What makes a gutter a box gutter
An eaves gutter hangs on a fascia board, outside the building, on an eaves overhang. A box gutter sits within the building’s footprint. The Commission’s practice note on box gutters is written for designers, builders, owner-builders, building surveyors, engineers, plumbers and trades, and this page reports what it says.
Whether a box gutter is brand new or replaces an old one, it has to meet the Plumbing Code of Australia’s performance requirements; that code is Volume Three of the National Construction Code. A gutter that complies with AS/NZS 3500.3 and the handbook HB 39 satisfies them automatically; otherwise compliance has to be shown through a performance solution.
The rules below are Victoria’s guidance, as its regulator has published it. Outside Victoria, check with your own state’s building or plumbing regulator.
The numbers in Victoria’s practice note
These are the parameters the Commission lists for a box gutter to count as a deemed-to-satisfy installation, for sole widths between 200 mm and 600 mm.
| Item | What the practice note says |
|---|---|
| Minimum width | 200 mm for domestic work, 300 mm for commercial. |
| Sole widths for sizing | 200, 300, 375, 450, 525 and 600 mm; a width in between may be used if it is sized to the lower width. |
| Flow at 200 mm | A 200 mm sole cannot take flow above 10.5 litres per second. |
| Flow per downpipe | Each downpipe’s maximum design flow has to fall between 3 and 16 litres per second for it to be plotted. |
| Design storm | Sizing is based on a 1% annual exceedance probability (a 100-year average recurrence interval). |
| Grade | 1:200, 1:150, 1:100 or 1:40. |
| Shape and line | Straight, with no change of direction, discharging at the downstream end. V-shaped gutters are not permitted. |
| Outlet | A sump or a rainhead, fixed and fully sealed to the gutter. |
| Rainhead overflow | A weir the full width of the rainhead, 25 mm below the gutter sole, fully open above the weir at the front. |
| Sumps | As wide as the gutter. With a high-capacity overflow, always 600 mm long and at least 150 mm deep; with a side overflow, at least 400 mm long. |
| Expansion | Required in every box gutter: where the distance between fixed points exceeds 6 metres, and at the intervals the standard prescribes for the material and situation. |
| Laps | 25 mm, sealed and fastened in the direction of fall. |
| Support | Must carry the whole gutter and sumps when full of water, plus a trafficable load. Continuous support suits all widths; bracket support, at intervals of no more than 750 mm, only suits sole widths under 450 mm. |
Why box gutters fail
The Commission’s documents name the failures directly.
- The wrong shape. A V-shaped gutter may fail early through inadequate drainage, permanent ponding, and debris gathering in the crevices, which may lead to intense localised corrosion.
- Overflows too small or in the wrong place. These can lead to serious damage to buildings and contents.
- A covered rainhead. A shroud fitted around a rainhead forms a vertical chute, for which there are no deemed-to-satisfy examples or sizing methods.
- No room to move. Roofing materials expand and contract with temperature; if the installation does not allow for it, the result can be unsightly oil-canning and structural problems.
- Mismatched metals. The support system must be compatible with the gutter, or the gutter must be protected against corrosion from an incompatible support.
A worked example
Suppose a house’s box gutter was drawn at a 200 mm sole width, its single downpipe is calculated to carry 12 litres per second, and the gutter runs 8.4 metres between fixed points. Checked against the practice note:
- 12 L/s sits inside the 3 to 16 L/s range a downpipe can be plotted for.
- 12 L/s is 1.5 L/s more than the 10.5 L/s a 200 mm sole can take, so a 200 mm gutter would fall outside the deemed-to-satisfy parameters. The next plotted width is 300 mm.
- 8.4 metres is more than the 6 metres between fixed points at which expansion provision is required. The practice note asks for provision in every box gutter as well, at the intervals the standard prescribes for the material and situation, so a shorter run is not exempt.
- If the outlet is a sump with a high-capacity overflow, that sump is 600 mm long and at least 150 mm deep.
The sizing itself uses the general methods in AS/NZS 3500.3, and the Commission says it is up to the licensed plumber to confirm the gutter complies when developing the incidental design for the site.
Replacing an old box gutter
There is no like-for-like shortcut. The Commission says the compliance pathway for roofing makes no provision for like-for-like replacement, so all new and replacement work has to show compliance with the National Construction Code’s performance requirements, either as deemed-to-satisfy or as a performance solution.
When a plumber spots a roof design that departs from the deemed-to-satisfy provisions, their first step must be to contact the builder and the relevant building surveyor and settle how the work will be certified. Where the roof frame will not take a compliant gutter, altering any structural element needs an approved building permit, new or amended, before that work starts. Box gutters 800 mm or 1000 mm wide are outside the scope of AS/NZS 3500.3, and can only be designed, installed and certified as a performance solution.
For who may do this work in Victoria and elsewhere, see who can do roof plumbing work.
More from the guide
- Signs a gutter needs repairWhat can be seen from the ground, and what official sources say about it.
- Ladders, roofs and guttersWhat the work health and safety regulators say about ladders and roof edges.
- Gutters and ember attackWhat the NSW RFS, the CFA and the QBCC ask of a gutter before fire season.
- Who can do roof plumbing workLicences for gutter and roof drainage work in NSW, Victoria and Queensland.
- Where downpipe water may goStormwater, the sewer, tanks, and the pipes an owner looks after.