Terry Thomas (2002)
In their classic forms, 'institutional RWH' uses run-off from institutional roofs to meet the water needs of that institution and 'domestic RWH' uses runoff from the roof of a house to meet the needs of its inhabitants. However there is a hybrid form of RWH that bridges these two forms by using the 'spare' or 'unused' capacity of some institutional roofs to collect water for domestic use by households outside that institution.
Not all households possess of roofs of sufficient size or quality to practice their own roofwater harvesting. However community buildings such as schools and places of worship, or commercial buildings, may be present that possess large and potentially clean collection surfaces suitable for rainwater harvesting. Such roofs are already sometimes used for institutional water supply - for example school roofs supply water to school pupils. The abbreviation IRWHDS stands for 'Institutional Roof Water-Harvesting System for Domestic Supply'. An IRWHDS system bears some operational similarities to public water supplies using rock catchments (e.g. as in Gibraltar), to the sale of roofwater to neighbours by householders with large roofs and to the sale of ground-runoff water stored in private ponds and tanks.
Roof run-off is almost always of higher quality than ground run-off, the latter usually requiring treatment if it is to achieve potable quality. This note addresses specifically rainwater harvesting from roofs rather than other catchment surfaces.