Part 7.4 Gutters and downpipes
New for 2022
- 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.
- Information on drainage requirements outside the allotment can be obtained from the appropriate authority.
- Where box gutters are proposed to be installed, AS/NZS 3500.3 may be used to calculate minimum sizes, falls and overflow requirements.
- For Class 10 buildings, it may not be necessary to comply with the requirements for removing surface water where the Class 10 building is not connected to or does not impact a Class 1 building. For example, where a Class 10 garage is attached to a Class 1 dwelling, the run-off from the garage would most likely directly impact the dwelling and therefore be required to be removed. However, a garage that is separated by a reasonable distance from the dwelling so as to not have an impact would not necessarily have to comply with the requirements for removal of surface water.
- The following are a number of other Clauses and Parts of the ABCB Housing Provisions that contain requirements related to drainage and roofing in addition to the provisions of this Part:
Explanatory information: Design of stormwater drainage systems
Stormwater drainage systems specified in the NCC Volume Two and the ABCB 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 annual exceedance probability of 5% 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 annual exceedance probability of up to 1%, 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 NCC Volume Two and the ABCB 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 7.4.4a and Table 7.4.4b were calculated using the following formulas:
- For continuous slots or rainhead:
= Area (m2)
= Discharge coefficient = 0.61
= Gravity = 9.81 m/s2
= Effective head (m)
= Flow rate (m3/s)
- For front face weir, end stop weir, inverted nozzle, front bead or controlled gap:
= Width (m)
= Discharge coefficient = 0.63
= Gravity = 9.81 m/s2
= Effective head (m)
= Flow rate (m3/s)
Gutters, downpipes and flashings must—
- be manufactured in accordance with AS/NZS 2179.1 for metal components; and
- be manufactured in accordance with AS 1273 for UPVC components; and
- be compatible with all upstream roofing materials in accordance with 7.2.2(2); and
- not contain any lead if used on a roof forming part of a drinking water catchment area.
Selection of guttering
The size of guttering must—
- for eaves gutters, be in accordance with Table 7.4.3a, Table 7.4.3b and Table 7.4.3c; and
- be suitable to remove rainwater falling at the appropriate 5 minute duration rainfall intensity listed in Table 7.4.3d as follows—
- for eaves gutters — 5% annual exceedance probability; and
- for eaves gutter overflow measures — 1% annual exceedance probability.
|Design rainfall intensity (mm/h) (as per Table 7.4.3d)||Roof catchment area per downpipe — 30 m2||Roof catchment area per downpipe — 40 m2||Roof catchment area per downpipe — 50 m2||Roof catchment area per downpipe — 60 m2||Roof catchment area per downpipe — 70 m2|
|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|
|Gutter type||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||N/A|
|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 x 50 mm||Yes||Yes||Yes||Yes||Yes|
|90 mm dia.||Yes||Yes||Yes||Yes||Yes|
|100 mm x 75 mm||Yes||Yes||Yes||Yes||Yes|
- Yes — downpipe is suitable for the eaves gutter selection.
- No — downpipe is not suitable for the eaves gutter selection.
|State||Locality||Annual exceedance probability, 5% (mm/h)||Annual exceedance probability, 1% (mm/h)|
|QLD||Victoria Point, Brisbane||245||320|
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.
The cross sectional area referred to in Table 7.4.3b is measured up to the lowest part of the relevant overflow facility including the lower edge of a slot, gutter back, end-stop weir, inverted nozzle, front-face weir or overflow opening in a rainhead.
Explanatory information: Worked example — determining appropriate overflow measures
The location of a proposed building is in Wollongong, NSW. Using Table 7.4.3d the 5 minute duration rainfall intensity for a 1% annual exceedance probability is 311 mm/h. The 5 minute duration rainfall intensities in Table 7.4.4a and Table 7.4.4b are provided in 25 mm/h increments, therefore for the purpose of the worked example 325 mm/h will be used.
Table 7.4.4a and Table 7.4.4b provide required overflow volumes in both litres per second for dedicated overflow measures and litres per second per metre for continuous overflow measures. Extrapolation of the values in these tables can be used to inform a Performance Solution complying with the Governing Requirements of the NCC. Where both dedicated and continuous measures are proposed, Table 7.4.4b can be used to determine the required overflow volume.
- Multiple overflow measures are proposed to be used with a roof catchment area of 60 m2, incorporating a 10 m eaves gutter.
- Using Table 7.4.4b 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.
- Select an acceptable dedicated overflow measure from 7.4.7.
- To achieve the required overflow volume a continuous overflow measure is also selected from 7.4.6.
- 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.
- The 5.5 L/s capacity provided by the selected overflow measures exceeds the required 5.4 L/s overflow volume.
Installation of gutters
- installed with a fall of not less than 1:500; and
- supported by brackets securely fixed at stop ends, corners and at not more than 1.2 m centres; and
- fitted with overflow measures capable of removing the overflow volume specified in Table 7.4.4a and Table 7.4.4b.
- be installed on a roof with a pitch more than 12.5 degrees; and
- have dimensions in accordance with Table 7.4.4c for the relevant rainfall intensity; and
- have minimum freeboard of not less than 15 mm; and
- have a side angle of not less than 12.5 degrees.
- has no lining; or
- is a raked verandah or a raked eave with a lining sloping away from the building.
|Design 5 minute duration rainfall intensity (mm/h) (from Table 7.4.3d)||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||0.78 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|
|Design 5 minute duration rainfall intensity (mm/h) (from Table 7.4.3d)||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|
|Design rainfall intensity mm/h||Sheet width (minimum, mm)||Effective depth (he), (minimum, mm)||Effective width (we), (minimum, mm)|
|>200 to ≤ 250||375||35||234|
|>250 to ≤300||395||38||254|
|>300 to ≤350||415||40||273|
|>350 to ≤400||435||43||292|
Explanatory information: Valley gutters
- Where roofs have pitches less than 12.5 degrees valley gutters may be designed as box gutters in accordance with AS/NZS 3500.3 or as a Performance Solution by a professional engineer or other appropriately qualified person.
- An example of a valley gutter profile is shown in Explanatory Figure 7.4.4.
Downpipes – size and installation
- not serve more than 12 m of gutter length for each downpipe; and
- be located as close as possible to valley gutters; and
- be selected in accordance with the appropriate eaves gutter section as shown in Table 7.4.3a, Table 7.4.3b and Table 7.4.3c.
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 annual exceedance probability of 5%.
Where a rainhead overflow device is incorporated in the top of the downpipe, its overflow discharge should be directed away from the building.
Acceptable continuous overflow measure
2019: Table 220.127.116.11a
- a minimum slot opening area of 1200 mm2 per metre of gutter; and
- the lower edge of the slots installed a minimum of 25 mm below the top of the fascia,
the acceptable overflow capacity must be 0.5 L/s/m, constructed in accordance with Figure 7.4.6a.
- a permanent minimum 10 mm spacer installed between the gutter back and the fascia; and
- one spacer per bracket, with the spacer not more than 50 mm wide; and
- the back of the gutter installed a minimum of 10 mm below the top of the fascia,
the acceptable overflow capacity must be 1.5 L/s/m, constructed in accordance with Figure 7.4.6b.
Acceptable dedicated overflow measure per downpipe
2019: Table 18.104.22.168b
- a minimum clear width of 100 mm; and
- the weir edge installed a minimum 25 mm below the top of the fascia,
the acceptable overflow is 0.5 L/s constructed in accordance with Figure 7.4.7a.
- a minimum nozzle size of 100 mm × 50 mm positioned lengthways in the gutter; and
- the top of the nozzle installed a minimum of 25 mm below the top of the fascia,
the acceptable overflow is 1.2 L/s constructed in accordance with Figure 7.4.7b.
- a minimum clear width of 200 mm; and
- a minimum clear height of 20 mm; and
- the weir edge installed a minimum of 25 mm below the top of the fascia,
the acceptable overflow capacity is 1.0 L/s constructed in accordance with Figure 7.4.7c.
- a 75 mm diameter hole in the outward face of the rainhead; and
- the centreline of the hole positioned 100 mm below the top of the fascia,
the acceptable overflow capacity is 3.5 L/s constructed in accordance with Figure 7.4.7d.