The sump that keeps working, even when it blocks.
A Dam Buster free flow sump gives your box gutter two independent escape routes for stormwater. If the first chamber is overwhelmed, or blocks with debris, the second chamber takes over automatically and sends the water safely outside. No back flooding, no complex calculations, and every unit is fully engineered and certified.
- ✓NCC conforming
- ✓Meets AS 3500.3
- ✓Sized by simple table lookup
- ✓Fits box gutters 200 to 600 mm
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What does a sump actually do?
A sump is a deeper collection box fitted into a box gutter, used where the water needs to drop straight down into a downpipe below the gutter. The Dam Buster free flow version has two chambers, and that second chamber is the safety net ordinary sumps lack.
Collects
The box gutter discharges into the sump's first chamber, which feeds the primary downpipe below. In normal rain, this is the whole story.
Overflows
In an extreme storm, or if the first chamber blocks with debris, the water simply crests the internal weir and flows into the second chamber.
Protects
The second chamber disposes of the excess through its own dedicated overflow downpipe, discharging to atmosphere exactly as the code requires. An extra overflow can be fitted to the end of the second chamber for multiple redundancy.
Why free flow matters
Standard code sumps rely on back flooding, meaning the water level has to rise inside the box gutter before the overflow starts working, and every installation needs complex custom design and certification. The Dam Buster design lets water pass straight through in both the normal and the overflow conditions, so the box gutter can be designed independently and the whole system is dramatically simpler to get right.
Watch the video to see the free flow sump handle a full overflow event.
Sizing a sump takes one table.
Find your 1:100 year flow rate in litres per second, then read your sump size straight off the selection table in our Product Technical Statement. Every size is factory made in 100 mm increments for box gutters from 200 to 600 mm.
One sump, many jobs.
Every free flow sump is part of the integrated Dam Buster roof drainage system and fits virtually any box gutter, new or retrofit. Here are the most common ways to put one to work.
Replace a code sump
Dam Buster sumps can be safely used in place of the code specified sump and side overflow devices. Selection is far simpler, performance is far better, and the installed cost is potentially a lot cheaper.
Where a rain head can't go
A window or door below the discharge point rules out a rain head. A sump handles the same job from inside the gutter, and both the normal and overflow downpipes can be concealed and directed internally away from the sump location.
Indicator overflow
Fit an additional indicator overflow to the end of the second chamber. Water appearing there signals that the primary chamber needs attention, and it adds yet another layer of redundancy.
Any box gutter
Factory made sizes suit every box gutter from 200 to 600 mm, and the sumps fit new builds and retrofits alike. Potential uses of these devices are limited only by the imagination.
Jump straight to what you need.
Plumbers & roof plumbers
Eliminate callbacks. Install faster. Products that go in straight out of the box and comply first time.
→ How-to installation videos → Install manuals & guides → Find a local supplierArchitects & specifiers
Specify with confidence. Design the box gutter independently of the sump, with full NCC compliance evidence behind every product.
→ Evidence of suitability → Performance solutions templates → Product dimensions & dataWant the detail? It's all here.
Everything below this line is for the people who like design tables: the free flow hydraulics, sizing detail, compliance pathways and certification documents.
Hydraulics, sizing and design detail.
Free flow hydraulics
- Box gutters discharging to a Dam Buster sump are designed for genuine free flow in accordance with Figure I1 of AS/NZS 3500.3, in both the normal and the overflow conditions.
- The box gutter can therefore be designed independently of the sump, in the same way as it is for a rain head. Sumps from all other manufacturers require back flooding, where the water level must rise in the box gutter before the overflow begins to operate.
- If the first chamber is overwhelmed or blocks with debris, water crests the internal weir into the second chamber and is disposed of via the dedicated overflow downpipe to atmosphere, as required by the code. An additional overflow can be fitted to the end of the second chamber for multiple redundancy.
Sizing & selection
- Free flow sumps come factory made in 100 mm size increments to suit all box gutters between 200 mm and 600 mm wide, for new builds and retrofits alike.
- Like all Dam Buster devices, sumps are selected on the required 1:100 year (1% AEP) flow rate in litres per second. The selection tables are in the Product Technical Statement, available at Technical Downloads.
- Full physical external dimensions are on the Product Dimensions page.
Certified. Independently tested. Documented.
Dam Buster free flow sumps are fully engineered and certified devices that meet the performance requirements of AS/NZS 3500.3 and the Conformance, Performance and Suitability requirements of the National Construction Code. They can be safely used in lieu of the code specified sump and side overflow devices.
Note: requirements vary from state to state, and generally a Performance Solution is required for installation certification of free flow sump devices. Documentation to assist the development of a suitable Performance Solution is available at Technical Downloads.
Common questions.
Are Dam Buster sumps compliant?
Absolutely yes. All Dam Buster products achieve full conformance and compliance with the relevant NCC Performance Requirements via two methods. Full details are under Compliance. Remember that "a Performance Solution is also compliance": if a proposed installation is considered outside Deemed to Satisfy, an appropriately prepared Performance Solution is a fully permitted alternative pathway.
All Dam Buster products meet the roof plumbing standards and codes across Australia, including AS/NZS 3500.3:2021 and all previous versions. For Victoria, the VBA has specifically advised (Fact Sheet, March 2023) that it is satisfied as long as products are fit for purpose and meet the relevant Performance Requirements of NCC Volume Three. All Dam Buster products do, along with the NCC Volume 1 and 2 requirements in every other state and territory.
One warning: fake copies of Dam Buster products meet no standards at all and are an automatic audit fail. Only buy from an authorised licensee.
How do I calculate the size of my sump correctly?
Follow our Quick Design Guide from pages 3 to 13. Use Method A or B to calculate your 1% AEP, calculate your flow rate from your catchment area (page 6), then size your sump from the table on page 14. AS 3500.3 has required these calculations for all roof drainage design nationally since 1998, whichever brand of product you use. Read every step carefully, because skipping them will likely result in a non-compliant system.
Can a Dam Buster sump replace the sump specified in AS/NZS 3500.3?
Yes. Dam Buster free flow sumps can be safely used in lieu of the very complex "sump / high capacity overflow" device described in the code. They require no custom design or certification of the device itself, they perform far better hydraulically, and they are potentially a lot cheaper to supply and install.
The only box gutter drainage solution you'll ever need.
Australian designed, Australian made, protected by Australian and international patents, and available from suppliers across the country.
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