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Snow Drift Loading

Lower Adjacent Roofs (NBC2020)
v1.0.0
File:
Project:
Designer:
Date:

Climatic Data

Source
Region
Location
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Ground snow load
Ss:
Rain Load
Sr:
Density
Snow density, γ:
kN/m3 *If left blank, γ will calc as per cl. 4.1.6.13

Factors

Importance factor
Is:
Factors
Cw:
snow drift distribution

Upper Roof geometry

Roof Plan dimensions
Longer dimension, l:
m
Shorter dimension, w:
m
Height of parapet, hp:
m
snow drift distribution

Lower Roof geometry:

Roof step, h:
m
Pitch:
/12
Slippery:
Roof Parapet:
* If "Yes" selected, Cs = 1.0.

Case 2: Source Area Dimensions
Longer dimension,l:
m
Shorter dimension,w:
m
Parapet Height,hp:
m * Leave 0 unless parapet along entire roof edge of source area.
Case 3: Source Area Dimensions
Longer dimension,l:
m
Shorter dimension,w:
m
Parapet Height,hp:
m * Leave 0 unless parapet along entire roof edge of source area.

Specified Snow Load

[4.1.6.2]
S = Is[Ss(CbCwCsCa)+Sr]

Factors

User input values:
Ss = 1 kPa
Sr = 1 kPa
Importance Factors:
ULS: Is = 1.0
SLS: Is = 0.9

Roof slope = 0 degrees
Slope Factor
For non-slippery roof:
Slope <= 30 degrees.
Cs = 1
Density = γ = 2.63 kN/m3
[NBCC 2015
Figure 4.1.6.5.-A]

Drift Factors & Distribution

Case I

Shorter roof dimension, w = 10 m
Longer roof dimension, l = 10 m
Lcs = 2*ws-ws2/Ls = 10
Cb = 0.8
h'p = hp-(0.8*Ss/γ) [0<=h'p<=(Lcs/5)] = 0
Factor F = Lesser of:
a) F = 5
b) F = 0.35*β(γ*(lcs-5h'p)/Ss)0.5+Cb = 2.59
F = 2.59
Ca,governing(0) = lesser of:
a) Ca(0) = β(γ*h)/(CbSs)= 3.288
b) Ca(0) = F/Cb = 3.244

Ca(0) = 3.24

Governing case: Case 1

xd = 5*CbSs/γ*(Ca0-1) = 3.412 m
h' = h - CbCwSs/γ = 0.696m
Min dist where (Cw = 1.0) = 10*h' = 6.958 m

Snow Load Summary

x = 0

Cw = 1
Ca(0) = 3.244

SULS = 1.0[1(0.8*1*1*3.244)+1] = 3.595 kPa

SSLS = 0.9[1(0.8*1*1*3.244)+1] = 3.235 kPa

0< x < (xd = 3.412)m

Cw = 1
Ca(x) decreases linearly over 0 < x < xd
Ca(x) = 3.2437 - 0.6575 * x (x in meters)

x = xd = 3.412m

Cw = 1
Ca(xd) = 1

SULS = 1.0[1(0.8*1*1*1)+1] = 1.8 kPa

SSLS = 0.9[1(0.8*1*1*1)+1] = 1.62 kPa

x > ( xd = 3.412m )

Cw = 1.0
Ca(x) = 1

SULS = 1.0[1(0.8*1.0*1*1)+1] = 1.8 kPa

SSLS = 0.9[1(0.8*1.0*1*1)+1] = 1.62 kPa

This information must be read or used in conjunction with Part 4 of the NBC 2020. In the event of discrepancies, Part 4 of the NBC 2020 shall govern.

© 2009–2025 Modulo Software Inc.
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