NCC 2016 Volume Two
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Part 3.5.2 Gutters And Downpipes

Part 3.5.2 Gutters And Downpipes

Appropriate Performance Requirements

Appropriate :

Where an alternative gutter and downpipe system is proposed as a Performance Solution to that described in Part 3.5.2, that proposal must comply with—

  1. P2.2.1 ; and
  2. the relevant Performance Requirements determined in accordance with 1.0.7.

Acceptable construction manuals

3.5.2.0

P2.2.1 is satisfied for gutters and downpipes if they are designed and constructed in accordance with one of the following:

(a)

AS/NZS 3500.3.

(b)

Section 5 of AS/NZS 3500.5.

Acceptable construction practice

3.5.2.1 Application

(a)

Compliance with this acceptable construction practice satisfies Performance RequirementP2.2.1 for gutters and downpipes provided the roof drainage system is connected to a stormwater drainage system that complies with Part 3.1.2.

(b)

This Part does not apply to the removal of surface water from a storm having an average recurrence interval of 100 years for a Class 10 building where in the particular case there is no necessity for compliance.

Explanatory information

Explanatory information:
  1. The requirement to install drainage systems from roofs and sub-soil drains should be confirmed with the appropriate authority. These provisions need only be applied when drainage systems are necessary.
  2. Information on drainage requirements outside the allotment can be obtained from the appropriate authority.

3.5.2.2 Materials

Gutters, downpipes and flashings must—

(a)

be manufactured in accordance with AS/NZS 2179.1 for metal; and

(b)

be manufactured in accordance with AS 1273 for UPVC components; and

(c)

be compatible with all upstream roofing materials in accordance with 3.5.1.3(c); and

(d)

not contain any lead if used on a roof forming part of a potable water catchment area.

3.5.2.3 Selection of guttering

The size of guttering must—

(a)

for eaves gutters, be in accordance with Table 3.5.2.2; and

(b)

for box gutters, be in accordance with AS/NZS 3500.3 or Section 5 of AS/NZS 3500.5; and

(c)

be suitable to remove rainwater falling at the appropriate 5 minute duration rainfall intensity listed in Table 3.5.2.1 as follows—

(i)

for eaves gutters — 20 year average recurrence interval; and

(ii)

for eaves gutter overflow measures — 100 year average recurrence interval; and

(iii)

for box and valley gutters — 100 year average recurrence interval.

3.5.2.4 Installation of gutters

(a)

Gutters must be installed with a fall of not less than—

(i)

1:500 for eaves gutters, unless fixed to metal fascias; and

(ii)

1:100 for box gutters.

(b)

Eaves gutters must be—

(i)

supported by brackets securely fixed at stop ends and at not more than 1.2 m centres; and

(ii)

be capable of removing the overflow volume specified in Table 3.5.2.3.

(c)

Overflow measures in accordance with Table 3.5.2.4 are deemed to be capable of removing the overflow volume specified in that Table.

(d)

Valley gutters on a roof with a pitch—

(i)

more than 12.5 degrees — must have width of not less than 400 mm and be wide enough to allow the roof covering to overhang not less than 150 mm each side of the gutter; or

(ii)

not more than 12.5 degrees — must be designed as a box gutter.

(e)

The requirement of (b)(ii) does not apply to eaves gutters fixed to a verandah or an eave that is greater than 450 mm in width, which—

(i)

has no lining; or

(ii)

is a raked verandah or a raked eave with a lining sloping away from the building.

Explanatory information

Explanatory information:

Worked example — Determining appropriate overflow measures

The location of a proposed building is in Wollongong, NSW. Using Table 3.5.2.1 the 5 minute duration rainfall intensity for a 100 year average recurrence interval is 308 mm/h. The 5 minute duration rainfall intensities in Table 3.5.2.3 are provided in 25 mm/h increments, therefore for the purpose of the worked example 325 mm/h will be used.

provides required overflow volumes in both litres per second for dedicated overflow measures and litres per second per metre for continuous overflow measures. Where both dedicated and continuous measures are proposed, Table 3.5.2.3b can be used to determine the required overflow volume.

  1. Multiple overflow measures are proposed to be used with a roof catchment area of 60 m2, incorporating a 10 m eaves gutter.
  2. Using Table 3.5.2.3b for a 325 mm/h 5 minute duration rainfall intensity, the overflow volume in litres per second (L/s) for a roof catchment area of 60 m2 is 5.4 L/s.
  3. Select an acceptable dedicated overflow measure from Table 3.5.2.4b.

    The selected dedicated overflow measure is an end-stop weir which provides 0.5 L/s.

    One end-stop weir does not achieve the required overflow volume of 5.4 L/s, and additional overflow measures are required to remove the overflow volume.

  4. To achieve the required overflow volume a continuous overflow measure is also selected from Table 3.5.2.4a.

    A front face slotted gutter is the selected overflow measure as it provides 0.5 L/s/m. Taking account of the eaves gutter length (10 m), the combined overflow measures (0.5 L/s for the end-stop weir and 0.5 L/s/m × 10 m) will remove up to 5.5 L/s.

  5. The 5.5 L/s capacity provided by the selected overflow measures exceeds the required 5.4 L/s overflow volume.

3.5.2.5 Downpipes — size and installation

Downpipes must—

(a)

not serve more than 12 m of gutter length for each downpipe; and

(b)

be located as close as possible to valley gutters; and

(c)

be selected in accordance with the appropriate eaves gutter section as shown in Table 3.5.2.2.

Explanatory information

Explanatory information:

A maximum 12 m gutter length served by each downpipe is to ensure effective fall and adequate capacity to discharge all water anticipated during a storm having an average recurrence interval of 20 years.

Where a rainhead overflow device is incorporated in the top of the downpipe, its overflow discharge should be directed away from the building.

Table 3.5.2.1 RAINFALL DURATION INTENSITIES

Locality 5 minute duration rainfall intensity (mm/h) Locality 5 minute duration rainfall intensity (mm/h)
Average recurrence interval, once in— Average recurrence interval, once in—
20 years 100 years 20 years 100 years
ACT SA
Canberra 143 193 Adelaide
 124
 184
Gungahlin
 137
 179 Gawler
 110
 158
Tuggeranong
 148 210 Mt Gambier 103 144
Murray Bridge 120 178
NSW Port Augusta 133 199
Albury 139 180 Port Pirie 122 181
Broken Hill 143 219 Yorketown 155 166
Goulburn 121 156
Kiama 226 319 TAS
Newcastle 226 316 Burnie 128 180
Orange 142 186 Flinders Island 122 166
Sydney 200 262 Hobart 85 116
Avalon 206 278 Launceston 90 121
Campbelltown 167 222 Queenstown 94 120
Penrith 180 244 St. Marys 146 203
Windsor 175 233
Tweed Heads 252 330 VIC
Wollongong 217 308 Ballarat 131 188
Benalla 146 194
NT Geelong 102 144
Alice Springs 166 239 Horsham 120 173
Darwin 233 274 Lakes Entrance 145 198
Katherine 216 250 Melbourne 132 187
Hastings 117 145
QLD Sorrento 106 140
Bamaga 252 298 Mildura 142 218
Brisbane 234 305 Stawell 130 186
Ipswich 211 278
Victoria Point 245 320 WA
Bundaberg 265 340 Albany 125 178
Cairns 229 278 Broome 232 287
Cloncurry 218 278 Bunbury 147 199
Innisfail 248 301 Derby 211 256
Mackay 250 316 Geraldton 138 193
Mt Isa 199 260 Kalgoorlie 137 204
Noosa Heads 258 331 Perth 130 172
Rockhampton 229 300 Joondalup 133 180
Toowoomba 203 268 Midland 122 163
Townsville 235 300 Port Hedland 168 230
Weipa 239 283 Tom Price 138 182
Note: Locations used in this table are based on the nearest Bureau of Meteorology grid cell latitude and longitude to the central Post Office of each city or town, with the exception of Tom Price in WA, which uses the Police Station.

Table 3.5.2.2 GUTTER AND DOWNPIPE SELECTION

Table a. Gutter sizes for various rainfall intensities and roof catchment areas per downpipe
Design Rainfall Intensity (mm/h) (as per Table 3.5.2.1) Roof Catchment Area per Downpipe — m2
30 40 50 60 70
Size of gutter required to drain roof catchment area into one (1) downpipe (A, B, C, D, E and F defined in Table b.)
90 A or C A or C A or C A or C A or C
120 A or C A or C A or C A or C A or D
140 A or C A or C A or C A or D B or E
160 A or C A or C A or C A or E B or E
175 A or C A or C A or D B or E E
200 A or C A or C A or D B or E F
225 A or C A or C A or B E F
255 A or C A or D B or E E F
275 A or C A or D B or E F F
325 A or C B or E F F F
425 A or C E F F F
Table b. Gutter sizes for various rainfall intensities
Gutter Type (as per Table a.) Gutter description Minimum Cross Sectional Area mm2
A Medium rectangular gutter 6500
B Large rectangular gutter 7900
C 115 mm D gutter 5200
D 125 mm D gutter 6300
E 150 mm D gutter 9000
F Gutter must be designed in accordance with AS/NZS 3500.3 or Section 5 of AS/NZS 3500.5
Table c. Downpipe selection
Downpipe Section Gutter Sections — (as per Table b.)
A B C D E
75 mm dia. Yes Yes Yes Yes No
100 mm × 50 mm Yes Yes Yes Yes Yes
90 mm dia. Yes Yes Yes Yes Yes
100 mm × 75 mm Yes Yes Yes Yes Yes

Legend:

Yes—downpipe is suitable for the eaves gutter selection; and
No—downpipe is not suitable for the eaves gutter selection.

Table 3.5.2.3 OVERFLOW VOLUME

Table a. Overflow volume for continuous measure (L/s/m)
Design 5 minute duration rainfall intensity (mm/h) (from Table 3.5.2.1) Ridge to Gutter Length (m)
2 4 6 8 10 12 14 16
150 0.08 0.17 0.25 0.33 0.42 0.50 0.58 0.67
175 0.10 0.19 0.29 0.39 0.49 0.58 0.68 0.78
200 0.11 0.22 0.33 0.44 0.56 0.67 0.78 0.89
225 0.13 0.25 0.38 0.50 0.63 0.75 0.88 1.0
250 0.14 0.28 0.42 0.56 0.69 0.83 0.97 1.1
275 0.15 0.31 0.46 0.61 0.76 0.92 1.1 1.2
300 0.17 0.33 0.50 0.67 0.83 1.0 1.2 1.3
325 0.18 0.36 0.54 0.72 0.90 1.1 1.3 1.4
350 0.19 0.39 0.58 0.78 0.97 1.2 1.4 1.6
375 0.21 0.42 0.63 0.83 1.0 1.3 1.5 1.7
400 0.22 0.44 0.67 0.89 1.1 1.3 1.6 1.8
Table b. Overflow volume for dedicated measure (L/s)
Design 5 minute duration rainfall intensity (mm/h) (from Table 3.5.2.1) Roof Catchment Area (m2)
30 40 50 60 70
150 1.3 1.7 2.1 2.5 2.9
175 1.5 1.9 2.4 2.9 3.4
200 1.7 2.2 2.8 3.3 3.9
225 1.9 2.5 3.1 3.8 4.4
250 2.1 2.8 3.5 4.2 4.9
275 2.3 3.1 3.8 4.6 5.3
300 2.5 3.3 4.2 5.0 5.8
325 2.7 3.6 4.5 5.4 6.3
350 2.9 3.9 4.9 5.8 6.8
375 3.1 4.2 5.2 6.3 7.3
400 3.3 4.4 5.6 6.7 7.8

Table 3.5.2.4 ACCEPTABLE OVERFLOW MEASURES

Table a. Acceptable continuous overflow measure
Description Overflow Capacity (L/s/m) Construction

Front face slotted gutter with—

  1. a minimum slot opening area of 1200 mm2 per metre of gutter; and
  2. the lower edge of the slots installed a minimum of 25 mm below the top of the fascia.
 0.5
v2_Tab3524a1_2016.svg

Controlled back gap with—

  1. a permanent minimum 10 mm spacer installed between the gutter back and the fascia; and
  2. one spacer per bracket, with the spacer not more than 50 mm wide; and
  3. the back of the gutter installed a minimum of 10 mm below the top of the fascia.
 1.5
v2_Tab3524a2_2016.svg
Controlled front bead height with the front bead of the gutter installed a minimum of 10 mm below the top of the fascia. 1.5
v2_Tab3524a3_2016.svg
Table b. Acceptable dedicated overflow measure per downpipe
Description Overflow Capacity (L/s/m) Construction

End-stop weirNote 1 with—

  1. a minimum clear width of 100 mm; and
  2. the weir edge installed a minimum 25 mm below the top of the fascia.
 0.5
v2_Tab3524b1_2016.svg

Inverted nozzle installed within 500 mm of a gutter high point with—

  1. a minimum nozzle size of 100 mm × 50 mm positioned lengthways in the gutter; and
  2. the top of the nozzle installed a minimum of 25 mm below the top of the fascia.
 1.2
v2_Tab3524b2_2016.svg

Front face weir with—

  1. a minimum clear width of 200 mm; and
  2. a minimum clear height of 20 mm; and
  3. the weir edge installed a minimum of 25 mm below the top of the fascia.
 1.0
v2_Tab3524b3_2016.svg

Rainhead with—

  1. a 75 mm diameter hole in the outward face of the rainhead; and
  2. the centreline of the hole positioned 100 mm below the top of the fascia.
 3.5
v2_Tab3524b4_2016.svg

Notes:

  1. An end-stop weir is not suitable where the end-stop abuts a wall.
  2. The rainhead should be detailed to avoid nuisance discharge from the overflow at rainfall intensities below the normal design level.

Explanatory information

Explanatory information:

Stormwater drainage systems specified in the Housing Provisions are not designed to remove all water to an appropriate outfall during exceptionally heavy rain, particularly in tropical areas. Specifically, eaves gutter systems are designed to remove water arising from rainfall events with an average recurrence interval of 20 years provided they are not blocked. Accordingly, it is necessary to design and install the system to incorporate overflow measures so that when overflowing occurs, during a rainfall event with an average recurrence interval of up to 100 years, any water is directed away in a manner which ensures it does not pond against, enter or damage the building, even if the stormwater drainage system is blocked.

Insufficient and poorly located downpipes are a frequent cause of poor roof drainage system performance. The installation of downpipes, especially near valley gutters, is designed to ensure rainwater from areas on the roof that have concentrated water flows is adequately removed.

Particular consideration needs to be given to box gutters, valley gutters etc. located above the internal areas of a building.

There are several options available to designers using the requirements of the Housing Provisions. The designer will need to choose an overflow system that will cope with the rainfall intensity for the particular location. Consideration needs to be given to the total capacity of overflow measures on lower level roofs where overflow measures adopted for a higher roof catchment will result in overflow to a lower one. Overflow discharge onto lower roofs may also require consideration of sarking, flashing and other weatherproofing precautions to the lower roof area.

The acceptable overflow measures in Table 3.5.2.3 were calculated using the following formulas:

For continuous slots or rainhead For front face weir, end stop weir, inverted nozzle, front bead or controlled gap
v2_Tab3523A_2016.svg
v2_Tab3523B_2016.svg

Where—

A = Area (m2)

Cd = Discharge coefficient = 0.61

g = Gravity = 9.81 m/s2

h = Effective head (m)

Q = Flow rate (m3/s)

Where—

b = Width (m)

Cd = Discharge coefficient = 0.63

g = Gravity = 9.81 m/s2

h = Effective head (m)

Q = Flow rate (m3/s)