape coefficients . 6
Annex A (informative) Background on the determination of some snow parameters .8
Annex B (normative) Snow load distribution on selected types of roof .13
Annex C (informative) Determination of the exposure coefficient for small roofs .28
Annex D (informative) Determination of thermal coefficient .31
Annex E (informative) Roof snow retention devices .34
Annex F (informative) Snow loads on roof with snow control .36
Annex G (informative) Alternative methods to determine snow loads on roofs not covered by the
prescriptive methods in this International Standard .38
Bibliography .39
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ISO 4355:2013(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 98, Bases for design of structures, Subcommittee
SC 3, Loads, forces and other actions.
This third edition cancels and replaces the second edition (ISO 4355: 1998), which has been technically
revised.
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ISO 4355:2013(E)

Introduction
The intensity and distribution of snow load on roofs can be described as functions of climate, topography,
shape of building, roof surface material, heat flow through the roof, and time. Only limited and local data
describing some of these functions are available. Consequently, for this International Standard it was
decided to treat the problem in a semi-probabilistic way.
The characteristic snow load on a roof area, or any other area above ground which is subject to snow
accumulation, is in this International Standard defined as a function of the characteristic snow load on
the ground, s , specified for the region considered, and a shape coefficient which is defined as a product
0
function, in which the various physical parameters are introduced as nominal coefficients.
The shape coefficients will depend on climate, especially the duration of the snow season, wind, local
topography, geometry of the building and surrounding buildings, roof surface material, building
insulation, etc. The snow can be redistributed as a result of wind action; melted water can flow into
local areas and refreeze; snow can slide or can be removed.
In order to apply this International Standard, each country will have to establish maps and/or other
information concerning the geographical distribution of snow load on ground in that country. Procedures
for a statistical treatment of meteorological data are described in Annex A.
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INTERNATIONAL STANDARD ISO 4355:2013(E)
Bases for design of structures — Determination of snow
loads on roofs
1 Scope
This International Standard specifies methods for the determination of snow load on roofs.
It can serve as a basis for the development of national codes for the determination of snow load on roofs.
National codes should supply statistical data of the snow load on ground in the form of zone maps,
tables, or formulae.
The shape coefficients presented in this International Standard are prepared for design application, and
can thus be directly adopted for use in national codes, unless justification for other values is available.
For determining the snow loads on roofs of unusual shapes or shapes not covered by this International
Standard or in national standards, it is advised that special studies be undertaken. These can include
testing of scale models in a wind tunnel or water flume, especially equipped for reproducing accumulation
phenomena, and should include methods of accounting for the local meteorological statistics. Examples
of numerical methods, scale model studies, and accompanying statistical analysis methods are described
in Annex G.
The annexes describing methods for determining the characteristic snow load on the ground, exposure
coefficient, thermal coefficient, and loads on snow fences are for information only as a consequence of
the limited amount of documentation and available scientific results.
In some regions, single winters with unusual weather conditions can cause severe load conditions not
taken into account by this International Standard.
Specification of standard procedures and instrumentation for measurements is not dealt with in this
International Standard.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
1)
ISO 2394 , General principles on reliability for structures
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
characteristic value of snow load on the ground
s
0
load with a specified annual exceedance probability
2
Note 1 to entry: It is expressed in kilonewton per square metre (kN/m ).
Note 2 to entry: In meteorology, the term “weight of the ground snow cover” is also used.
1) In process of revision.
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ISO 4355:2013(E)

3.2
shape coefficient
μ
coefficient which defines the amount and distribution of the snow load on the roof over a cross section
of the building complex and primarily depends on the geometrical properties of the roof
3.3
value of snow load on roofs
s
function of the characteristic snow load on the ground, s , and appropriate shape coefficients
0
Note 1 to entry: The value of s is also dependent on the exposure of the roof and the thermal conditions of the
building.
Note 2 to entry: It refers to a horizontal projection of the area of the roof.
2
Note 3 to entry: It is expressed in kilonewton per square metre (kN/m ).
3.4
basic load coefficient
µ
b
coefficient defining the reduction of the snow load on the roof due to a slope of the roof, β, and the
surface material coefficient, C
m
3.5
drift load coefficient
µ
d
coefficient which defines the amount and redistribution of additional load on a leeward side or part of
a roof, depending on the exposure of the roof to wind, C , and the geometrical configurations of the roof
e
3.6
slide load coefficient
µ
s
coefficient defining the amount and distribution of the slide load on a lower part of a roof, or a lower
level roof
3.7
exposure coefficient
C
e
coefficient which accounts for the effects of the roof’s exposure to wind
3.8
exposure coefficient for small roofs
C
e0
exposure coefficient for small roofs with effective roof length shorter than 50 m
3.9
effective roof length
l
c
length of the roof influenced by exposure coefficient given as a function of roof dimensions
3.10
thermal coefficient
C
t
coefficient defining the change in snow load on the roof as a function of the heat flux through the roof
Note 1 to entry: C , in some cases, can be greater than 1,0. Further guidance is given in 6.2 and Annex D.
t
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ISO 4355:2013(E)

3.11
surface material coefficient
C
m
coefficient defining a reduction of the snow load on sloped roofs made of surface materials with low
surface roughness
3.12
equivalent snow density
ρ
e
density for calculating the annual maximum snow load from annual maximum snow depth
3.13
snow density
ρ
ratio between snow load and snow depth
4 Snow loads on roofs
4.1 General function describing intensity and distribution of the snow load on roofs
Formally, the snow load on roofs can be defined as a function, F, of several parameters:
sF= sC,,CC,, μμ,, μ
()
0 et mb ds
(1)
where the symbols are as defined in Clause 3.
While C , C , and C are assumed constant for a roof or a roof surface, µ , µ , and µ generally vary
e t m b d s
throughout the roof.
4.2 Approximate formats for the determination of the snow load on roofs
This International Standard defines the snow load on the roof as a combination of a basic load part, s ,
b
a drift load part, s , and a slide load part, s . Thus, for the most unfavourable condition (lower roof on
d s
leeward side):
ss=+"" ss""+
bd s
(2)
where “+” implies “to be combined with”.
Effects of the various parameters are simplified by the introduction of product functions.
ss=08, CC μ
be0 tb
(3)
ss= μμ
db0 d
(4)
ss= μ
ss0
(5)
[1] [2]
The basic roof snow load, s , is uniformly distributed in all cases,  except for curved roofs, where
b
the distribution varies with the slope, β (see B.4).
The basic load defines the load on a horizontal roof, and the load on the windward side of a pitched
roof. Since any direction can be the wind direction, the basic load is treated as a symmetrical load on a
symmetrical roof, thus defining a major part of the total load on the leeward side as well.
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ISO 4355:2013(E)

The drift load is the additional load that can accumulate on the leeward side due to drifting.
The slide load is the load that can slide from an upper roof onto a lower roof, or a lower part of a roof.
4.3 Partial loading due to melting, sliding, snow redistribution, and snow removal
A load corresponding to severe imbalances resulting from snow removal, redistribution, sliding, melting,
etc. (e.g. zero snow load on specific parts of the roof) should always be considered.
Such considerations are particularly important for structures which are sensitive to unbalanced loading
(e.g. curved roofs, arches, domes, collar beam roofs, continuous beam systems) which are addressed in
other clauses of this International Standard.
4.4 Ponding instability
Roofs shall be designed to preclude ponding instability. For flat roofs (or with a small slope), roof
deflections caused by snow loads shall be investigated when determining the likelihood of ponding
instability from rain-on-snow or from snow meltwater.
5 Characteristic snow load on the ground
The characteristic snow load on the ground, s , is determined by statistical treatment of snow data.
0
Snow load measurements on the ground should be taken in an undisturbed area not subject to localized
drifting.
Methods for the determination of the characteristic snow load on the ground, s , are described in
0
Annex A.
For practical application, the characteristic snow load on the ground will be defined in standard step
values, which will yield basic values for the preparation of zone maps as described in Annex A.
6 Snow load coefficients
6.1 Exposure coefficient
The exposure coefficient, C , should be used for determining the snow load on the roof. The choice of
e
C should consider the future development around the site. For regions where there are no sufficient
e
winter climatological data available, it is recommended to set C = 1,0.
e
For most cases, the exposure coefficient, C , is equal to the exposure coefficient for small roofs, C .
e e0
However, for very large flat roofs, wind is less effective in removing snow from the whole roof. To
compensate for this, the exposure coefficient for large roofs is higher than for smaller roofs.
Cl ≤50m
 e0 c
C =

e −−()l 50 /200
c
1251−−25 50m
,,()Ce l >

e0 c

(6)
where

2
W
l
is the effective roof length equal to 2W − in metres;
c
L
C is the exposure coefficient for small roofs.
e0
Methods for the determination of C are given in Annex C.
e0
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ISO 4355:2013(E)

In the expression for l , W is the length of the shorter side of the roof and L is the length of the longer side
c
(see Figure 1).
W
L
Figure 1 — Rectangular roof dimensions
For non-rectangular roofs, W and L can be taken as the shorter and longer side of the roof’s major
dimensions along two orthogonal axes. For example, for an elliptical shape, W is measured along the
short axis and L along the long axis.
An overview of the exposure coefficient is shown in Figure 2.
1,30
1,20
1,10
1,00
C = 1,2
e0
C = 1,0
e0
0,90
C = 0,8
e0
0,80
0,70
0 100 200 300 400 500
l [m]
c
Figure 2 — Exposure coefficient, C , as a function of effective roof length, l
e c
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C
e

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ISO 4355:2013(E)

6.2 Thermal coefficient
The thermal coefficient, C (see 3.10), is introduced to account for effect of thermal transmittance of the
t
roof.
The snow load is reduced on roofs with high thermal transmittance because of melting caused by heat
loss through the roof. For such cases and for glass-covered roofs in particular, C , can take values less
t
than unity.
For buildings where the internal temperature is intentionally kept below 0 °C (e.g. freezer buildings, ice
skating arenas), C , can be taken as 1,2. For all other cases, C = 1,0 applies.
t t
Bases for the determination of C are the thermal transmittance of the roof, U, and the lowest temperature,
t
θ, to be expected for the space under the roof, and the snow load on the ground, s .
0
Methods for the determination of C for roofs with high thermal transmittance are described in Annex D.
t
NOTE The intensity of snowfall for short periods, approximately 1 d to 5 d, is often a more relevant parameter
than s for roofs with considerable heat loss, since the melting is too rapid to allow accumulation throughout the
0
winter. Since only s , however, is available, it has been used with the modifications given in Annex D.
0
6.3 Surface material coefficient
The amount of snow which slides off the roof will, to some extent, depend on the surface material of the
roofing; see 6.4.2.
The surface material coefficient, C (see 3.11), is defined to vary between unity and 1,333, and takes the
m
following fixed values:
— C = 1,333 for slippery, unobstructed surfaces for which the thermal coefficient C <0,9 (e.g. glass
m t
roofs);
— C = 1,2 for slippery, unobstructed surfaces for which the thermal coefficient C >0,9 (e.g. glass roofs
m t
over partially climatic conditioned space, metal roofs, etc.);
— C = 1,0 corresponds to all other surfaces.
m
NOTE C = 1,2 could also be applied for C <0,9 if this is assumed to be more reasonable.
m t
6.4 Shape coefficients
6.4.1 General principles
The shape coefficients define distribution of the snow load over a cross section of the building complex
and depend primarily on the geometrical properties of the roof.
For buildings of rectangular plan form, the distribution of the snow load in the direction parallel to the
eaves is assumed to be uniform, corresponding to an assumed wind direction normal to the eaves.
The shape coefficients presented for selected types of roof (see Annex B), are illustrated for one specific
wind direction. Since prevailing wind directions can not correspond to the wind directions during
heavy snow falls, the condition that the wind during snow fall can have any direction with reference to
the roof location should be considered when designing roofs.
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ISO 4355:2013(E)

6.4.2 Basic load coefficient
When snow on sloped roofs can slide off unobstructed, snow load on the roof will be reduced. The
reduction of the snow load on the roof due to the slope, β, of the roof and the surface material coefficient,
C , is defined by the shape coefficient, μ (see 3.4), which is given by Formula (7):
m b
β < 30 1/C
()
1
m

μβ=−60 C /30   30 11//C <<β 60 C
() () ()

bm mm

0
β > 6001/C
 ()
m
(7)
An overview of the basic load coefficient is shown in Figure 3.
1,2
1
0,8
C = 1,33
0,6
m
C = 1,2
m
0,4
C = 1,0
m
0,2
0
0 10 20 30 40 50 60 β [°]
Figure 3 — Basic load coefficient, µ, as a function of surface material coefficient, C
b m
6.4.3 Drift load coefficient
The drift load coefficient, µ (see 3.5), is dependent on the roof geometry and the exposure coefficient,
d
C , and is described in Annex B.
e
6.4.4 Slide load coefficient
Slide load from an upper part of a roof onto a lower part of a roof, or onto a lower roof of a multilevel roof,
will depend on the amount of snow that can slide down, and on the geometrical configuration of the roof.
The distribution of the slide load and the spreading out of the load will, in addition to the geometrical
shape of the roof, depend on the properties of the sliding snow and on the friction on the upper roof from
which the snow is sliding.
The slide load magnitude and distribution is incorporated in the slide load coefficient, µ (see 3.6).
s
In the cases when slide load should be considered, the slide load coefficient for different roof types is
described in Annex B.
The impact loading due to slide load should be considered.
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μ
b

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ISO 4355:2013(E)

Annex A
(informative)

Background on the determination of some snow parameters
A.1 Snow zones and maps
The characteristic snow load on the ground, s , with an annual exceedance probability of 0,02 or other
0
values taking into account the importance of the building and the limit state considered, should be
available in national standards.
Due to the nature of the variation of snow load, a snow zone mapping with basic values throughout the
zones, often related to a fixed altitude, is preferred rather than a continuous field with isolines. This
approach is recommended since a specific snow load variation with altitude can often be developed
within climatologically defined zones.
Investigations have shown that near the coasts, not only the altitude but also the distance from the coast
can influence the snow load.
NOTE 1 When more appropriate, an annual exceedance probability of less than 0,02 can apply.
NOTE 2 Important studies on defining the characteristic snow load on load the ground have been carried out
and are discussed in References [3],[4],[5],[6], and [7]. On the treatment of statistical values, see A.3.
A subdivision of a country into zones of basic s values should be constructed in a logical set of steps.
0
2
Recommended interval values, in kN/m , are: 0,25 – 0,5 – 0,75 – 1,0 – 1,5 – 2,0 – 2,5 – 3,5 – 4,5 – [.].
A.2 Use of basic meteorological data
To determine snow load on the ground, s , a sequence of maximum yearly snow loads is used. This
0
parameter can be determined on the basis of recordings of water equivalents, snow depths, precipitation,
etc. For areas where there is snow every year, the recommended recording length is 20 years. For areas
with larger variability, a longer recording length is recommended. Snow sampling equipment and the
[8]
observation procedure should be in accordance with WMO recommendations. Preferably, snow
courses with records of water equivalents should be used. However, if water equivalent data are scarce,
available data on snow depth can be used.
A.2.1 Snow load on the ground related to snow depth
[9]
In the USA , the following relationship between snow load and snow depth is used:
1,36
sd=1,97
50
50
(A.1)
where
2
s is the snow load on the ground (kN/m ) with a return period of 50 years;
50
d is the snow depth on the ground (m) with a return period of 50 years.
50
Formula (A.1) takes into account that the maximum ground load does not necessarily occur on the same
day as the maximum ground snow depth.
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ISO 4355:2013(E)

A.2.2 Density of snow
The average density of snow layer is an important parameter for determining snow load, since the snow
depth has more recordings than the water equivalent at many stations.
When determining annual maximum snow load by means of snow depth and density, it should be
considered that these two parameters usually have a significant positive correlation before the
occurrence of a year’s snow depth maximum, and negative afterwards. In heavy snow regions, there is
usually a time lag between the annual maximum snow depth and the annual maximum snow load. This
difference is due to densification of snow layers. Therefore, an equivalent density of snow needs to be
[11]
used for determining s when based on the annual maximum snow depth.
0
Many formulae have been proposed due to different climates in different countries. A snow density of
3
300 kg/m should be used if no other information is given.
[10]
In Russia, former USSR , Formula (A.2) has been proposed:
3
ρ =+()90 130 dT()15,,+0171()+01, v
(A.2)
where
3
ρ is the snow density (kg/m );
d is the snow depth (m);
T is the average temperature (°C) over the period of snow accumulation (assumed to be not
below –25 °C);
v is the average wind speed (m/s) over the same period.
Another formula for equivalent snow density on the ground with a return period of 100 years used in
[11]
Japan is
d
ρ =+73 240
e
d
ref
(A.3)
where
3
ρ is the equivalent snow density (kg/m );
e
d is the snow depth (m);
d is the reference snow depth of 1 m.
ref
Formula (A.4) is for snow density in the USA. It relates the snow density at one point in time to the snow
load at the same point in time:
ρ =+43,5 s 224≤480
0
(A.4)
where
3
ρ is the snow density (kg/m );
2
s is the snow load on the ground (kN/m ).
0
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Formula (A.4), written in terms of snow depth, is
270/(12,,−<0511dd)m,25
ρ =
{
480 d,≥125m
(A.5)
where
3
ρ is the snow density (kg/m );
d is the snow depth (m).
[12] [13]
Based on observations of the German Weather Service (Deutscher Wetterdienst DWD) the
following approach has been developed:
 
 
ρρd  
d
∞ ref0
ρ =+ln11exp − 
 
 
d ρ d
 
∞  ref 
 
 
(A.6)
where
d is the snow depth (m);
3
ρ is the density of snow at the surface (kg/m );
0
ρ is the upper limiting value of the snow density;
∞
d is the reference snow depth of 1 m.
ref
3 3
For Germany, the snow density at the surface usually is in the range from 170 kg/m to 190 kg/m , and
3 3
the upper limiting value ranges from 400 kg/m to 600 kg/m . The latter value is valid for wet climates.
[14]
Figure A.1 shows a comparison of Formulae (A.1), (A.2), (A.3), (A.5), and (A.6) for snow density.
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ISO 4355:2013(E)

600
500
400
300
200

A.1 A.2 a A.2 b
100 A.3 A.5 A.6 a
A.6 b
0
0,0 1,0 2,0 3,0 4,0 5,0
d (m)
NOTE 1 For Formula (A.2), two alternatives are shown: a) average temperature T = −10 °C and average wind
speed v = 4 m/s and b) average temperature T = −20 °C and average wind speed v = 4 m/s.
3 3
NOTE 2 For Formula (A.6): a) dry climate with surface density 170 kg/m and upper limit density 400 kg/m ,
3 3
b) wet climate with surface density 190 kg/m and upper limit density 600 kg/m .
Figure A.1 — Snow density, ρ, as a function of snow depth, d, according to Formulae (A.1), (A.2),
(A.3), (A.5), and (A.6)
A.2.3 Snow intensities for short periods of time
For roofs with high values of heat loss, the snow fall intensity for short periods of time, 24 h or even
shorter, can be of particular interest of des
...