Roof Runoff Coefficient Explained: Why Metal Roofs Collect More

The roof runoff coefficient is the share of rain that leaves the roof surface. A value of 0.95 means 95 litres out of every 100 litres that fall on the roof leave the surface, and 5 litres are lost to splash, evaporation and absorption. About 90 percent efficiency then applies, so about 85.5 litres out of 100 reach the tank.

What the roof runoff coefficient is

Every roof catches rain, but no roof delivers every drop to the tank. Some water splashes off the edges before it reaches the gutter. Some evaporates from a warm surface. Some soaks into the roofing material itself. The roof runoff coefficient sums up all three losses in one figure, a share between zero and one. Multiply the rain that falls on your roof by this number and you get the water that leaves the roof surface. Size and rainfall set the ceiling, and the coefficient decides how close to that ceiling you get.

The calculator uses the coefficient with efficiency, which the page puts at about 90 percent for gutter and first flush losses. The coefficient covers the roof surface. Efficiency covers the gutters and the first flush diverter. Both are applied in the same formula, and leaving either one out overstates the harvest.

The three ways water is lost from a roof

The page names three losses that the coefficient accounts for: water that splashes, water that evaporates, and water that soaks into the roof material.

Splash

Rain does not always land gently. Heavy drops hit the roof hard and bounce, and wind can lift thin sheets of water clear over the edge before they reach the gutter. Short sharp storms lose more this way than long gentle rain, and splash is worst at the start of a storm, when drops strike a dry surface and rebound.

Evaporation

A roof in strong sun can be far hotter than the air around it. Thin films of water left on a warm surface after a light shower can dry before they ever start to move. In a long steady rain the roof cools and stays wet, so later rain runs off with far less lost to the air.

Absorption

Some roofing materials drink water. Porous surfaces hold rain in tiny pores and cracks, and that water only leaves slowly by drying, long after the storm has passed. Smooth hard surfaces give water little to grip and little to soak into, so far more of it keeps moving toward the gutter.

Why a smooth hard surface loses less water

Picture rain falling on a sheet of metal. Drops join into a thin sheet that slides quickly toward the gutter. There is almost nowhere for the water to hide, the surface stays slick, and only a small share is lost to splash and to drying. That is the physical picture behind a high coefficient: little absorption, a fast path to the gutter, and little time for evaporation.

On a rough or porous surface the same rain behaves differently. Drops catch in the texture, spread into pores, and sit in small dips. The water moves more slowly, so more of it dries before it reaches the edge. The page gives no coefficient number for any rough or porous material, so this article assigns none.

How the coefficient fits into the collection formula

The calculator uses one clear formula: collectable water equals roof area times rainfall times runoff coefficient times efficiency. In metric units the arithmetic is simple, because 1 mm of rain on 1 square metre of roof is 1 litre. The worked example uses an 80 square metre metal roof in an area with 800 mm of rain a year, at 90 percent efficiency, and the page states about 54,700 litres a year and about 1,710 litres from a single 25 mm storm.

The page does not state a coefficient value, but the arithmetic implies one. Dividing 54,700 by 80 times 800 times 0.9 gives 0.9497, which rounds to 0.95. The per storm figure confirms it: 80 times 25 times 0.95 times 0.9 equals 1,710 litres exactly. So 0.95 is the coefficient implied for a metal roof, derived from the numbers on the page, not stated there. No other roof type is given a number here, because the page gives no other values.

Rain falling on the roof is 80 square metres times 800 mm, or 64,000 litres a year if nothing were lost. Apply the derived coefficient of 0.95 and the total drops to 60,800 litres. Apply 90 percent efficiency and it drops again to 54,720 litres, which the page rounds to about 54,700 litres. Apply both, because either one alone overstates the harvest by thousands of litres.

How roof size changes the yearly total

Roof size changes the yearly total in direct proportion, because the coefficient multiplies the whole rainfall figure. The table below keeps 800 mm of rain a year, the derived coefficient of 0.95 and 90 percent efficiency fixed, and changes only the roof size. A larger roof catches more rain, so the same coefficient and the same rain give a larger harvest.

Roof size	Yearly collection	Water from one 25 mm storm	Weeks of watering at 500 L a week
40 square metres	27,360 L	855 L	54
80 square metres	54,720 L, stated on the page as about 54,700 L	1,710 L	109
120 square metres	82,080 L	2,565 L	164

Doubling the roof from 40 square metres to 80 square metres doubles the yearly collection from 27,360 litres to 54,720 litres, and a 120 square metre roof collects 82,080 litres. That is the difference between 54 weeks of watering and 164 weeks, from the same rain and the same coefficient. When you compare two roofs, roof size deserves as much attention as the coefficient: a large roof with the same coefficient collects far more than a smaller roof.

An imperial example, kept separate

The metric and imperial figures in this article are separate examples with rounded equivalents, not exact conversions of each other. The page states that 1 inch of rain on 1,000 square feet is about 620 gallons, or roughly 0.62 gallons per square foot per inch, minus losses.

Take a 1,000 square foot roof, 1 inch of rain in a storm, the derived coefficient of 0.95, and 90 percent efficiency. The rain before losses is 1,000 times 1 times 0.62, which is 620 gallons. Apply the coefficient and the total is 589 gallons, then apply 90 percent efficiency for about 530 gallons from that one storm. Over a year with 32 inches of rain, the same roof collects about 17,000 gallons. The table below shows the same roof at three yearly rainfall levels.

Yearly rainfall	Water from one 1 inch storm on 1,000 sq ft	Yearly collection
16 inches	About 530 gallons	About 8,500 gallons
32 inches	About 530 gallons	About 17,000 gallons
48 inches	About 530 gallons	About 25,400 gallons

Do the sums in one system from start to finish.

What the coefficient does not tell you

The coefficient answers one question: how much of the rain that falls on the roof leaves the roof surface. It says nothing about how much of that water you can store, because storage size decides how much you actually use. The calculator covers this with its weeks of watering figure, which shows how far the water goes, not how much of it you can keep.

The coefficient also says nothing about water quality. The first flush diverter exists because the first water off a roof carries dust, leaves and bird droppings, and that water is discarded before the rest reaches the tank. Finally, it does not tell you whether collecting rainwater is allowed where you live: the page advises readers to check local rules, and it notes that rainwater for drinking needs treatment.

Common mistakes with the roof runoff coefficient

The most common mistake is to leave the coefficient out entirely. Multiply roof area by rainfall and stop there, and you have the rain that fell, not the water you can collect. On the worked example roof that error alone adds more than 9,000 litres a year to the estimate. The second mistake is to treat the coefficient and the efficiency as the same number, though one covers the roof surface and the other covers the gutters and the first flush diverter.

A third mistake is to borrow a coefficient from a different roof and apply it to your own. The page gives no coefficient for any roof type except the 0.95 derived for metal from its worked example, so there is no page based number to borrow for any other material. If your roof is not metal, treat the result as an estimate to be checked against what your tank actually collects.

A fourth mistake is to mix metric and imperial figures in the same sum. The page gives 1 litre per square metre per millimetre and about 0.62 gallons per square foot per inch. Those belong to different sums. Keep one system through the whole calculation. A fifth mistake is to plan storage from the yearly total alone, since a tank cannot hold a year of rain at once. Size it from the per storm figure and your weekly use instead. The rain barrel calculator on this site turns a per storm volume into barrel numbers, and the garden watering schedule calculator helps you set a weekly watering figure.

Frequently asked questions

Q: What is the roof runoff coefficient?
A: The share of rain that leaves the roof surface after losses to splash, evaporation and absorption. A value of 0.95 means 95 litres out of every 100 litres that fall on the roof leave the surface, before about 90 percent efficiency applies.

Q: Why does a metal roof have a high runoff coefficient?
A: A smooth hard metal surface gives water little to soak into and a fast path to the gutter. The worked example implies a coefficient of about 0.95 for metal, derived from its arithmetic and not stated on the page.

Q: Does the coefficient change with the weather?
A: Yes, in effect. Light showers on a hot roof lose more to evaporation and heavy storms lose more to splash. The calculator uses one coefficient for the whole year, so treat the yearly total as an estimate.

Q: Is the runoff coefficient the same as efficiency?
A: No. The coefficient covers losses on the roof surface. Efficiency, about 90 percent on the calculator page, covers losses after the water leaves the roof, in the gutters and in the first flush diverter.

Q: What coefficient should I use for my roof if it is not metal?
A: The page gives no coefficient for any roof type other than the 0.95 derived for metal. Do not use an invented number. Check the result against what your tank actually collects.

Q: How much difference does the coefficient make to my yearly total?
A: A large difference. On an 80 square metre roof with 800 mm of rain a year, 64,000 litres fall on the roof, and the derived coefficient of 0.95 alone brings that total to 60,800 litres, before 90 percent efficiency brings it to about 54,700 litres.