Questions & concepts

As a rule of thumb, one millimetre of rain on one square metre of roof yields about one litre, before losses. After runoff and system losses you typically keep a little over half of that. So a 150 m² roof in a 900 mm/year climate might collect very roughly 70,000 litres a year — but local rainfall, roof condition and system efficiency move that figure substantially.

Yes, mainly through cleanliness and runoff. Smooth, non-absorbent roofs like metal shed water efficiently and stay relatively clean, while rougher or more absorbent surfaces lose a little more to wetting and hold more debris. Material can also influence runoff chemistry, which matters more for drinking-water use than for the garden.

It sets aside the first volume of runoff from each rain — the water carrying the most roof dust, leaf litter and droppings — so it never enters the tank. It is one of the simplest and most effective ways to keep stored water clean, because most contamination arrives in that initial flush.

It depends entirely on the catchment, the system, treatment and local guidance, so this reference does not certify any rainwater as potable. For non-drinking uses — garden, laundry, toilets — good roof and storage hygiene is usually the main concern. For drinking, seek qualified, local advice.

Yes, a full tank overflows, and that is the design working rather than a fault. What matters is how often it overflows and where the surplus goes — ideally routed safely away from foundations to a garden, soak or stormwater point. Frequent overflow simply means inflow exceeds storage in that climate.

There is no single correct size, only a sensible range. It is set by four interacting variables: roof area, rainfall and its variability, household demand, and how much you can store. The right tank is large enough to bridge typical dry spells but small enough that it fills regularly.

A larger tank only helps if inflow can fill it and demand can empty it. Past a certain point, extra capacity sits part-empty most of the year and adds little usable yield. Capture scales with roof area and rainfall; storage scales with the dry gaps you need to ride out.

The annual total matters, but timing matters more. A peaky climate — a short wet season then months of little rain — needs more buffer to carry a household through the dry gap. An even climate refills storage often, so a smaller tank works harder.

Demand is what the water is used for — garden, laundry, toilet flushing — which sets how fast storage draws down. Comparing annual capture to annual demand shows whether your constraint is collection (not enough rain reaches the roof) or storage (plenty of rain, nowhere to keep it).

It estimates annual capture as roof area × annual rainfall × a runoff coefficient (~0.8) × a system efficiency factor, giving litres per year. That is a learning tool for seeing how variables interact — not a property-specific design or a commercial calculator.

Greywater is gently used water from showers, baths, hand basins and the laundry — water that has not contacted toilet waste, and usually not kitchen waste either. It is distinct from blackwater (toilet and often kitchen water), which carries far higher contamination and is handled separately.

As a principle, no — untreated greywater is best used quickly, typically within about a day. Its organic load lets bacteria multiply rapidly if it sits, so it is treated as a fresh, perishable resource rather than something to stockpile.

In concept it can be, when applied below the surface or under mulch, kept away from edible parts of plants, and paired with low-salt, low-phosphorus detergents. This reference covers those principles only; actual reuse is regulated and varies by location, and design is a job for licensed professionals.

Because what is washed into greywater ends up in soil. Sodium can degrade soil structure over time and phosphorus can over-feed waterways, so low-sodium, low-phosphorus products make reused greywater far gentler on a garden.

No. We explain greywater concepts — sources, safe-reuse principles and environmental considerations — but deliberately give no plumbing or installation instructions. Greywater systems are regulated and should be designed and installed by qualified people to local rules.

Rain itself is close to distilled; almost all contamination is acquired on the way down and in storage. The roof contributes dust, pollen, leaf litter and droppings, the first flush carries most of it, and warmth, light and debris allow growth inside an unmanaged tank.

Keep the tank dark, screened and undisturbed. Excluding light suppresses algae, intact inlet and overflow screens keep out leaves and mosquitoes, and drawing water from above the base leaves settled sediment where it belongs — on the floor, out of supply.

Only occasionally, and gently. A settled sediment layer is normally harmless if left undisturbed, and over-frequent desludging can stir contamination back into the water. Check periodically and remove build-up only when it becomes significant.

Filtration is graded, not a single product. Coarse inlet screening handles leaves, first-flush diversion handles the dirtiest runoff, and finer treatment — sediment filters and disinfection for drinking use — is matched to the intended use rather than applied uniformly.

Yes — it is one of the main influences. Leaves and organic matter in gutters rot and wash into storage, feeding growth and discolouring water. This is why most quality measures target the roof and inlet, and why gutter maintenance is really a water-quality task.

Because both the amount and the timing of rain vary enormously across Australia. Even-rainfall cities refill storage often, so a smaller tank copes; monsoonal and Mediterranean climates concentrate rain into one season, so storage must capture a short wet period and ration it through a long dry one.

Of the cities covered here, Darwin receives the most on average — very roughly 1700 mm a year — but almost all of it arrives in the monsoonal wet season, with months of near-zero rainfall in the dry. Adelaide is the driest of the set, around 545 mm.

It is the Mediterranean pattern seen in Perth and Adelaide: wet winters and very dry summers. Storage charges over winter and must then carry through a long dry summer, so buffer size matters more than the annual total alone.

Hot, dry climates lose stored and surface water faster, so the same rainfall stretches less far. Cool Hobart, with low evaporation, makes a modest annual total go further than the raw millimetre figure suggests — evaporation is the quiet third variable alongside amount and timing.

No — they are rounded long-term averages used to illustrate patterns and relative differences. Real planning uses local, year-specific rainfall records rather than a single average number.

Clearing gutters and inlet screens before the main rains, so the first heavy fall is captured cleanly instead of carrying a season of debris into the tank. Most water-quality problems trace back to a blocked or neglected upstream component.

A light seasonal rhythm suits most systems: gutters and screens before the wet season, an inlet check after heavy leaf-fall or bushfire smoke, and an occasional look at pumps and fittings for leaks. Sediment removal is rare — often years apart.

Most faults announce themselves: overflowing gutters, a tank filling slower than it used to, insects at the inlet, or a damaged screen. Treat those as signals to inspect the upstream chain — roof, gutters, first-flush, inlet — rather than the tank itself.

Because almost every quality problem starts as a maintenance lapse upstream — a blocked first-flush, a torn screen, a gutter full of rotting leaves. Keep the catchment and inlet in order and stored water largely looks after itself.

No. Monterivajersy is an independent educational reference. It does not sell products or services, recommend installers, generate leads, or provide quotes or bookings — it explains the underlying engineering concepts so you can ask better questions of qualified professionals.

They are how a community shares a shrinking resource through a dry period, stretching stored supply until the rains return. They scale up as dam and aquifer levels fall and ease as they recover, so they are temporary by design.

Most schemes tighten in steps. Early stages target the highest-volume, most discretionary uses — especially outdoor watering — because they save the most water for the least hardship. Later stages tighten further while preserving essential indoor use.

No. Restriction stages, permitted hours and exemptions are set by your water authority and change with conditions. We explain the general logic only and make no claim about current rules anywhere — always check your local authority for what applies now.

Often, yes — rainwater from a private tank is frequently treated differently from mains water under restriction rules, which is part of why harvesting and conservation are connected topics. Specifics depend on local rules, so confirm with your water authority.