ISO 4355:2013

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 2013
© ISO 2013
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ii © ISO 2013 – All rights reserved

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
Foreword
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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.
iv © ISO 2013 – All rights reserved

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
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.
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
load with a specified annual exceedance probability
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.
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
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.
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
2 © ISO 2013 – All rights reserved

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.
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.
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
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
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
4 © ISO 2013 – All rights reserved

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
C
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 .
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
winter. Since only s , however, is available, it has been used with the modifications given in Annex D.
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.
6 © ISO 2013 – All rights reserved

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

β > 6001/C
 ()
m
(7)
An overview of the basic load coefficient is shown in Figure 3.
1,2
0,8
C = 1,33
0,6
m
C = 1,2
m
0,4
C = 1,0
m
0,2
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.
μ
b
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
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.
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
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
(A.1)
where
s is the snow load on the ground (kN/m ) with a return period of 50 years;
d is the snow depth on the ground (m) with a return period of 50 years.
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.
8 © ISO 2013 – All rights reserved

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.
Many formulae have been proposed due to different climates in different countries. A snow density of
300 kg/m should be used if no other information is given.
[10]
In Russia, former USSR , Formula (A.2) has been proposed:
ρ =+()90 130 dT()15,,+0171()+01, v
(A.2)
where
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
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
(A.4)
where
ρ is the snow density (kg/m );
s is the snow load on the ground (kN/m ).
Formula (A.4), written in terms of snow depth, is
270/(12,,−<0511dd)m,25
ρ =
{
480 d,≥125m
(A.5)
where
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);
ρ is the density of snow at the surface (kg/m );
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.
10 © ISO 2013 – All rights reserved

A.1 A.2 a A.2 b
100 A.3 A.5 A.6 a
A.6 b
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 design.
Normally, only recordings from various kinds of rain gauges can be obtained for this purpose. Such data
on snow fall should never be used without corrections. The data shall be corrected for errors caused by
wind effects at the gauge. Recommendations on such adjustments of the data, based on observations in
Nordic countries, are available in Reference [15].
A.2.4 Rain on snow surcharge load
For locations where 0 < s < 1 kN/m , all roofs with slopes less than W/15,2 (in degrees) shall have
0,25 kN/m rain-on-snow surcharge. This rain-on-snow-augmented design load applies only to the basic
load case and need not be used in combination with drift, sliding, unbalanced, or partial loads.
NOTE Formulation from ASCE 7–10.
A.2.5 Climate change
When developing national or regional maps for ground snow loads, it is important to note that confined
ensembles of annual extremes or peaks over a specific threshold can contain random positive or negative
trends. The evaluation of possible climate change effects has to consider this randomness. Climate
change scenarios can provide information on the basic shape of trends which should be considered in
the analysis.
ρ (kg/m )
A.3 Statistical treatment of basic data
When applying statistical methods to basic snow measurement data, it should generally be noted that the
regional significance of such data is highly dependent on the method of observation and the sheltering
of the observation area. Whether or not a meteorological station typifies a region shall therefore be
carefully considered in snow load calculations.
A.3.1 Statistical distributions
For snow climates with a permanent snow cover over the complete winter season in every winter, the
annual maximum snow loads provide the appropriate basis for extreme value statistics. For those snow
climates which have more than one independent period of permanent snow cover over the winter season,
the statistical stability of the estimated parameters can be increased by using peaks over a specific
threshold. Since confined ensembles inevitably contain random information, it is extremely difficult to
identify the “true” probability distribution and the corresponding “true” parameters. Therefore, it is
recommended to use the Type I extreme value distribution for the annual non-exceedance probabilities.
For snow climates not having snow every year, the fitting of data should only use non-zero snow load
amplitudes. Special care has to be taken if the observations include unusual large values in terms of
outliers.
A.3.2 Possible climatic dependence in choice of distribution
Research indicates that the best fit of local data to the Lognormal or the Type 1 distribution is governed
[4]
by certain climatic conditions of the region.
If detailed analyses comparing different distributions are not available, it is recommended that in
regions with an annual extreme snow load resulting from accumulation during a long part of the winter
season, the Type 1 should be selected. In other regions with extreme load as a result of only one, or a few,
snowfalls, the Lognormal distribution applies.
The conservatism of the two distributions depends on the magnitude of the coefficient of variation, i.e.
for low values, Type 1 is more conservative, and for high values, Lognormal is most conservative when
calculating long return period loads.
The standard error of estimate for the return period considered can be used in comparing different
parameter estimation methods.
Often the return period considered is greater than the number of maximum snow load recordings
available. The degree of goodness of fit for a theoretical distribution to the sample data cannot always
be relied upon for extrapolated values corresponding to long return periods. It is recommended also to
consider climatic conditions in the decision making.
12 © ISO 2013 – All rights reserved

Annex B
(normative)
Snow load distribution on selected types of roof
B.1 Simple pitched roofs
Snow load distribution for simple pitched roofs is described in Figure B.1. For asymmetrical simple
pitched roofs, each side of the roof shall be treated as one-half of a corresponding symmetrical roof.
s
b
s
d
β
x
w
W
Figure B.1 — Snow load distribution on simple pitched roof
Basic load case:
— Windward side: s = s
b
— Leeward side: s = s
b
Drifted load case:
— Windward side: s = 0
— Leeward side: s = s + s
b d
Basic load part:
ss=08, CC μ
be0 tb
(B.1)
Drifted load part:
ss= μμ
dd0 b
(B.2)
 β 
μ =+01, 2 00, 55−+C 6
()
de 
42,5
 
(B.3)
where

56<<β 0
WW ≤ 20 m
w=
{
20 m W > 20 m
s μμ xw≤
0d b
s =

d
xw>

0,3
0,25
C = 1,2
e
0,2
C = 1,0
e
0,15
C = 0,8
e
0,1
0,05
0102030405060 β [°]
Figure B.2 — µ µ as a function of roof slope β in case of C = 1,0
b d m
B.2 Simple flat and monopitched roofs
For this roof shape only the basic roof snow load, s , need to be considered.
b
ss=
b
ss=08, CC μ
be0 tb
(B.4)
14 © ISO 2013 – All rights reserved
μ *μ
b d
s
b
β
x
Figure B.3 — Snow load distribution on monopitched roof
B.3 Multipitched roofs
For a simple pitched roof, one expects snow to slide off the roof when the slope is steep. However, for
a multipitched roof, the snow slides and results in a redistribution of load on the same roof. This is
covered by separate basic and sliding load cases as shown in Figure B.4. The sliding load case accounts
for the potential for sliding snow and possible drifted snow. It is recommended to set ρ = 300 kg/m .
External slopes of multipitched roofs is according to B.1.
s
b
s
s1max
s
s1min
s
s2max
s
s2min
β β
1 2
W
w
Figure B.4 — Snow load distribution — multipitched roof
Basic load case:
ss=08, CC μ
b0 et b
(B.5)
Sliding load case 1:
 
08, sC CW
0e t
h< 
ρβtant90- + an 90-β
 ()() () 
 
(B.6)
04, sC C h
0e t
d=+
ρ 2
(B.7)
04, sC C h
0e t
s =−2ρ
s1min  
ρ 2
 
(B.8)
sd=2ρ
s1max
(B.9)
Sliding load case 2:
 
08, sC CW
0e t
 
h≥
ρβtant90- + an 90-β
 ()() () 
 
(B.10)
 
08, sC CW
0e t
d = 
ρβtan 90- +tan 90-β
 ()() () 
 
(B.11)
s =0
s2min
(B.12)
sd=2ρ
s2max
(B.13)
B.4 Simple curved roofs, pointed arches, and domes
For curved and pointed arch roofs with h/b ≥ 0,05, the basic load and drifted load distributions are
determined according to Figure B.5. For h/b <0,05, the snow loads are determined according to B.2.
Pointed arches with β ≥5° at the ridge line (x = 0) should be treated as pitched roofs, see B.1.
For domes of circular plan form, the basic load is that in Figure B.6 applied in an axially symmetric
manner. The drifted load along the central axis parallel to the wind is the same as for an arch and µ
d
varies as shown in Figure B.5 with distance y from this axis.
16 © ISO 2013 – All rights reserved

s
b
s
d
s = 0
β
β
b
x
Figure B.5 — Basic and drifted snow load distribution on curved roofs
Basic load case:
s,=08sC C μ
b0 et b
(B.14)
Drifted load case:
s =0
b
(B.15)
ss= μμ ()x
d0 b d
(B.16)
μ = 2 xx/   xx≤ h
d30 30
> 01, 2
}
μ = 2        xx≥
b
d 30
(B.17)
μ = 16,7/xx/ hb
()() xx≤ h
d30
0,05≤≤ 0,12
}
xx≥
μ = 16,7/hb
() b
d
(B.18)
where
xx≡= at 3β 0 °≤and xb /2
30 30
Drift load coefficient on domes (see Figure B.6):
 y 
μμ(,xy)(=−x,)01
 
dd
r
 
(B.19)
hh
µ (x,y)
d
r
µ (x,0)
y
d
x
NOTE The arrow indicates wind direction.
Figure B.6 — Plan view of drift load coefficient on dome
B.5 Multilevel roofs (lower roofs with slope β )
l
For lower roofs, Figure B.7, the basic part of the load is determined from
ss=0,8 CC μ
b0 et b
(B.20)
Cx≥<1,0 for 10h
e
(B.21)
where
x is the horizontal distance from the step;
h is the height of step.
The drift part of the snow load, s (x), is determined as the most severe of three possible cases, illustrated
d
in Figure B.8:
Case a: Step faces in the downwind direction and snow drifts off the upper roof into the sheltered zone
at the step.
Case b: Step faces into the wind and snow drifts over the lower roof into the step region.
Case c: End region of a step that faces in the downwind direction where snow drifts into the sheltered
step region from around the corner.
s (x) is a triangular function of x, being a maximum at x = 0 and decreases linearly to zero at the tail of
d
the drift, at x = l . Where the tail of the drift would extend beyond the edge of the lower roof, the drift is
d
truncated to a trapezoidal form.
18 © ISO 2013 – All rights reserved

x
a
> >
C 1,0 C 0,8
e e
l
d
x = 10h
Figure B.7 — Snow distribution and snow load coefficients for lower level adjacent roofs
The length of the drift is as follows:
s ()0
d
l =5 (B.22)
d
ρ g
The maximum of the drift load is given by the following formulae:
ss()00= μμ () (B.23)
d0 bd
'
lh−5 ρ g
()
cp
μξ()0 =0,35 (B.24)
d
s
ξρgh−s
0<≤μ (B.25)
d
s μ
0b
4ξ
μ ≤ (B.26)
d
2,5
C
e
h
p
s
d
s
b
h
h
β
l
s
'
bs
hh=− (B.27)
pp
ρg
where
h is the height of the roof perimeter parapet of the source area.
p
s is the basic snow load of the source area.
bs
ρ is the snow density.
g is the acceleration due to gravity.
h shall be taken as zero unless all the roof edges of the source area have parapets.
p
It is recommended to set ρ = 300 kg/m . Alternatively, see Annex A for density formulae.
Table B.1 — Coefficients C and µ
e d
x C µ
e d
0 C ≥1,0 µ (0)
e d
0 < x ≤ l C ≥1,0 x
d e
μ ()01()−
d
l
d
l < x ≤ 10h C ≥1,0 0
d e
x > 10h C ≥0,8 0
e
The appropriate values of the parameter ξ for each of cases a), b), and c) are tabulated in Table B.2. l
cs
is the representative length of the appropriate source area for snow drifting for each of the three cases
shown in Figure B.8.
W
lW=−2 (B.28)
cs
L
where
L is the longer dimension of the source area.
If the upper roof is pitched, the dimensions W and L are based on the overall dimensions of the upper
roof for case a).
20 © ISO 2013 – All rights reserved

2 2
h
W
W
1 1
L
(b) (c)
(a)
Key
1 lower roof
2 upper roof
3 length of step
source area for snow in drift
snow drift
wind direction
NOTE The directions of the dimensions L and W shown in the diagrams will interchange depending on which
is larger.
Figure B.8 — Snowdrift cases and parameters for lower level roofs
Table B.2 — Values of the parameter ξ for each of cases a), b), and c)
Parameter Case a) Case b) Case c)
ξ 1,0 0,67 0,67
Parapet height of Parapet height of Parapet height of
h
p
upper roof lower roof lower roof
W and L taken as W and L taken as W and L taken as the
the shorter and the shorter and shorter and longer
longer dimensions, longer dimensions, dimensions, respec-
l respectively, of the respectively, of tively, of the source
cs
upper roof the source area on area on the lower
the lower roof for roof for downwind-
upwind-facing step facing step
Figure B.9 shows µ (0) plotted as a function of ρgl /s for cases a) and b) for several values of ρgh /s .
d cs 0 p 0
For case c) the plot for case b) with ρgh /s = 0 applies.
p 0
L
W
L
A
B
C
ρgh s
p/ 0
110 100 ρgl s
cs/ 0
(a)
A
B
C
ρgh s
p/ 0
110 100 ρgl s
cs/ 0
(b)
Key
22 © ISO 2013 – All rights reserved

μ (0)
μ (0)
d
d
l
d
A upper limit for C = 0,8
e
B upper limit for C = 1,0
e
C upper limit for C = 1,2
e
NOTE For case (c), the plot for case (b) applies with ρgh /s = 0.
p 0
Figure B.9 — Variation of µ (0) with ρgl /s for cases a) and b)
d cs 0
At an outside corner where two step faces meet (see Figure B.10) the triangular drift load from the
more lightly loaded step region shall be assumed to extend radially from the corner. At an inside corner
the drift loads calculated for each step face shall be applied as far as the bisector of the corner angle, as
shown in Figure B.11.
r = l
d
Key
1 step face with higher µ (0)
d
2 step face with lower µ (0)
d
NOTE Radius of drift surcharge, r, is equal to l .
d
Figure B.10 — Drift loading at outside corner
Key
1 lower roof
2 upper roof
3 step face 1
4 step face 2
5 bisector of angle between two step faces
Figure B.11 — Drift loading at inside corner
When a building roof is closer than 5 m to a higher level roof of an adjacent building, it shall be designed
for the tail portion of the triangular drift load, as shown in Figure B.7.
If the upper roof is sloped greater than 5° and has no edge parapet or snow fence to prevent sliding, the
additional sliding snow load, s (x), on the lower roof shall be assumed to take a triangular form (see
s
Figure B.11) and is calculated as follows:
sx()=sxμμ () (B.29)
s0 bs
h
u
μ ()0 = (B.30)
s
l tanβ
su
ρgh
μ ()00≤−μ () (B.31)
s d
s μ
0b
 
h
lh=+2 cosβ pp− (B.32)
 
su u
 
h
u
 
where
p=−sintββan cosβ (B.33)
uul
hs<=3f/(ρμg or 0) 1 (B.34)
0s
 
x
μμ()x =−()0 1 for 0≤≤xl (B.35)
 
ss s
l
 s 
24 © ISO 2013 – All rights reserved

β
u
x
l
s
Figure B.12 — Sliding snow load factor
The sliding snow load should be considered as simultaneous with the basic load and 50 % of the drift
load. Note that the sliding snow load defined above does not include the effect of the impact of the snow
as it lands on the lower roof.
B.6 Additional drift load and sliding load on ground or on lower level roof, acting
against the upper arch or pitched roof
A lower level roof should be checked for the sliding load as an alternative load case as compared with the
load cases of B.5. Impact effects shall be considered (see Figure B.13).
l
l
d
l
d
Figure B.13 — Additional drift load and sliding load acting on upper arch or pitched roof
ss=+μμ (B.36)
()
0 db
6s
l = (B.37)
d
ρg
μμ+ s
()
bd 0
l = (B.38)
ρβgtan
μ (0)
s
h
h h
u
β
l
s μ s μ s μ
...