623 gallons per thousand square feet per inch
This is the number worth carrying around. An inch of rain falling on 1,000 square feet of roof plan area is 83.3 cubic feet of water, and a cubic foot is 7.48 gallons, so the total is 623 gallons. Every other figure on this page is that one scaled.
It scales two ways and both are linear. Double the roof and you double the water; double the rainfall depth and you double it again. A 1,120 square foot footprint in a two inch storm is about 1,400 gallons. A 2,400 square foot ranch in a half inch shower is about 750. There is no threshold effect and no diminishing return; the roof is a funnel and it collects exactly what falls on its shadow.
The shadow is the point people get wrong. Roof pitch does not change the volume. A 12/12 roof over a 40 by 28 footprint catches precisely as much rain as a flat roof over the same footprint, because rain falls vertically and the horizontal projection is identical. Pitch changes how fast water arrives at the gutter and how hard it is thrown past it, which is why the gutter and downspout sizing calculator applies a pitch factor to capacity, but the total volume comes off the plan area alone.
The even split is a fiction
Divide the roof by the number of downspouts and you get a tidy number that no roof produces. Gutters run to one end, valleys dump into one point, a long run with a single outlet at the far corner takes far more than its nominal share, and a downspout serving a low-slope section of a complicated roof can be handling half the building. The share field exists so you can put in what you observe rather than what the arithmetic assumes.
The easiest way to find the busiest one is to stand outside during a storm. The downspout that roars is the one carrying the roof, and it is also the one whose discharge point has the settled ground, the splashed siding and the moss. If you would rather work it out on paper, sketch the gutter runs, mark where each one falls to, and assign the roof area that drains to each run.
Volume, rate, and which one you need
| Question | Number you need | Unit |
|---|---|---|
| How much water lands beside the wall in a storm | Rainfall depth | Inches of rain |
| Whether the gutter and downspout can carry it | Rainfall intensity | Inches per hour |
| How big a dry well or storage has to be | Rainfall depth | Inches of rain |
| Whether a pipe or swale passes the flow | Rainfall intensity | Inches per hour |
Depth and intensity are not interchangeable. A one inch storm spread over eight hours and a one inch storm delivered in twenty minutes contain the same water and behave completely differently: the first soaks in, the second overtops the gutter, jumps the valley and cuts a channel. Storage sizing runs off depth, capacity sizing runs off intensity, and both numbers come from rainfall frequency data for your own location rather than from anywhere on this site.
What happens at the outlet
The equivalent depth line is the one that tends to change how people look at a splash block. If 156 gallons arrives on forty square feet, that patch has received the equivalent of six and a quarter inches of rain in the time the storm took. Soil that would have handled an inch of rain without complaint is being asked to take six times that, concentrated, and repeatedly, at the exact spot where the backfill around the foundation is loosest.
Three things follow. The ground there settles, which reverses the grade and sends the next storm toward the wall rather than away from it. The soil stays saturated between storms, so the next one arrives on ground that cannot absorb anything. And the water that does soak in goes down beside the foundation wall rather than out into undisturbed soil, which is the condition a footing drain and a sump pump then spend their lives dealing with.
Extending the discharge is the cheapest intervention available on the whole list, and spreading it matters as much as the distance. A pipe that releases four feet out onto a bare hollow has moved the problem four feet. The same pipe releasing onto a spread gravel apron on ground that falls away has actually solved something. If the water needs to go underground instead, the dry well sizing calculator puts a volume on what that would take, and the french drain calculator covers the trench-and-stone route.
Before any of it is dug
Before digging anywhere on the property, have the underground utilities located. In much of North America that is a free call-before-you-dig service and using it is a legal requirement, not a courtesy. Separately: excavating alongside a foundation can undermine it. Soil next to a footing is carrying load, a trench cut beside or below the footing can let that soil move, and the failure does not always show up on the day. This page gives no excavation procedure and none should be inferred from it. Work close to a foundation, below the footing line, or in soil that will not hold a face belongs with a contractor or an engineer who is standing on the site.
Where roof water and site water are allowed to go is set locally and it varies enormously. Discharging onto a neighbouring property, into a street gutter, into a storm sewer, into a sanitary sewer, or into a watercourse may be required, permitted, permitted only with approval, or prohibited outright depending on where you are, and some jurisdictions also set minimum setbacks and grading requirements at the building. None of that is stated on this page as fact, because none of it can be. Your local building department or stormwater authority is the source, and it is worth a phone call before anything is dug, because water arriving on someone else's land is one of the more common ways a drainage project turns into a dispute.
Questions people ask
How many gallons of water comes off my roof in a storm?
Multiply the roof plan area in square feet by 0.623 for each inch of rain. A 1,200 square foot footprint in a one inch storm gives roughly 750 gallons; the same roof in a three inch storm gives about 2,240. Use the plan area, meaning the outline the building casts on the ground including the eave overhangs, not the sloped surface area of the roof. Rain falls vertically, so a steep roof and a flat roof over the same footprint collect identical volumes.
Does a steeper roof shed more water?
It sheds the same volume faster. Pitch has no effect on how much rain lands on the roof, because the catchment is the horizontal projection. What pitch changes is velocity: water arrives at the eave sooner and with more momentum, which is why steep roofs overshoot gutters in heavy rain and why gutter capacity calculations apply a pitch factor. For working out volumes, ignore pitch entirely. For working out whether the gutter copes, it matters.
How far from the house should a downspout discharge?
This page will not give you a distance, and you should be wary of any that does, because minimum discharge setbacks and grading requirements are set locally and differ between jurisdictions. What is not in dispute is the direction of the effect: further out is better than closer in, spread is better than concentrated, and the ground it lands on should fall away from the building rather than toward it. Ask your building department what applies to you, and while you have them, ask where the water is permitted to end up.
Is a splash block enough?
It depends entirely on the volume and on what is under it. A splash block breaks the fall of the water and stops it scouring a hole, which is a real job, but it moves the discharge point roughly a foot and a half. Run the equivalent depth figure on this page for the area a splash block actually wets and you will usually see a number several times the rainfall depth of the storm. Where the ground falls away steadily that may be fine. Where the ground is flat, or has settled toward the house, it will not be.
Can I pipe the downspouts underground?
Physically yes, and it is a common approach, but three things decide whether it works. The pipe needs continuous fall to a point where water can leave, which many flat lots do not have. It needs to be able to be cleaned, because downspout lines fill with shingle grit and leaf debris and a buried line that cannot be rodded is a line that will eventually back up at the foundation. And the discharge point has to be somewhere the water is allowed to go, which is a local regulatory question rather than a technical one. Freezing matters too in cold climates, since a line that ices at the outlet backs the water up to the wall.