Page 37 - Build 152

USING NEW ZEALAND BUILDING CODE CLAUSE E1 SURFACE WATER TO SIZE GUTTERS AND DOWNPIPES
DESIGN
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
2.5 m
1.5 m
11 m
downpipe
RIGHT
TOM EDHOUSE,
BRANZ TECHNICAL ADVISOR
3m
NEW ZEALAND BUILDING CODE clause E1 uses plan areas of the roof to nd the appropriate down- pipe and gutter sizes. Individual roof plan areas 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 adjacent high point on one side of that gutter.
We will use Figure 1 as an example.
Total roof plan area = 88 + 10 = 98 m2
Roof plan area for A =
2 × 2.5 = 5
4 × 8.5 = 34
1.5 × 0.75 = 1.125
Total A = 5 + 34 – 1.125 = 37.875 m2
Roof plan area for B =
7 × 4 = 28
4 × 2 = 8
Total B = 28 + 8 = 36 m2
Roof plan area for C =
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
5 m
fall C fall
fall
roof pitch 45°
AB
fall
fall
downpipe
Figure 1
2 m
8 m
10 m
An example of a roof with a total plan area of 98 m2.
Sizing downpipes
Downpipes are sized for the areas that discharge into them. Usually, the size for the largest collection area on the roof is used to size downpipes throughout.
Using clause E1 Table 5 (see Figure 2) at a roof pitch of 45° for this example:
● 63 mm diameter downpipes serve roof plans
up to 35 m2
● 74 mm diameter downpipes serve roof plans
up to 50 m2.
Areas A and B require 74 mm diameter downpipes and Area C requires a 63 mm diameter downpipe (see Figure 2). Therefore, 74 mm downpipes are adequate for A, B and C.
Sizing gutters
In this example, the rainfall intensity is 100 mm per hour.
The cross-sectional area of each section of gutter is determined from clause E1 Figure 15 for external gutters or Figure 16 for internal gutters.
Build 152 — February/March 2016 — 35