Where can I harvest water from?

In the urban environment, the roof of the house and garage are the largest and easiest surfaces to harvest water from – the Catchment Area.  The vast majority of houses already have eavestroughs and downspouts installed to gather and direct the water.


Is the water coming off the roof contaminated?

It depends on many factors.  The contamination load in the rainwater depends on the amount, type, and proximity of contamination sources.  Rainwater can pick up contaminants as it falls through the air.  For example, soot, industrial pollution, dust and soil particles.  Rainwater can also pick up contaminants that accumulate on the roof between rains.  Examples are fecal material from birds and animals and organic material.  All of these substances are picked up and transferred to the water.


Can I harvest water from my asphalt shingles?

Again, it depends.  Every shingle manufacturing company has MSDS (Material Safety Data Sheets) for their product.  This will tell you the composition and any treatments applied to the shingles.


Here are some general guidelines if you don’t know the specific roofing you have:

ARCSA – Rainwater Harvesting Manual 2015 (page 8-7)

Can I harvest 100% of the rainwater that falls on my roof?

No.  Some water will be lost to evaporation, splashing, and being absorbed by the roofing material.

Run off coefficients are estimations of the amount of rainwater that runs off the roof after accounting for the above factors.


What are the best roofing materials for rainwater harvesting?

The Texas Water Development Board did a study and concluded that no roofing material selected was clearly superior for rainwater harvesting.  They found that the water quality varied widely by roofing materials, location, time between rain, rainfall pH, surrounding land uses and other conditions.

Widely recommended roofing materials:  Galvalume (zinc-aluminum alloy, can have baked or enameled finish) and other metal roofs with non toxic finishes

What roofing materials are not recommended?

Roofs treated with biocide, shake and wood shingles, uncoated copper roofs