Designing Roofs to Catch Enough

Many of our grandparents understood collecting rainwater off their roofs and storing it in cisterns. They also understood how much water was available and how to avoid running out. Today, most of us have no idea how much water we use or how much our bio-region can supply. There are some people who still collect, store and clean rainwater that falls on their roofs, and they offer us some of the best and most simple lessons about what sustainable building design and living mean. Their homes' roofs still collect rainwater and their cisterns still store it. Occupants in these kinds of houses typically learn to tailor their water usage according to how much water is in storage.

How can you determine how much water you use?

I often get calls from people living in the city, who are now considering country life beyond the public water lines. Although they've heard of catching and storing rainwater, one of their main worries is whether they can collect enough and not run out. So I start by doing some simple calculations.

Let's assume your household has two people and your water usage is 7 CCF (hundred cubic feet) for 3 months. (If you live in a town or city, this is a typical way your water utility bills show usage.) Converting the 7 CCF to gallons by multiplying by 748 will show that you used 5236 gallons. To further define your usage into more commonly used terms, divide by 91 days (about 3 months) and 2 persons. This produces 28.8 gallons per person per day of average use.

USE= 7 CCF x 748 gal/CCF = 28.8 gallons

91 days x 2 people per day per person

Is this more or less than you could collect from your roof?

Let's assume that your house roof area is 1,250 square feet and you're in or near Cincinnati, Ohio. According to that city's local weather data, annual rainfall averages 40", but has ranged from 28" to 57" over the years. Converting these inches to gallons by multiplying by 0.623, available rainwater in an average year for your house is 1250 x 40 x .623 x .67 (one-third may be lost to evaporation, leaks and roof washing), or 20,870 gallons. In the driest year on record, the available water would have been 14,609 gallons. To convert to gallons per person per day, divide by 2 people and 365 days to get 28.6 in an average year and 22.0 in the driest year.

Avg RAIN= 20,870 gal/yr = 28.6 gallons

365 days/yr x 2 people per day per person

In this example, consumption just about equals water collection in an average year. However, if consumption is reduced by a fourth or if collection and storage is increased by a third, the collected rainwater could meet all water needs even in the driest year.

This demonstrated method of balancing consumption with available renewable resources, in this case rainwater, exemplifies how we can prepare and move toward sustainable resource usage. I've found that understanding rainwater collection and sustainable water usage is a simple way to begin understanding off-the-grid solar electricity (or PV) collection and sustainable energy usage. Both are relatively simple functions of collection area and occupant use.

If you are interested in more info about rainwater collection and usage, read THE HOME WATER SUPPLY by Stu Campbell, a Garden Way Publishing book.

John F. Robbins, CEM CSDP

3519 Moffett Road

Morningview, KY 41063-8748

Phone: (859) 363-0376

E-mail: johnfrobbins@insightbb.com