Most people think of a downspout as a trickle. Stand next to one in a July thunderstorm and you'll change your mind. It runs like a garden hose turned all the way up, and it hits the same spot every rain, every thaw, every year.
When a stoop, a sidewalk square, or an AC pad drops, there's usually a downspout within a few steps of the low corner. This page is the "why," with real numbers for a Green Bay house. For the how-to, go to how to extend downspouts.
The Number to Remember: 20 Gallons per Square Foot
Start with the U.S. Geological Survey's figure: a 1-inch rain drops 27,154 gallons on an acre. Divide by the 43,560 square feet in an acre and you get about 0.62 gallons per square foot for every inch of rain, or 623 gallons per 1,000 square feet.
A shingled roof soaks up next to none of it. Two details make the math work for your house:
- Use the roof's footprint, not its sloped surface. Rain falls more or less straight down, so a steep roof catches no more than a flat one covering the same ground. Measure the area the roof covers, overhangs included.
- Use Green Bay's annual total. The 1991–2020 normal is 31.61 inches of precipitation a year, according to the Wisconsin State Climatology Office. That already includes the 55.6 inches of normal snowfall, counted as the water it melts into.
Multiply them and you get about 20 gallons per square foot of roof, per year. A downspout draining 500 square feet sends roughly 10,000 gallons a year to wherever it ends.
What Green Bay's Weather Puts on a Roof, Season by Season
The water doesn't arrive evenly. Here's how a normal year breaks down for one downspout draining 600 square feet of roof.
| Season | Normal precipitation | Arrives as | Gallons at one 600 sq ft downspout | What it does at the foundation |
|---|---|---|---|---|
| December–March | 6.30" | Mostly snow (about 47.5" in a normal winter) | About 2,360 | Sits on the roof, then comes off on thaw days while the ground is frozen |
| April–May | 6.35" | Rain and the last snow | About 2,380 | Lands on soil already soaked from the melt |
| June–August | 11.11" | Thunderstorms; June is the wettest month at 4.10" | About 4,160 | Big bursts in short storms, when washouts can dig fastest |
| September–November | 7.85" | Fall rain, early snow | About 2,940 | Loads the soil with water right before it freezes |
| Full year | 31.61" | About 11,800 |
Normals: Wisconsin State Climatology Office, Green Bay, 1991–2020.
About two-thirds of the year's water falls from April through September. That's washout season. The winter third causes a different kind of trouble, covered below.
About snow: "10 inches of snow equals an inch of water" is only a rule of thumb. The National Weather Service notes it's right only about a quarter of the time, and heavy, wet March snow holds far more water than January fluff. The totals above measure the melted water directly, so they skip the guesswork.
Worked Example: One Green Bay House, Four Corners
Take a fairly typical layout: a 1,600-square-foot ranch (32 by 50 feet) with an attached two-car garage (24 by 24) and about a foot of overhang, for a roof footprint of roughly 2,400 square feet. Its four downspouts don't split the roof evenly, because garage roofs and valleys funnel more water to some corners.
| Downspout | Roof area it drains | Gallons in a 1-inch storm | Gallons in a normal year | Where it often ends on a house like this |
|---|---|---|---|---|
| Front corner, by the stoop | 500 sq ft | About 310 | About 9,850 | A short elbow aimed at the stoop, or a splash block in the mulch |
| Back corner, by the patio | 500 sq ft | About 310 | About 9,850 | Beside the patio slab |
| Garage front corner | 700 sq ft | About 440 | About 13,800 | Onto the edge of the driveway apron |
| Inside corner, where garage meets house | 700 sq ft | About 440 | About 13,800 | Right next to the AC pad |
| Whole house | 2,400 sq ft | About 1,500 | About 47,300 |
Knock off 10 to 20 percent for evaporation, splash, and overshoot in downpours, and it's still around 38,000 to 43,000 gallons a year. Other ways to picture it:
- Weight. Water weighs about 62.4 pounds per cubic foot, a little over 8 pounds a gallon. The 47,300 gallons off this roof weigh roughly 394,000 pounds: nearly 200 tons a year, landing within a few feet of the house.
- One storm. A 1-inch storm sends about 440 gallons out of each garage-side downspout in a few hours. That's eight 55-gallon drums poured onto one corner.
- Concentration. Say a downspout draining 600 square feet empties onto a patch 2 feet by 3 feet. That patch gets 100 times the water the rest of the yard gets: the equivalent of about 3,160 inches, more than 260 feet of rain a year, on 6 square feet of soil.
That last one is the whole case for extensions. Your yard can handle 31 inches of rain a year. No patch of soil can handle 260 feet.
Why It Lands on the Worst Soil on Your Lot
A short downspout drops all that water into the strip right along the foundation, the weakest ground you own. When your house was built, the hole was dug wider than the basement walls, and the gap was backfilled afterward, often loosely. That ring of backfill is softer and more porous than the ground around it.
Around Green Bay, the undisturbed ground is often red, clay-rich glacial till or old lake clay, which lets water through slowly. So the loose backfill fills up like a bathtub. Your stoop, front walk, AC pad, and the house end of your patio and driveway all sit on top of that tub.
The Damage Chain, From First Rain to Ice Sheet
A short downspout doesn't sink concrete overnight. It works in stages:
| Stage | What's happening underground | What you see from the yard |
|---|---|---|
| Saturated backfill | Soil under the slab edge stays wet for days after every rain and loses strength | Soft, muddy ground at the outlet; a splash block that has tipped |
| Washout | Moving water carries fine soil out from under the slab edge, a little each storm | A gully in the mulch; a gap under the slab edge you can slide fingers into |
| Sinking | The slab loses support at the corner nearest the downspout and settles into the hollow | A stoop, walk square, or AC pad lower at one corner; a new trip edge |
| Tipping | The slab now slopes toward the house | Puddles against the foundation; a widening gap where the slab meets the wall |
| Water at the basement | Even more water heads straight down the foundation wall | A damp basement corner, a musty smell, or white mineral crust on the inside of the wall |
| Winter ice | The low spot holds water, and thaw-day melt runs across the walk | The same ice patch every morning, and the salt you keep throwing on it |
The chain feeds itself: a tipped slab sends more water at the foundation, which softens more backfill, which lets the slab drop further. That's why the cost of waiting tends to climb. Wet basements have plenty of causes, and we don't do foundation work, but roof water beside the wall is a usual suspect and the cheapest one to rule out.
Spring Melt: Why March Is Rough on Your Concrete
From December through March, about 2,360 gallons fall on that 600-square-foot roof section, mostly as snow. Much of it sits on the roof and comes off in pulses on sunny late-winter afternoons.
The timing is the problem. Soil thaws from the top down. Wisconsin's Department of Transportation puts weight limits on roads every spring for exactly this reason (they average roughly March 9 to May 9): meltwater gets trapped in the thawed top layer, above ground that's still frozen.
Your backfill does the same thing. Roof melt pours onto a few inches of thawed soil over a frozen layer. It can't soak down, so it turns that layer to mush or runs sideways under your slabs, while the concrete sits on its weakest support of the year.
Add the snow shoveled against the house all winter, plus the extension flipped up in November for the snowblower, and it's no surprise that spring is when so many homeowners first notice a stoop or walk has dropped. For winter habits that help, see winter concrete care.
The 15-Minute "Where Does My Water Go?" Audit
Do this on a dry day, then repeat the last step during the next good rain.
- 1Sketch the house and mark every downspout, including the garage and any porch roof.
- 2Estimate the roof area each one drains. Pace off the gutter that feeds it and multiply by the horizontal distance back to the roof peak (on a simple gable, about half the building's width plus the overhang). Split shared gutter runs. A valley counts toward the downspout it empties into.
- 3Multiply by 20 for roughly how many gallons a year it handles in Green Bay.
- 4Follow the water. How far from the house does it come out, what concrete is nearby, and which way does the ground slope?
- 5Look for evidence: gullies, washed-out mulch, a gap under a slab edge, a tipped splash block, mud on the siding, or a dark stain on the concrete.
Fill in a row for each one:
| Downspout | Roof area | × 20 = gallons a year | Comes out how far from the house? | Nearest concrete | Evidence | Priority |
|---|---|---|---|---|---|---|
| Example: front corner | 25 ft × 18 ft = 450 sq ft | About 9,000 | 1 ft (short elbow) | Stoop, 2 ft away | Gully in mulch, stoop corner low | Fix this weekend |
How to rank them:
- Fix this weekend: it comes out within 6 feet of the house and near any slab, or you found a gully, a gap, or a tipped slab.
- Fix this season: it comes out 6 feet or more away, but onto flat ground or a low spot that drains back toward the house.
- Fine for now: it comes out 6 to 10 feet away onto ground that clearly slopes away, past the end of any concrete.
Then the rain check. During the next steady rain, walk the house with an umbrella. Water that runs back toward the house, travels along a slab edge, or disappears under concrete goes to the top of the list, whatever the math said.
Fixing It Is the Cheap Part
You don't need us for this. The full how-to, with extension types compared for Wisconsin winters, is in how to extend downspouts. The short version:
- Get the water at least 6 feet out, 10 if you can, past the far edge of any slab, onto ground that slopes away.
- Keep the gutters flowing, or the water dumps along the roofline before it reaches a downspout. See gutter cleaning to protect your concrete.
- Fix the grade where soil has settled into a trough along the foundation. See soil grading around concrete.
- Audit the sump discharge too. It's another downspout, one that starts in the basement.
Where We Come In
We don't install gutters, extensions, or drain lines. We fix the concrete the water already got to:
- Foam lifting raises a sunken stoop, walk, patio, or AC pad through 5/8" holes about the size of a dime, filling the hollows the water carved as it lifts. Cured foam is waterproof, so it won't wash away like the soil did. Here's how concrete lifting works.
- Joint and crack caulking closes the gap where the slab meets the house, so water on the concrete runs off instead of down. See caulking where concrete meets the house.
Fix the downspout either way. Lift a slab and leave the elbow dumping beside it, and next spring's melt starts carving a new hollow.