Calculation 2: Basic Water HarvestingHarvesting rain water from building roofs is often considered a simple way to achieve net-zero water usage. The realities of quantity, storage and use are likely more complicated than you might anticipate.

Please determine the following:

How much rain water can be harvested annually off the roof of:

An average, detached, single family home within the Portland inner city? Is this enough for average consumption patterns?
An average apartment building in downtown Portland? Is this enough for average consumption patterns?
Portland’s annual rainfall is not distributed equally across each month. Water storage will be needed to get through the dry months.

How much water will need to be stored to get through these months, respective to the usage of both our average single family home and our average apartment building?
What size storage tank will be needed to accomplish this goal respective to each building type?