NCC 2019 Volume Two
Search the National Construction Code editions
Classification
Building class 1a Building class 1b Building class 2 Building class 3 Building class 4 Building class 5 Building class 6 Building class 7a Building class 7b Building class 8 Building class 9a Building class 9b Building class 9c Building class 10a Building class 10b Building class 10c

Filter

Classification
Building class 1a Building class 1b Building class 2 Building class 3 Building class 4 Building class 5 Building class 6 Building class 7a Building class 7b Building class 8 Building class 9a Building class 9b Building class 9c Building class 10a Building class 10b Building class 10c
Part 3.5.3 Gutters and downpipes

Part 3.5.3 Gutters and downpipes

Appropriate Performance Requirements

Appropriate Performance Requirements

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. Performance RequirementP2.1.1; and
  2. the relevant Performance Requirement s determined in accordance with A2.2(3) and A2.4(3) as applicable.

Acceptable Construction Manuals

3.5.3.0 Application

Performance RequirementP2.1.1 is satisfied for gutters and downpipes if they are designed and constructed in accordance with AS/NZS 3500.3.

State and Territory Variations

State and Territory Variations

In Victoria, except for 3.5.3.0 - Acceptable construction manuals, Part 3.5.3 does not apply.

Acceptable Construction Practice

3.5.3.1 Application

(a)

Compliance with this acceptable construction practice satisfies Performance Requirement P2.1.1 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.

3.5.3.2 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 3.5.1.2(b); and

(d)

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

3.5.3.3 Selection of guttering

The size of guttering must—

(a)

for eaves gutters, be in accordance with Table 3.5.3.2a to Table 3.5.3.2c; and

(b)

for box gutters, be in accordance with AS/NZS 3500.3; and

(c)

be suitable to remove rainwater falling at the appropriate 5 minute duration rainfall intensity listed in Table 3.5.3.1a to Table 3.5.3.1h 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.

3.5.3.4 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 3.5.3.3a and Table 3.5.3.3b.

(c)

Overflow measures in accordance with Table 3.5.3.4a and Table 3.5.3.4b 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 3.5.3.3a and Table 3.5.3.3b are provided in 25 mm/h increments, therefore for the purpose of the worked example 325 mm/h will be used.

Table 3.5.3.3a and Table 3.5.3.3b 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 3.5.3.3b 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 3.5.3.3b 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 3.5.3.4b.

    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 3.5.3.4a.

    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.

3.5.3.5 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 3.5.3.2a to Table 3.5.3.2c.

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 3.5.3.1a 5 minute duration rainfall intensities for the Australian Capital Territory

Locality

average recurrence interval , once in 20 years (mm/h)

average recurrence interval , once in 100 years (mm/h)

Canberra

143

193

Gungahlin

137

179

Tuggeranong

148

210

Note to Table 3.5.3.1a: 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.

Table 3.5.3.1b 5 minute duration rainfall intensities for New South Wales

Locality

average recurrence interval , once in 20 years (mm/h)

average recurrence interval , once in 100 years (mm/h)

Albury

139

180

Broken Hill

143

219

Goulburn

121

156

Kiama

226

319

Newcastle

226

316

Orange

142

186

Sydney

200

262

Avalon, Sydney

206

278

Campbelltown, Sydney

167

222

Penrith, Sydney

180

244

Windsor, Sydney

175

233

Tweed Heads

252

330

Wollongong

217

308

Note to Table 3.5.3.1b: 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.

Table 3.5.3.1c 5 minute duration rainfall intensities for the Northern Territory

Locality

average recurrence interval , once in 20 years (mm/h)

average recurrence interval , once in 100 years (mm/h)

Alice Springs

166

239

Darwin

233

274

Katherine

216

250

Note to Table 3.5.3.1c: 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.

Table 3.5.3.1d 5 minute duration rainfall intensities for Queensland

Locality

average recurrence interval , once in 20 years (mm/h)

average recurrence interval , once in 100 years (mm/h)

Bamaga

252

298

Brisbane

234

305

Ipswich, Brisbane

211

278

Victoria Point, Brisbane

245

320

Bundaberg

265

340

Cairns

229

278

Cloncurry

218

278

Innisfail

248

301

Mackay

250

316

Mt Isa

199

260

Noosa Heads

258

331

Rockhampton

229

300

Toowoomba

203

268

Townsville

235

300

Weipa

239

283

Note to Table 3.5.3.1d: 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.

Table 3.5.3.1e 5 minute duration rainfall intensities for South Australia

Locality

average recurrence interval , once in 20 years (mm/h)

average recurrence interval , once in 100 years (mm/h)

Adelaide

124

184

Gawler, Adelaide

110

158

Mt Gambier

103

144

Murray Bridge

120

178

Port Augusta

133

199

Port Pirie

122

181

Yorketown

155

166

Note to Table 3.5.3.1e: 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.

Table 3.5.3.1f 5 minute duration rainfall intensities for Tasmania

Locality

average recurrence interval , once in 20 years (mm/h)

average recurrence interval , once in 100 years (mm/h)

Burnie

128

180

Flinders Island

122

166

Hobart

85

116

Launceston

90

121

Queenstown

94

120

St. Marys

146

203

Note to Table 3.5.3.1f: 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.

Table 3.5.3.1g 5 minute duration rainfall intensities for Victoria

Locality

average recurrence interval , once in 20 years (mm/h)

average recurrence interval , once in 100 years (mm/h)

Ballarat

131

188

Benalla

146

194

Geelong

102

144

Horsham

120

173

Lakes Entrance

145

198

Melbourne

132

187

Hastings, Melbourne

117

145

Sorrento, Melbourne

106

140

Mildura

142

218

Stawell

130

186

Note to Table 3.5.3.1g: 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.

Table 3.5.3.1h 5 minute duration rainfall intensities for Western Australia

Locality

average recurrence interval , once in 20 years (mm/h)

average recurrence interval , once in 100 years (mm/h)

Albany

125

178

Broome

232

287

Bunbury

147

199

Derby

211

256

Geraldton

138

193

Kalgoorlie

137

204

Perth

130

172

Joondalup, Perth

133

180

Midland, Perth

122

163

Port Hedland

168

230

Tom Price

138

182

Note to Table 3.5.3.1h: 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.

Table 3.5.3.2a Size of gutter required to drain roof catchment area into one (1) downpipe for various rainfall intensities and roof catchment areas (A, B, C, D, E and F defined in Table 3.5.3.2b )

Design rainfall intensity (mm/h) (as per Table 3.5.2.1a to Table 3.5.2.1h )

Roof catchment area per downpipe — 30 m 2

Roof catchment area per downpipe — 40 m 2

Roof catchment area per downpipe — 50 m 2

Roof catchment area per downpipe — 60 m 2

Roof catchment area per downpipe — 70 m 2

90 mm/h

A or C

A or C

A or C

A or C

A or C

120 mm/h

A or C

A or C

A or C

A or C

A or D

140 mm/h

A or C

A or C

A or C

A or D

B or E

160 mm/h

A or C

A or C

A or C

A or E

B or E

175 mm/h

A or C

A or C

A or D

B or E

E

200 mm/h

A or C

A or C

A or D

B or E

F

225 mm/h

A or C

A or C

A or B

E

F

255 mm/h

A or C

A or D

B or E

E

F

275 mm/h

A or C

A or D

B or E

F

F

325 mm/h

A or C

B or E

F

F

F

425 mm/h

A or C

E

F

F

F
Table 3.5.3.2b Gutter sizes for various rainfall intensities

Gutter type

Gutter description

Minimum cross sectional area mm 2

A

Medium rectangular gutter

6500 mm2

B

Large rectangular gutter

7900 mm2

C

115 mm D gutter

5200 mm2

D

125 mm D gutter

6300 mm2

E

150 mm D gutter

9000 mm2

F

Gutter must be designed in accordance with AS/NZS 3500.3

N/A
Table 3.5.3.2c Downpipe selection for gutter types (A, B, C, D, E and F defined in Table 3.5.3.2b )

Downpipe section

Gutter type A

Gutter type B

Gutter type C

Gutter type D

Gutter type 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

Notes to Table 3.5.3.2c :

  1. Yes—downpipe is suitable for the eaves gutter selection.
  2. No—downpipe is not suitable for the eaves gutter selection.
Table 3.5.3.3a Overflow volume for continuous measure (L/s/m)

Design 5 minute duration rainfall intensity (mm/h) (from Table 3.5.2.1a to Table 3.5.2.1h )

Ridge to gutter length — 2 m

Ridge to gutter length — 4 m

Ridge to gutter length — 6 m

Ridge to gutter length — 8 m

Ridge to gutter length — 10 m

Ridge to gutter length — 12 m

Ridge to gutter length — 14 m

Ridge to gutter length — 16 m

150 mm/h

0.08 L/s/m

0.17 L/s/m

0.25 L/s/m

0.33 L/s/m

0.42 L/s/m

0.50 L/s/m

0.58 L/s/m

0.67 L/s/m

175 mm/h

0.10 L/s/m

0.19 L/s/m

0.29 L/s/m

0.39 L/s/m

0.49 L/s/m

0.58 L/s/m

0.68 L/s/m

0.67 L/s/m

200 mm/h

0.11 L/s/m

0.22 L/s/m

0.33 L/s/m

0.44 L/s/m

0.56 L/s/m

0.67 L/s/m

0.78 L/s/m

0.89 L/s/m

225 mm/h

0.13 L/s/m

0.25 L/s/m

0.38 L/s/m

0.50 L/s/m

0.63 L/s/m

0.75 L/s/m

0.88 L/s/m

1.0 L/s/m

250 mm/h

0.14 L/s/m

0.28 L/s/m

0.42 L/s/m

0.56 L/s/m

0.69 L/s/m

0.83 L/s/m

0.97 L/s/m

1.1 L/s/m

275 mm/h

0.15 L/s/m

0.31 L/s/m

0.46 L/s/m

0.61 L/s/m

0.76 L/s/m

0.92 L/s/m

1.1 L/s/m

1.2 L/s/m

300 mm/h

0.17 L/s/m

0.33 L/s/m

0.50 L/s/m

0.67 L/s/m

0.83 L/s/m

1.0 L/s/m

1.2 L/s/m

1.3 L/s/m

325 mm/h

0.18 L/s/m

0.36 L/s/m

0.54 L/s/m

0.72 L/s/m

0.90 L/s/m

1.1 L/s/m

1.3 L/s/m

1.4 L/s/m

350 mm/h

0.19 L/s/m

0.39 L/s/m

0.58 L/s/m

0.78 L/s/m

0.97 L/s/m

1.2 L/s/m

1.4 L/s/m

1.6 L/s/m

375 mm/h

0.21 L/s/m

0.42 L/s/m

0.63 L/s/m

0.83 L/s/m

1.0 L/s/m

1.3 L/s/m

1.5 L/s/m

1.7 L/s/m

400 mm/h

0.22 L/s/m

0.44 L/s/m

0.67 L/s/m

0.89 L/s/m

1.1 L/s/m

1.3 L/s/m

1.6 L/s/m

1.8 L/s/m
Table 3.5.3.3b Overflow volume for dedicated measure (L/s)

Design 5 minute duration rainfall intensity (mm/h) (from Table 3.5.2.1a to Table 3.5.2.1h)

Roof catchment area — 30 m2

Roof catchment area — 40 m2

Roof catchment area — 50 m2

Roof catchment area — 60 m2

Roof catchment area — 70 m2

150 mm/h

1.3 L/s

1.7 L/s

2.1 L/s

2.5 L/s

2.9 L/s

175 mm/h

1.5 L/s

1.9 L/s

2.4 L/s

2.9 L/s

3.4 L/s

200 mm/h

1.7 L/s

2.2 L/s

2.8 L/s

3.3 L/s

3.9 L/s

225 mm/h

1.9 L/s

2.5 L/s

3.1 L/s

3.8 L/s

4.4 L/s

250 mm/h

2.1 L/s

2.8 L/s

3.5 L/s

4.2 L/s

4.9 L/s

275 mm/h

2.3 L/s

3.1 L/s

3.8 L/s

4.6 L/s

5.3 L/s

300 mm/h

2.5 L/s

3.3 L/s

4.2 L/s

5.0 L/s

5.8 L/s

325 mm/h

2.7 L/s

3.6 L/s

4.5 L/s

5.4 L/s

6.3 L/s

350 mm/h

2.9 L/s

3.9 L/s

4.9 L/s

5.8 L/s

6.8 L/s

365 mm/h

3.1 L/s

4.2 L/s

5.2 L/s

6.3 L/s

7.3 L/s

400 mm/h

3.3 L/s

4.4 L/s

5.6 L/s

6.7 L/s

7.8 L/s
Table 3.5.3.4a 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_Tab3534a1_2019.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_Tab3534a2_2019.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_Tab3534a3_2019.svg

Note to Table 3.5.3.4a :

For the controlled back gap option, the spacer can be a proprietary clip or bracket that provides the required offset of the gutter from the fascia.

Table 3.5.3.4b Acceptable dedicated overflow measure per downpipe

Description

Overflow Capacity (L/s)

Construction

End-stop weir Note 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_Tab3534b1_2019.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_Tab3534b2_2019.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_Tab3534b3_2019.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_Tab3534b4_2019.svg

Notes to Table 3.5.3.4b:

  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 anaverage 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 3.5.3.3a and Table 3.5.3.3b were calculated using the following formulas:

For continuous slots or rainhead
v2_Tab3533a_2019.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)

For front face weir, end stop weir, inverted nozzle, front bead or controlled gap
v2_Tab3533b_2019.svg

Where—

b = Width (m)

Cd = Discharge coefficient = 0.63

g = Gravity = 9.81 m/s2

h = Effective head (m)

Q = Flow rate (m3/s)