A roof is a collector
Rainwater catchment, also called rainwater harvesting, means catching rain from a surface and storing it to use later. On a homestead the surface is almost always a roof. That surface is the catchment area.
What matters is the roof’s footprint: the area it covers when seen from above. A steep roof and a flat roof over the same floor catch about the same rain.
Text description
Side views of two houses of the same width. The left one has a steep roof and the right one a low-slope roof. Rain falls straight down on both. Dashed lines rise from each eave to show that each roof intercepts only the rain falling inside its footprint. Under each house, a dimension line marks the footprint, measured at the outer edge of the eaves. Because the footprints are the same, both roofs catch about the same amount of rain.
To find the footprint, measure the length and width of the house at the outer edge of the eaves. Multiply them.
The rain math
One inch of rain falling on one square foot of roof gives about 0.62 gallons. That is the core number in most US rainwater manuals.
roof footprint (sq ft) × rainfall (inches) × 0.62 = gallons
In metric, 1 mm of rain on 1 square meter gives 1 liter.
Not all of it reaches the tank. Some splashes off, evaporates, or is diverted on purpose. The share that makes it is the runoff coefficient. For metal and shingle roofs it is often 0.75 to 0.9.
Set your roof footprint and a month’s rainfall. How many gallons could you catch?
In theory: 498 gallons.
After losses (splash, evaporation, first flush): about 370 to 450 gallons.
Assumptions: One inch of rain on one square foot is 0.623 gallons. Runoff coefficient of 0.75 to 0.9 is typical for metal and shingle roofs (Texas rainwater manual).
Rainfall is not steady
Your yearly total is less useful than your monthly pattern. Two places with 30 inches (760 mm) a year can be very different. One gets rain every month. The other gets most of it in winter and almost none in summer.
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Two bar charts of monthly rainfall, each adding up to 30 inches (760 millimeters) a year. The numbers are illustrative. Place A gets rain most months, about 2 to 3 inches each month. Place B gets 4 to 5.5 inches a month in winter, and almost none from June to August, which are marked as the dry summer. The yearly total is the same, but the monthly pattern is very different.
NOAA publishes climate normals: 30-year averages of rainfall for each month at weather stations across the US. Find the station nearest you and note the driest months.
Remember that normals are averages. A dry year can bring much less. Many manuals suggest planning around a below-average year, not a typical one.
The parts of the system
A rain system has a few main parts. Each one has a job.
- Gutters and downspouts carry water off the roof edge.
- A leaf screen keeps leaves and twigs out.
- A first-flush diverter holds back the first, dirtiest water from each rain. It carries dust, pollen and bird droppings off the roof.
- The storage tank holds the water. It must be sealed from light, insects and animals.
- An overflow sends extra water safely away when the tank is full.
- A pump or gravity moves water to where you use it (lesson 17).
Pick the right name for each numbered part.
Text description (gives the answers)
Side view of a small house and a storage tank. Rain lands on the roof (1) and runs into the gutter (2). It passes a leaf screen and then a first-flush diverter (3), a standpipe that fills first with the dirtiest water and then lets cleaner water pass. Clean water enters the storage tank (4). An overflow pipe (5) near the top of the tank carries extra water away from the foundation. A pump (6) beside the tank sends water to the house.
First flush and screens
The first water off a roof is the dirtiest. A first-flush diverter fills a small chamber or pipe with that water, then lets cleaner water pass to the tank. It then drains slowly, ready for the next rain.
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Three side views of the same downspout, first-flush chamber and tank. Stage 1, rain starts: water from the roof runs down the downspout and fills a vertical chamber with the first, dirtiest water, which carries dust, pollen and bird droppings. Stage 2, the chamber is full: cleaner water now passes along the side pipe into the storage tank, and the tank level rises. Stage 3, after the rain: a small opening at the bottom lets the chamber drain slowly, so it is empty and ready for the next rain. Screens and the diverter need cleaning every few weeks in the rainy season.
Manuals give different sizing rules. The Texas rainwater manual recommends about 1 to 2 gallons diverted for every 100 square feet of roof (4 to 8 liters per 10 square meters). Dustier sites, or roofs under trees, need more.
Diverters and screens only work if you clean them. Plan to check them every few weeks in the rainy season and after storms.
Choosing a roof and tank size
Roof material matters. Smooth metal roofs shed water cleanly and are the common choice. Asphalt shingles work but shed grit. Wood shakes and some treated materials can add chemicals. Avoid them for any water you might later treat for drinking.
The tank must cover your longest dry stretch, not just your average month. You will size yours in lesson 20.
A rain barrel you can set up
A rain barrel on a downspout is a safe, low-cost first project. It waters a garden and teaches you how your roof behaves.
- Put it on a firm, level base. A full 55-gallon (208-liter) barrel weighs about 460 lb (210 kg).
- Fit a fine screen on every opening to keep out mosquitoes and leaves.
- Lead the overflow at least a few feet away from the foundation.
- Use the water for plants. It is not safe to drink without treatment (lesson 18).
- Drain it before hard frost, or the barrel and fittings can crack.
Check
A cabin has a 600 sq ft roof footprint. In a month with 2 inches of rain, about how much could it catch before losses?
Summary
- Catch = roof footprint × rainfall × 0.62 gallons, then reduce for losses with a runoff coefficient of about 0.75 to 0.9.
- Plan with monthly rainfall and a dry year, not the yearly average.
- Gutters, a leaf screen, a first-flush diverter, a sealed tank and an overflow each have a job, and each needs cleaning.
- A rain barrel for the garden is a safe first project. Rain is not drinking water until it is treated and tested.
Next, you will look at storing water and getting it to the tap with pressure.
Go deeper: why 0.62 gallons
One inch of rain on one square foot is 144 cubic inches of water, a block 12 by 12 by 1 inch. A US gallon is 231 cubic inches. So 144 ÷ 231 gives about 0.623 gallons. The metric version is simpler: a millimeter of rain on a square meter is exactly one liter. Manuals such as the Texas Manual on Rainwater Harvesting build on this and add runoff coefficients for each roof type.