What A Roof Actually Delivers
One millimetre of rain on one square metre of roof yields one litre. That is the whole calculation. Everything after it is subtraction.
Put numbers on it. A 100 square metre roof under 25mm of rain offers 2,500 litres on paper. You will capture less. Gutters splash. Leaves hold water. The first flush runs straight down the drain. Roof runoff coefficients sit between 0.75 and 0.95 depending on the surface, so a realistic haul from that storm lands nearer 1,900 to 2,400 litres. Texas A&M AgriLife’s harvesting manual lists the coefficients by material if you want to run your own figure.
Here is where most people get it backwards. They assume the roof is the problem. It is not.
I collect off a 140 square metre bungalow roof into a 1,200 litre butt behind the shed. A 20mm shower fills it inside ten minutes. The overflow then runs for the rest of the afternoon. The roof delivered far more than the tank could hold, and that is the usual story. Storage, not catchment, is the pinch point on nearly every domestic system I have looked at.
The Number That Decides Everything: Litres Per Week
Start on the demand side. That is the figure nobody measures.
A cool-season lawn wants about 25mm of water a week in summer. The classic one inch. Purdue Extension puts Indiana lawns at 1 to 1.5 inches weekly, applied as one or two deep soakings instead of a light sprinkle every day. That publication is worth reading before you size anything.
Volume is where it gets sobering. Twenty-five millimetres over 100 square metres is 2,500 litres. Per week, not per month. A lawn that size in a hot July is asking for two and a half tonnes of water every seven days.
Hold that against the supply. A dry spell can run twelve days without useful rain. The tank has to bridge the gap alone. A 1,000 litre barrel covers about three days of that lawn and then sits empty while the grass browns off. The barrel is not faulty. That is arithmetic.
Sizing Storage Against A Lawn’s Thirst
The working rule is simple. Multiply weekly demand by your longest likely dry spell. If the lawn needs 2,500 litres a week and your region regularly goes a fortnight between rain events, you want 5,000 litres of storage before you add any margin.
Use it or lose it. A full tank is a dead tank. The EPA’s municipal harvesting handbook makes the point that collected rainwater has to be used in good time so the store is free for the next storm. A tank that fills in April and never empties is a pond with a lid.
Cost does not scale neatly either. A 200 litre barrel is cheaper than a 1,000 litre slimline tank, but nowhere near five times cheaper. Once you accept that a lawn wants thousands of litres, the barrel stops being the right tool and you are pricing a cistern. Aboveground plastic holds its price best up to roughly 5,000 litres. Past that, and especially if you want the space back, buried or bladder tanks start to compete.
One more trap. Storage is only useful if it refills. A 10,000 litre tank under a 30 square metre porch roof will spend most of the summer half empty. Match tank volume to connected roof area, not to the size of your ambition.
First-Flush Diversion: The Dirtiest Water Of The Storm
The first rain off a dry roof is filthy. Dust and pollen settle on the shingles between storms. Birds leave deposits up there. Roof grit washes loose. The CDC states plainly that this first flow carries the most dirt, germs and chemicals, and that it is worst after a run of dry days.
A first-flush diverter throws that water away before it reaches the tank. Texas A&M suggests diverting 1 to 2 gallons for every 100 square feet of catchment. Scale that to a 100 square metre roof and you are sacrificing roughly 50 to 100 litres per storm. That stings. It is still the right call, because a tank full of roof grit is a tank you will have to empty and scrub by hand.
Mine is a standpipe with a screw cap on the bottom. After each rain event I unscrew it and let the chamber dump into a bed of ferns. Forget that job for a month and the chamber stays full. The next storm then pushes the dirty water straight past the diverter and into the store. The device only works if you empty it.
Gravity Is A Liar
Height makes pressure, and almost nobody guesses how little. One metre of fall gives you about 0.1 bar. A sprinkler wants something near 1.5 to 2 bar to throw a proper pattern. To get that from gravity alone you would need a tower fifteen metres tall.
North Carolina State Extension is blunt here. Gravity flow from a cistern is generally enough to fill a bucket or water a few nearby plants. Anything needing steady pressure wants a pump, and their pump selection guide walks through flow and total head, measured in feet of water or psi.
So budget for one. A small pressure pump will lift water from a buried tank and push it through a hose or a couple of pop-up heads. Add a float switch so the pump cuts out when the tank runs dry. That cheap switch saves more pumps than any warranty claim.
Gravity still has a role on small plots. A soaker hose on a gentle slope will trickle for hours with no electricity at all. Slow delivery suits turf, because it lets clay soils absorb the water instead of shedding it down the path.
Downspout Sizing, Screens And Overflow
Downspout capacity gets ignored until a storm backs water up under the shingles. The AgriLife manual gives a workable ratio. Allow one square inch of downspout for every 100 square feet of roof. A 2×3 inch unit carries roughly 600 to 700 square feet. A 3×4 inch one handles up to 1,200.
Every inlet needs a screen. Leaf litter chokes everything downstream of it. I run a coarse guard on the gutters and a finer mesh basket at the tank inlet, and I empty that basket after heavy autumn rain. A blocked basket turns a harvesting system into an overflow pipe with extra steps.
Overflow routing deserves the same thought as the inlet. When the tank is full the surplus has to go somewhere harmless. Penn State Extension advises leading the overflow away from the building to a garden bed or a rain garden. Get that wrong and you have invented a permanent damp patch against the wall.
Diverter kits usually suit standard downspouts, but the sizes vary more than the listings admit. Mine came with a 68mm inlet and a 3×4 inch adapter, and the fit was the fiddliest part of the whole job. Measure your downspout before you order anything.
What Rainwater Does To Turf Chemistry
Rainwater is soft. It arrives with no chlorine and no dissolved calcium or magnesium, so it will not leave the pale crust that hard mains water can build on leaves and in the topsoil. The USGS puts normal rainfall near pH 5.6, mildly acidic because carbon dioxide dissolves in the drops and forms weak carbonic acid.
That acidity is gentle. Most soils buffer it without complaint. It matters more if your ground is already sour, so test before assuming anything. Do not treat rainwater as a feed either. It picks up a little atmospheric nitrogen between the clouds and the downspout, but nowhere near enough to replace a feeding programme.
The roof surface changes the chemistry more than the rain does. Penn State warns against watering edible crops with water off old tar roofs, treated timber shingles, copper, or a zinc anti-moss strip. For a lawn this is a small concern, since you are not eating the grass.
Algae is the practical worry. Light plus nutrients equals green slime on the tank walls. The EPA handbook recommends dark containers or buried tanks for exactly that reason, along with leaf screens and pre-filtration ahead of the barrel.
Mosquitoes And The Lid Question
An open tank in summer is a mosquito nursery, and the fix is not complicated. The CDC says to cover water storage containers tightly, or to fit wire mesh with holes smaller than an adult mosquito. Their fallback is emptying the container at least every ten days, which is far more work than a lid.
I keep a stainless screen on the inlet and a sealed lid on top. In four summers I have never dipped a wriggler out of that tank. When a neighbour left his barrel open for a fortnight, the surface went black with larvae.
A tight lid does a second job. It keeps leaves and light out, so the water stays clear instead of turning to soup.
Rainwater Rules Change At The County Line
This is the part that catches people out. The law is local, and it moves.
Colorado’s Division of Water Resources allows two rain barrels with a combined 110 gallons, no permit needed, for outdoor use on the property where the rain fell. Anything larger, feeding an exempt well, needs a rooftop precipitation collection permit.
Colorado State University Extension lays out the fine print. Lids must seal. The water cannot be drunk or used indoors. And the state engineer can curtail barrels if a senior water right is harmed, because barrel use is not a water right in Colorado.
Utah went the other way in 2010 and legalised rooftop harvesting. Utah State University Extension still meets homeowners who believe harvesting is illegal, a myth left over from the old rule.
Your own rule sits somewhere. It might be a state statute, a water authority bylaw, or a clause buried in an HOA covenant. Check before you cut a downspout. An afternoon of reading beats an enforcement letter.
Cut The Thirst Before You Buy A Tank
Collecting is the costly half of this. Cutting demand is nearly free.
Water deeply and less often. A lawn trained on daily sprinkles grows shallow roots and wilts fast when you skip a week. One proper soaking every four days pushes roots deeper and hardens the turf.
Raise the cut. Longer blades shade the soil and slow evaporation. Leave the clippings where they fall, because they are water held in place.
Then look at the controller. EPA’s WaterSense programme reckons American households pour nearly 8 billion gallons a day onto their outdoors, most of it onto lawns. Swapping a clock timer for a weather-aware controller saves an average home around 7,600 gallons a year, according to the agency’s own fact sheet. A rain sensor costs a fraction of that and simply switches the sprinkler off when the sky is already doing the job.
There is a harder reason to fix demand first. Storage gets sized against the dry spell, and a dry spell is exactly when your roof delivers nothing. Every litre you stop wasting is a litre the tank does not have to hold. Every square metre of thirsty turf you swap for something tougher is a demand that never arrives.
Only then decide whether a tank earns its place. For a small lawn it is a decent gesture and a modest saving. For a large one, the storage needed to ride out a real drought runs into thousands of litres and serious money, and the honest conclusion is that a tank stretches your supply rather than replacing it.
Build the cheap parts first. Fix the thirsty lawn. Then size the tank around whatever demand is left.