Rainwater tanks can improve a STORM rating when they capture runoff from a suitable roof catchment and are connected to regular water reuse.
Yes. A rainwater tank can improve a STORM rating by intercepting roof runoff and storing it for reuse rather than allowing all of that water to discharge directly into the stormwater system.
Its modelled benefit depends on three connected factors:
A larger tank does not automatically produce a better result. If only a small roof area is connected, the tank may receive limited runoff. If there is little reuse, the tank may remain full and have less capacity available during the next rainfall event.
The strongest result usually comes from coordinating the tank capacity, roof catchment and reuse demand as one stormwater treatment measure.
A STORM assessment examines runoff from the impervious areas of a proposed development and calculates the treatment performance provided by the nominated measures.
Roof surfaces are a major source of runoff on many residential and small development sites. Where roof water flows directly into the drainage system, it contributes to the project’s untreated runoff.
A rainwater tank changes this pathway by:
The assessment tool models this interaction and attributes treatment performance to the tank based on its inputs.
For a broader overview of stormwater treatment assessments, visit the Certified Energy STORM Knowledge Hub.
The effect of a rainwater tank cannot be understood from its nominal capacity alone.
Three inputs work together to determine its contribution to the STORM rating.
The tank capacity determines how much water can be stored at a particular time.
A larger tank can potentially capture more runoff before overflowing. However, the additional capacity only helps where enough water reaches the tank and enough stored water is subsequently reused.
The connected roof catchment determines how much rainfall can enter the tank.
A large tank connected to a small roof section may fill slowly and make limited use of its full capacity. A moderately sized tank connected to a substantial roof catchment may capture runoff more effectively.
Reuse removes water from the tank and creates room for later rainfall.
Without sufficient drawdown, a full tank behaves largely as an overflow point during subsequent rainfall. Regular demand can therefore be as important as storage volume.
A rainwater tank has the greatest capacity to capture runoff when it is partly empty before rainfall begins.
Consider two identical tanks:
The toilet-connected tank is likely to experience more frequent drawdown. This creates storage space more consistently, including during cooler or wetter periods when garden irrigation demand may be low.
The garden-only tank may remain full for longer periods. Once full, additional roof water passes through the overflow and into the site drainage system.
This is why the STORM result depends on the pattern of reuse rather than simply confirming that a tank is shown on the plans.
The reuse demands included in the assessment must be credible for the proposed development and supported by the project documentation.
Common non-potable uses may include:
The exact inputs available can depend on the assessment tool and project type.
Reuse should not be added to the model solely to improve the rating if the required plumbing or irrigation connection will not be constructed.
Toilet flushing can provide a relatively consistent demand because it occurs throughout the year and is not dependent on seasonal irrigation needs.
Where toilet reuse is included in the assessment, the design documentation should identify that the rainwater tank will supply the relevant toilets.
This may require coordination between:
If the toilet connection is later removed, the tank may no longer provide the same modelled performance and the assessment may need to be updated.
Garden irrigation can also contribute to rainwater reuse, particularly where the development contains a meaningful landscaped area.
The modelled benefit depends on the realistic irrigation demand associated with that area.
Important considerations include:
A small garden should not be assigned an unrealistic water demand simply to increase tank drawdown.
Where garden irrigation is the only reuse source, its contribution may be less consistent than regular internal demand.
No. Increasing the rainwater tank capacity can improve the result, but the relationship is not unlimited or directly proportional.
A larger tank may provide little additional benefit where:
For example, increasing a tank from 3,000 litres to 5,000 litres may not solve a low rating if most of the roof is not connected or the development has no regular reuse demand.
Before increasing the tank size, it is often more useful to review the complete catchment and reuse arrangement.
There is no single roof-catchment percentage that is correct for every project.
The appropriate connected area depends on:
Only roof areas that can credibly drain to the tank should be included.
A report should not assign the complete roof to a tank where parts of the building have separate gutters, incompatible levels or no practical connection.
Yes. Multi-dwelling developments often use separate tanks for individual dwellings or groups of roof catchments.
For example, a townhouse project may include:
The assessment should identify:
The arrangement used in the model should match the physical design rather than combining unrelated tanks and demands into an artificial result.
A shared tank may provide a strong and consistent reuse demand where it serves several dwellings or common landscape areas.
It may also reduce the number of separate assets that need to be installed and maintained.
However, shared systems require careful coordination of:
The assessment result alone does not establish whether a shared system is the most practical long-term arrangement.
A rainwater tank may be sufficient to achieve the required benchmark for some developments.
This is more likely where:
Other projects may not reach the target through rainwater harvesting alone.
Where a development contains a large shared driveway, extensive paving or other untreated impervious areas, additional measures such as a raingarden or permeable paving may be needed.
A 100% result should be achieved through a practical treatment response rather than by relying on unrealistic tank inputs.
A proposed rainwater tank can produce less improvement than expected for several reasons.
Common causes include:
The most effective improvement is not always a larger tank. It may instead involve increasing the connected roof area, adding consistent reuse or treating another impervious catchment.
Consider a three-townhouse development with three dwelling roofs and a shared driveway.
The original design provides one small tank per dwelling, but only a limited section of each roof is connected and the tanks supply garden taps only.
The STORM assessment may show that the tank contribution is limited because:
The project team might then investigate:
The best response would depend on the available space, building services and site drainage arrangement.
A rainwater tank requires an overflow connection.
Once the tank reaches capacity, additional roof runoff passes through the overflow into the nominated drainage system or another approved part of the stormwater strategy.
This is another reason regular reuse matters. Drawing water from the tank creates available capacity before the next rainfall event.
The STORM assessment does not remove the need to design an appropriate overflow pathway.
The hydraulic or civil documentation may still need to address:
A rainwater tank does not need to work alone.
Many developments achieve a stronger and more balanced treatment result by combining rainwater harvesting with other measures.
The tank may treat roof runoff while a raingarden treats runoff from a driveway or paved area.
The tank may capture roof water while permeable paving reduces or treats runoff from courtyards, paths or parking areas.
Removing unnecessary paving can reduce the overall runoff load and the amount of treatment infrastructure required.
This combined approach can be more practical than installing an oversized tank solely to compensate for untreated hard surfaces elsewhere on the site.
The original Melbourne Water STORM Calculator has been replaced by BlueFactor for suitable small Victorian developments.
Older permit conditions, reports and project discussions may continue to refer to:
A new application may instead require a BlueFactor assessment or another stormwater treatment method accepted by the responsible authority.
The underlying design principle remains relevant: a rainwater tank performs most effectively when its storage, connected catchment and reuse demand are properly coordinated.
A tank may contribute to the stormwater treatment result, but it does not automatically resolve every stormwater requirement for a project.
It does not ordinarily replace:
The STORM assessment remains focused on the modelled treatment contribution provided by the tank and other nominated measures.
Only include roof sections that can physically drain to the tank through the proposed gutter and downpipe arrangement.
Toilet, laundry or irrigation uses included in the assessment should be supported by the final design.
The tank must fit within the site without obstructing access, private open space, windows, services or required setbacks.
The nominated storage capacity should represent usable tank volume and remain consistent across the architectural, hydraulic and assessment documents.
Internal rainwater uses may require a pump, appropriate controls and mains-water backup. These should be coordinated by the relevant designer.
The tank overflow must connect to an appropriate drainage pathway and should not be assumed to disappear from the site.
If the rating remains low, check whether driveway and paved runoff requires separate treatment rather than continuing to increase the tank size.
Tank capacity, roof catchment and reuse commitments should match the architectural plans, hydraulic notes and final STORM report.
The assessor will generally need:
Where these details are not final, a preliminary assessment may test reasonable options before the design is fixed.
Certified Energy can assess how proposed rainwater tanks contribute to the stormwater treatment performance of a suitable Victorian development.
The assessment may include:
Where separate drainage, detention or civil engineering documentation is required, that broader scope should be identified rather than presenting the tank as a complete stormwater solution.
Explore the STORM Assessment Knowledge Hub or send through the current plans and council correspondence for an initial project review.
Yes. It can improve the result by capturing runoff from a connected roof catchment and creating storage capacity through regular reuse.
No. The result also depends on the connected roof area and reuse demand. Additional storage may provide little benefit where either is limited.
Reuse removes water from the tank and creates space for later rainfall. A tank that remains full has less capacity to capture the next runoff event.
Toilet flushing often provides more regular year-round demand. Garden irrigation can still contribute, but its demand may vary seasonally and depends on the landscaped area.
No. However, only roof areas that can realistically drain to the tank should be included as connected catchment.
Yes. A shared tank may serve several dwellings or common areas, provided its plumbing, access, reuse and maintenance arrangements are properly coordinated.
It may be possible for some developments. Projects with large driveways or other untreated surfaces may also need raingardens, permeable paving or another treatment response.
The connected roof area or reuse demand may be too small, the tank may already be oversized for the available catchment, or other untreated surfaces may be limiting the overall result.
Yes. Once a tank is full, additional runoff leaves through its overflow. The overflow must connect to an appropriate drainage arrangement.
Not automatically. Treatment storage and detention storage serve different assessment purposes. Council or the drainage designer should confirm the project’s detention requirements.