Rain to the garden.
Rain to the toilets.

Test the demand

Garden irrigation and toilet flushing are confirmed uses. Occupancy, irrigated area and application rate below are provisional. Editing them changes this calculation only, not the 3D model.

Toilets: 24 L/person/day assumed. Irrigation: May–September, the specified daily volume is applied throughout these months; rainfall-responsive control can reduce this allowance. Default garden area is the sum of the nine modelled terrace footprints, not a confirmed planting schedule.

Unmet demand requires toilet backup or reduced irrigation; it does not prescribe potable-water irrigation. Monthly pooling is optimistic: rain falling late in a month cannot supply earlier demand. These results compare mean-year options and are not a daily reliability calculation.

When water arrives and when it is needed

Blue: captured water. Ochre: demand. Monthly estimates, m³.

What was recovered, and what changed

Only the main roof is counted. The rejected perimeter canopy pipes have been removed, so canopy runoff is excluded from the yield estimate. Lower accessible terraces, balconies and retaining-wall drains are also excluded. Four source scuppers connect to straight downpipes; the south pair sits 110 mm farther out than the previous top section to clear the balcony beam without a mid-height bend. Short connections remain at roof outlets and buried collectors.

The old 6.08 m³ retention volume is kept as a separate overflow reservation. Infiltration and the final outfall remain unconfirmed. No rainwater route connects to the domestic sewage pumping station.

Calculation basis

Green-roof monthly runoff is not measured: 0.40 is an explicit screening assumption, not an ANQIP monthly coefficient. Use a daily local rain series and the selected roof system’s measured response before fixing the tank size. The 30% drier scenario is a sensitivity test, not a return-period drought.

A monthly net balance repeats the reference year until initial and final storage agree. Storage = previous storage + collection − delivered demand − overflow. No free full tank is added each January. Tank options are compared using the same inflow and demand.

System provisions to detail

Provide a covered, inspectable tank, inlet filtration, controlled first-flush diversion, calming inlet, floating draw-off and a screened vent. The non-potable delivery network needs a pump/controller, separate marking and protected backup through an appropriate air gap. Confirm water quality for toilet use, including potential green-roof colour and dissolved contaminants.

These are provisions for the next design stage: pressurised supply pipes to the toilets and garden, pump head, treatment, vent termination and bypass disposal are not yet modelled. Storm intensity must separately size gutters, outlets, filters, collectors, independent emergency roof drainage and overflow. Tank wall design, buoyancy and excavation are also unresolved.

ANQIP ETA 0701 is technical guidance, not a law. Its sections 4.1–4.5 address collection/storage and 4.6–4.8 address use, water quality and backup. It recommends gross volume at least 20% above usable volume. This model reserves 30%. No compliance certification is claimed.