Roof Runoff Calculator: How Much Water Hits Your Foundation?

One inch of rain on a 2,000 square foot roof footprint sends about 1,246 gallons of water to the ground around your home. That water runs off the roof, through the gutters, and out the downspouts. If the ground slopes toward your house, much of it ends up against your foundation.

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What Your Number Means

Your roof is a funnel. Every square foot of it catches rain, and the gutters gather all of it into a few downspouts. That is why the totals in this calculator look so big. A 2,000 square foot home does not get a little water in a storm. It gets more than a thousand gallons, and all of it comes down in a few narrow streams at the corners of the house. If those streams dump right beside the foundation, the soil there gets soaked while the rest of the yard stays drier. Over time, that uneven soaking is what moves a foundation.

The number also explains why small drainage fixes matter so much. Moving a downspout outlet a few feet farther from the house, or fixing the slope of the ground beside one wall, can redirect hundreds of gallons per storm. You do not need to stop the rain. You need to give it a path away from the house. In Shreveport, that job is harder than it looks, because the soil itself works against you. Caddo Parish sits on expansive clay that swells when it soaks up water and shrinks when it dries out. September 2026 was the 8th driest September in 132 years for the area, followed by rain. That is the shrink-swell cycle in one season: dry clay pulls away from the foundation and opens gaps, then a downpour fills those gaps and swells the clay back up, pushing on the walls. Homes in South Highlands, Broadmoor, Cedar Grove, and Queensborough all sit on this same clay, and they all feel the same cycle.

Use your result as a way to look at your own yard with fresh eyes. After the next rain, walk around the house and watch where the water goes. Does it run away from the walls, or does it sit in a puddle against the brick? Do the downspouts end right at the foundation, or do they carry water out into the yard? Does one corner stay wet long after the rest of the yard dries? Each yes is a clue, and the calculator tells you how much water is behind each clue. A single downspout pouring 300 gallons beside one corner in one storm can keep that corner of soil swollen while the other corners dry and shrink, and that uneven movement is what cracks walls and sticks doors.

Gutters, Extensions, and Grading

Gutters only help if they are clear and pitched right. Leaves, pine needles, and roof grit collect in them, and a clogged gutter spills water over the edge in a sheet, right beside the foundation. That is often worse than having no gutter at all, because the water is concentrated along the whole wall instead of spread out. Cleaning gutters before the rainy season and after big storms is one of the cheapest things a homeowner can do for a foundation. While you are up there, look for sections that sag or pull away from the fascia, and for seams that drip. A gutter that holds standing water is pitched wrong, and it will spill at the low spot every time it rains.

Downspout extensions are the next step. A downspout that ends at the foundation puts the whole roof load from that corner into one small patch of soil. An extension carries that water out into the yard, where it can spread and soak in over a wide area. Extensions can be simple splash blocks, hinged plastic sections, or buried pipe that pops up in the lawn. What matters is the landing spot: dry, open ground that slopes away from the house, not a low spot that sends the water right back. In many Shreveport yards, the ground has settled over the years so it now slopes toward the house instead of away from it. That is common near the foundation, where the backfill soil from construction settles more than the undisturbed soil farther out. Adding soil and shaping it so water runs away from the walls is grading work, and it is often the first fix an inspector suggests before any bigger drainage system.

Grading does not need to be dramatic. The goal is a steady slope away from the house with no dips where water can sit against the wall. Flower beds piled high against the brick can trap water instead of shedding it, and mulch that dams up against the siding holds moisture where you least want it. Keep soil a few inches below the siding line, keep beds shaped so water runs off them, and watch what happens in the first heavy rain after any change. If water still ponds, the yard may need a drain system rather than more dirt. That is a judgment call an inspector can make on site, because the fix depends on how much water arrives, where it comes from, and where it can legally and safely go.

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French Drains and Catch Basins

When gutters and grading are not enough, the next step is a system that collects water and moves it. A French drain is a buried trench filled with gravel around a pipe with holes in it. Water in the soil seeps into the pipe and flows to a safe outlet, instead of sitting against the foundation. It works well for yards where water moves through the soil toward the house, or where a low spot holds water after every storm. A catch basin is a box set into the ground at a low point, with a grate on top. Surface water flows into the box and out through a buried pipe. It works well for driveways, patios, and yard low spots where water ponds on the surface.

Both systems live or die on the outlet. A drain that has nowhere to go will fill up and stop working, and a catch basin that clogs with leaves becomes a small pond. The outlet must run downhill to daylight, a ditch, or a storm drain, and it must stay clear. That is why drainage work starts with a walk of the whole lot, not just the wet spot. The inspector looks at the roof lines, the neighbor lots, the street grade, and the natural low points of the yard, then plans a path for the water that works with the slope instead of against it. Homes in Bossier City, Benton, Haughton, and the other towns around Shreveport face the same clay and the same storms, but each lot drains differently, so each plan is drawn for that lot.

One mistake to avoid is fixing only the symptom you can see. Filling a low spot with dirt can push water onto a neighbor or against a different wall. A drain that stops short of a real outlet can dump water right back at the foundation on the other side of the house. And a sump pump in a crawl space, while useful for water that gets under the house, does not fix water that is soaking the soil outside. The right order is simple: roof water first, then surface water, then water in the soil. Gutters and extensions handle the roof. Grading and catch basins handle the surface. French drains handle the soil water. An inspector who follows that order will usually find the cheapest fix that actually works.

The table shows the same math the calculator uses, for one common roof size. If your home is bigger or smaller, the totals scale in direct proportion. A 3,000 square foot footprint collects half again as much water as a 2,000 square foot one in the same storm. That is why large homes need more downspouts, longer extensions, and sometimes a drain system, even when the roof and gutters look fine.

Signs Water Is Reaching Your Foundation

Water leaves clues before it cracks anything. Inside, look for a white powdery film on basement or crawl space walls, which is mineral residue left behind when water seeps through concrete and dries. Look for damp spots, musty smells, and stains on the walls of a crawl space, and for insulation that sags or falls because it got wet. Outside, look for mulch washed out of beds, soil eroded in lines from the roof edge, and green algae or dark stains on the foundation wall near the ground. After a storm, look for standing water against the house that is still there hours later, and for one corner of the yard that stays muddy while the rest dries.

Cracks tell the water story too. A crack that widens after heavy rain, then seems to close up in dry weather, is tracking the soil moisture cycle. Stair-step cracks in brick that grow a little with each wet season are another sign. Doors and windows that stick more in wet months than in dry months point the same way. None of these signs alone proves a foundation failure, but together they show water is driving the movement, and water is the part you can often fix first. If you want the full picture, read our signs of drainage problems post and our guide to Louisiana clay soil and foundation damage. For the fix itself, see our drainage correction service, crawl space repair service, and foundation repair costs guide. Homes in ZIP codes 71106, 71111, 71112, 71006, 71037, 71033, 71047, and 71009 all share the same clay and the same storm patterns, so the signs and the fixes travel well across the area.

Roof Runoff Calculator Questions

How many gallons of water come off my roof in a storm?

Multiply your roof footprint in square feet by the rainfall in inches, then by 0.623. One inch of rain on a 2,000 square foot footprint gives about 1,246 gallons. The calculator above does the same math for your numbers.

Does roof size or rainfall matter more?

They matter equally, because they multiply. Doubling either one doubles the total. A big roof in a light rain can send as much water to the ground as a small roof in a heavy storm, which is why both numbers go into the calculator.

How far should downspouts carry water from the house?

Far enough that the water soaks into open ground that slopes away from the house, not into the soil right beside the foundation. Extensions, splash blocks, or buried pipe all work when the landing spot is right. The free inspection checks every downspout outlet on your home.

Can runoff really crack a foundation?

Yes, through the soil. Water that soaks one side of a foundation swells the clay there, while the dry sides shrink. That uneven movement cracks walls and slabs over time. The drought then downpour cycle in northwest Louisiana makes the effect stronger, because the swings between dry and soaked are so large.

What if my gutters overflow in heavy rain?

Overflow usually means a clog, a sag, or gutters and downspouts that are too small for the roof area. Start with cleaning and a look at the pitch. If clean gutters still spill, the system may need a larger downspout or an extra one. An inspector can tell you which it is during the free visit.

Is this calculator exact?

The math is exact for the numbers you enter. Real runoff varies with roof slope, gutter condition, and ground slope, so treat the total as a planning figure. It shows the scale of the water your drainage must handle, not a measurement of a past storm.

What does the free inspection cover for drainage?

Gutters and downspout outlets, grading and slope around the house, spots where water ponds, crawl space moisture, and signs of soil movement. You get a written plan for the fixes that matter, with the price for your home. Qualifying repairs carry a 20-year structural warranty. Call (318) 585-2003 to book yours.

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Pelican Foundation Repair serves Shreveport and the surrounding areas, including Bossier City, Blanchard, Greenwood, Keithville, Benton, Stonewall, Haughton, Red Chute, Eastwood, Mooringsport, Minden, and Ruston, Louisiana. Learn more about crawl space encapsulation and all of our services.

Contact Pelican Foundation Repair

Pelican Foundation Repair
6118 Fairfield Ave, Shreveport, LA 71106, USA
Phone: 318-585-2003
Hours: Monday–Saturday, 8:00 AM–6:00 PM
Website: foundationrepairshreveport.com