Gutter size calculator

How it works

Choose the state or territory and the nearest locality listed in NCC 2022 Housing Provisions Table 7.4.3d. The reference provides the site design rainfall as a 5-minute, 5% annual exceedance probability (AEP) intensity in millimetres per hour. Locality values follow the NCC table, which uses the nearest Bureau of Meteorology grid cell to each town or city central post office.

Measure the horizontal roof catchment that drains to each downpipe. If the roof is split among several downpipes, determine the area flowing to each one and enter the largest catchment for the downpipe layout being checked. Do not enter the whole roof area as though it drains to one downpipe unless that is actually the catchment. The NCC table selects gutter type using catchment area per downpipe, not roof area alone.

Enter how many downpipes have that same catchment and the length of gutter served by each. The NCC 2022 Housing Provisions limit each downpipe to 12 m of gutter. If the downpipes have different catchments, check the largest one separately and have the overall layout reviewed.

The calculator uses Table 7.4.3a with the next equal or higher rainfall row and catchment column. For a listed locality below the table’s minimum rainfall, it uses the lowest published row (90 mm/h) as a conservative ceiling band. It does not extrapolate above the table’s highest row. Table 7.4.3b names each gutter type and its minimum cross-section, and Table 7.4.3c lists compatible downpipe sections.

Formula

• Design rainfall = the selected locality’s 5-minute, 5% AEP intensity from NCC Table 7.4.3d.

• Table catchment band = the smallest listed area column that is equal to or greater than the roof catchment draining to one downpipe.

• Table rainfall band = the smallest listed intensity row that is equal to or greater than the locality’s design rainfall.

• Gutter type = the Table 7.4.3a cell at the selected intensity row and per-downpipe catchment column. The cell may offer alternatives such as A or C. Type F means the gutter must be designed to AS/NZS 3500.3, this calculator does not estimate its dimensions.

• Total eaves gutter length for this matching layout = gutter length served by each downpipe × the number of downpipes with that same arrangement. This quantity does not size or verify any other roof drainage component.

Worked example

1. 1

   The ABCB explains that Mackay, Queensland has a 5-minute 5% AEP intensity of 250 mm/h. For one downpipe serving a 30 m² roof catchment, use the next higher table intensity row, 255 mm/h, and the 30 m² column. Table 7.4.3a gives gutter type A or C. Table 7.4.3b identifies these as Medium rectangular gutter (6500 mm² minimum cross-section) or 115 mm D gutter (5200 mm² minimum cross-section).

2. 2

   If that same arrangement is used on 1 downpipe, the layout contains 6 m of gutter when each serves 6 m. The entered catchment is per downpipe, so the example does not divide a whole-roof total or assume how runoff is distributed.

Assumptions and limits

This calculator follows the NCC 2022 ABCB Housing Provisions Part 7.4 table method for Class 1 and Class 10 buildings, where accepted by the relevant jurisdiction. The NCC 2022 Victoria H2D6 variation requires AS/NZS 3500.3, so this calculator does not provide a Victorian result. NCC 2025 adoption and other state or territory changes may affect which edition or method applies. Confirm the applicable code edition, variations and approval requirements with the local building authority before using the estimate.

Part 7.4 is an eaves gutter method. It does not size box gutters, valley gutters, gutter overflow measures or Type F gutters. Those require separate NCC provisions or design to AS/NZS 3500.3. This tool also does not design the roof, calculate downpipe flow capacity outside Table 7.4.3c, size stormwater drains, or verify site drainage.

The table covers per-downpipe catchments from 30 to 70 m² and design rainfall rows from 90 to 425 mm/h. Catchments outside the published area columns and rainfall above the highest row are unsupported. Hobart’s published 86 mm/h value uses the minimum 90 mm/h row as a conservative ceiling. For rainfall above 255 mm/h, NCC 2022 Volume Two H2D2 does not allow the Part 3.3 drainage alternative, the complete drainage system must use an applicable standard or accepted design route.

The number of downpipes is entered by the user. It is not inferred from total roof area. Roof geometry, multiple planes, valleys, obstructions, gutter falls, outlet positions and uneven runoff can change each downpipe catchment. Check the roof drainage layout with a licensed plumber or suitably qualified designer.

This tool reports gutter type alternatives rather than selecting a commercial profile. Verify that the product meets the minimum cross-section, overflow configuration and downpipe compatibility in the relevant NCC tables, and confirm current local product dimensions with the supplier.

Frequently asked questions