How to fix grading, downspouts and window wells to stop a wet basement

Where the water comes from, in order

Roof water first. One inch of rain on a 1,000 sq ft roof is about 620 gallons; on a 1,500 sq ft roof, about 930. A gutter that overshoots at the valley, or a downspout that ends on a splash block tilted toward the house, puts all of it into the two feet of soil along the wall. That soil is backfill, looser than the undisturbed ground around it, and water that lands there runs down the outside of the foundation to the footing, where it finds the drain tile if the drain tile works and the floor-wall joint if it does not.

Lot water second. A yard that slopes toward the house, a patio or driveway that has settled toward the wall, a neighbor's runoff, and a window well with no drain all deliver water to the same place, more slowly.

Ground water last. A seasonal water table that rises to the footing shows up at the cove joint on every wall after days of rain or during the spring thaw, not under one downspout. It is the only one of the three that an interior drain system is built for.

Our waterproofing checklist is the diagnosis: the walk-around and the foil test that tell you which of the three you have. This guide is the repair for the first two, in the order water travels, with the measurements that decide whether each fix is done.

Tools and materials

Tools: a straight 10-ft board and a 4-ft level (or a string line, line level and tape), round-point shovel, rake, hand tamper or a rented plate compactor, wheelbarrow, garden hose, ladder and gutter scoop; for a buried line, a trenching shovel or rented trencher, a PVC saw and a torpedo level.

Materials: downspout extensions in the same profile as the downspouts (2x3 or 3x4 in.) with hinged elbows, straps, screws and splash blocks; for a buried line, 4-in. solid (not perforated) PVC drain pipe, a downspout adapter, a cleanout tee, long-sweep elbows and a pop-up emitter or grated outlet with a rodent screen; fill soil with some clay in it (screened fill or a clay-loam mix, not topsoil or compost), seed and straw or sod; window well covers, washed 3/4 to 1-1/2 in. gravel, 4-in. perforated pipe or a drain grate, and polyurethane sealant.

Call 811 before you trench; downspout lines share the yard with gas, electric and irrigation lines.

Gutters: the roof has to land in the pipe

Gutters fail in a hard rain for three reasons: clogged, pitched wrong, or not enough outlet. The Building America guidance published by the Department of Energy puts numbers on the last one: about 1 sq in. of downspout cross-section for every 100 sq ft of roof, downspouts spaced 20 to 50 ft apart, and the gutter sloped at least 1/16 in. per foot toward them. A 2x3 in. downspout is 6 sq in., good for roughly 600 sq ft of roof; a 3x4 is 12 sq in. and handles about 1,200. A 40-ft gutter under a deep roof plane with one 2x3 at the far end overflows every time it rains hard, exactly where you do not want it to.

The overshoot test is to stand outside in a downpour and watch. Sheeting over the front edge at a valley means a splash guard or a 6-in. gutter there; pouring over an end cap means the pitch runs the wrong way or the outlet is blocked; water behind the gutter means no drip edge or a gutter pulled off the fascia.

Measuring the result: the sump and the hygrometer

The outside work shows up inside in two instruments you already own.

Sump run time. During one good rain before the work, count how many times the pump cycles in an hour and how long it keeps cycling after the rain stops. A pump that runs every few minutes for two days after a storm is pumping roof water that soaked down the outside of the wall. After the gutters and downspouts are fixed, the same storm should produce fewer cycles and a pump that goes quiet within hours. If it still runs on a steady rhythm days later, that is ground water; our running constantly guide covers what it means for the pump.

Basement humidity. A hygrometer in the middle of the basement, read daily for the week after a rain, climbs when wet walls are evaporating into the room. Fix the water and the summer reading falls, often by 10 points, before any dehumidifier is involved; the target and the method are in our basement humidity guide.

A sensor where it used to come in. A leak sensor on the floor under the window that used to leak, or at the base of the wall below the worst downspout, is a cheap witness: dry through the next storm season means the fix held. The kit below reaches a basement corner that Wi-Fi does not.

What a landscaper or waterproofer does differently

The homeowner version of this job is a board, a level and a weekend. A grading or landscape contractor shoots the lot with a laser or a transit, which finds fall a board cannot see and settles whether a swale or a dry well is the answer; brings soil by the truckload and compacts it in lifts so it does not settle back into a dish; cuts swales and berms that move water across a lot with less than 6 in. of fall; installs curtain drains (gravel and perforated pipe in a trench uphill of the house) where water arrives from a slope or a neighbor's yard; trenches downspout lines by machine at a consistent grade with cleanouts; and excavates to the footing to tie a window well drain into the drain tile instead of guessing with a gravel column.

A basement waterproofer starts where the landscaper stops. If the exterior is right and water still enters at the floor-wall joint on every wall after days of rain, that is ground water, and the fixes are inside or below: an interior drain tile system feeding a sump, or, for block walls that weep across their face and walls with structural cracks, an exterior membrane with a new footing drain. The checklist explains when each is warranted and the sump pump cost guide what the pit and pump add. If you end up with a sump, the pump is the whole system, and the one below is the primary we would specify.

When to call a pro

Call a landscaper or grading contractor when the lot needs more than a few wheelbarrows of soil, when there is no visible fall and a swale or dry well has to be designed, when a downspout line has to cross a driveway or run more than 50 ft, when water arrives from a slope or a neighbor's yard, or when a window well drain has to be tied into the footing drain by excavation. Call a basement waterproofer when the outside is right and water still enters at the cove on every wall after days of rain, when a wall shows horizontal or stair-step cracks (an engineer first), or when the house has no footing drain at all. Call a plumber if the sump is the end of the line. If the basement is wet right now and the water has reached finished walls or floors, the first 24 to 48 hours decide whether it is a cleanup or a mold job; our 72-hour guide covers that, and the form below reaches vetted local landscapers, waterproofers and plumbers through HelpMeNow.com.