USING NEW ZEALAND BUILDING CODE CLAUSE E1 SURFACE WATER TO SIZE GUTTERS AND DOWNPIPES
DESIGN
RIGHT
TOM EDHOUSE,
BRANZ TECHNICAL
ADVISOR
Sizing gutters
and downpipes
We work through an example to show how to size gutters and downpipes using
New Zealand Building Code clause E1 Surface water.
downpipe
3m
fall
C
2.5 m
fall
5m
NEW ZEALAND BUILDING CODE clause E1 uses plan
areas of the roof to find the appropriate down-
fall
pipe and gutter sizes. Individual roof plan areas
1.5 m
roof pitch 45°
contribute to the total roof plan area.
Work out the roof plan areas
When working out gutter sizes, clause E1 requires
gutters to be divided into sections. A section is
the length of gutter between a downpipe and the
A
B
fall
fall
11 m
adjacent high point on one side of that gutter.
We will use Figure 1 as an example.
downpipe
Total roof plan area =
88 + 10 = 98 m²
downpipe
2m
Roof plan area for A =
2 × 2.5 = 5
8m
10 m
4 × 8.5 = 34
1.5 × 0.75 = 1.125
Figure 1
Total A = 5 + 34 – 1.125 = 37.875 m2
An example of a roof with a total plan area of 98 m².
Sizing downpipes
Areas A and B require 74 mm diameter downpipes
Roof plan area for B =
Downpipes are sized for the areas that discharge
and Area C requires a 63 mm diameter downpipe
7 × 4 = 28
into them. Usually, the size for the largest
(see Figure 2). Therefore, 74 mm downpipes are
4×2=8
collection area on the roof is used to size
adequate for A, B and C.
Total B = 28 + 8 = 36 m²
downpipes throughout.
Sizing gutters
Using clause E1 Table 5 (see Figure 2) at a roof
Roof plan area for C =
pitch of 45° for this example:
7.5 × 2.5 = 18.75
●●
2.5 × 1.25 = 3.125
3.0 × 0.75 = 2.25
Total C = 18.75 + 3.125 + 2.25 = 24.125 m²
●●
63 mm diameter downpipes serve roof plans
In this example, the rainfall intensity is 100 mm
per hour.
The cross-sectional area of each section of
up to 35 m²
gutter is determined from clause E1 Figure 15
74 mm diameter downpipes serve roof plans
for external gutters or Figure 16 for internal
up to 50 m².
gutters.
Build 152 — February/March 2016 — 35
d by Table 5,
than that require
here.
mm diameter sp
passage of a 50
Table
5:
Pitch and Area
for Given Roof
Downpipe Sizes
Paragraph 4.2.1
(mm)
Downpipe size
al sizes)
(minimum intern
Amend 1
Sep 1993
inte
gutter size
/hr the required
exceeds 100 mm
read
by taking the value
shall be increased
by the ratio
and multiplying it
from the figures
es how to
aph 3.2.2 describ
of “I”/100. Paragr
lue of “I”.
determine the va
0-25°
60
85
155
350
60
100
110
150
63 mm diameter
74 mm diameter
100 mm diameter
150 mm diameter
lar
65 x 50 rectangu
lar
gu
tan
rec
50
x
0
10
lar
75 x 75 rectangu
lar
100 x 75 rectangu
Roof pitch
25-35°
Figure 2
2
wnpipe (m )
served by the do
Plan area of roof
40
50
60
70
110
130
250
290
40
50
70
80
80
90
105
0
12
42
/A S1
lu tio n E1 Code clause E1 Table 5.
New
Zealand
ab le SoBuilding
Ac ce pt
er 2011
D
Amend 1
Sep 1993
45-55°
35-45°
SURFACE
Amend 2
Aug 1994
35
50
90
200
35
60
65
90
Amend 2
Aug 1994
WAT E R
SING
G AND HOU
OF BUILDIN
E PA R T M E N T
10 Octob
Figure 15:
r
ternal Gutte
nal Area of Ex
Cross-sectio
.3
5.1
.2 and
Paragraphs 5.1
Amend 1
Sep 1993
Figure 3
r
ternal Gutte
nal Area of In
Cross-sectio
.3
5.1
d
an
.2
hs 5.1 E1 Figure 15.
ragrap
New Zealand BuildingPa
Code
clause
Figure 16:
Gutters will be the same size for all the roof
This is an external gutter, so using clause E1
In this case, the cross-sectional area of a
so use the largest plan area to work out the
Figure 15, plot 38–40 m² plan area on the vertical
section of gutter is approximately 7,000–8,000
appropriate size gutter. The largest plan area is
axis. Then read off the gutter size for the roof
mm², so a 125 × 70 mm gutter (8,750 mm²) will
area A at 38 m².
pitch of 45° (see Figure 3).
meet the requirements of E1.
36 — February/March 2016 — Build 152