Designing Roofs to Catch Enough
Many of our grandparents understood collecting rainwater off roofs and storing it in cisterns. They 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. But 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 who've lived in a city but who are considering country life beyond 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, according to your last city water utility bill, is 7 CCF (hundred cubic feet) every 3 months. Converting 7 CCF to gallons by multiplying by 748 shows that you used 5236 gallons every 3 months. Divide by 91 days (about 3 months) and 2 persons and you get 28.8 gallons per person per day of average use.
How does this compared to what you could collect from your roof?
Let's assume your house roof area is 1,250 square feet and you're in or near Cincinnati, Ohio. According to 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.
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, unless household water consumption rises.
This method of trying to balance water consumption with available renewable resources and the size of the collector shows how we can use calculations to design a roof for a natural sustainable water supply. Understanding rainwater collection and sustainable water usage is also a simple way to improve your understanding of off-grid solar electricity (or PV), since both are best when there's an intentional balance between collection and usage use.
There are many other aspects of a roof-based rainwater collection system, including water filtering, disinfection and storage. If interested in more information about rainwater collection systems, I recommend reading THE HOME WATER SUPPLY by Stu Campbell, a Garden Way Publishing book.