Water, Drainage and Foundations: Grading, Gutters and French Drains

Last reviewed October 28, 2026 · 6 sources cited

Almost every foundation cause runs through water. Too much of it, in the wrong place, pushes on walls, swells clay and washes soil away; too little shrinks clay. Moving roof and surface water away from the house is the cheapest and most broadly useful measure.

Surface water: grading and gutters

A University of Missouri Extension publication gives a simple standard: grade the ground to fall 6 inches over 10 feet away from the house, and use gutters and downspouts with extensions. It estimates that without extensions each 1,000 square feet of roof can dump about 27,000 gallons of water next to the house per year.^[1] The publication is undated and written for Missouri conditions, but the arithmetic of roof runoff is universal.

The concrete industry gives a similar figure for new construction: final grade should slope 1/2 to 1 inch per foot for at least 8 to 10 feet from a basement wall.^[2] The Colorado Geological Survey's 1978 guidance for swelling soils adds that downspouts and splash blocks should carry roof runoff at least 10 feet from the building, grass, shrubs, flowers and sprinklers should stay at least 5 feet from the foundation, and any soil settlement next to the foundation should be corrected promptly because low spots pond water.^[3]

Subsurface water: hydrostatic pressure and the water table

Oak Ridge National Laboratory's foundation handbook says the goal is to keep water away from the foundation by sloping the ground, and notes that water pushing on walls from rainfall, irrigation or snowmelt (hydrostatic pressure) can enter through the wall and that a damp-proof coating does not stop it. It recommends perimeter drains where the crawl space floor is below the surrounding grade, discharging to daylight or an internal sump.^[4] The University of Missouri publication similarly describes a buried tile drain along the footing for severe cases and a sump pump on flat sites with no outlet.^[1]

French drains

A French drain is a perforated pipe in a gravel-filled trench that collects groundwater and carries it away. It addresses subsurface water, not roof runoff. The ASCE Texas Section guidelines list subsurface drains among non-structural remedies, and specifically caution against running downspouts into perforated subsurface drains.^[5] A drain that is poorly sloped, clogged or discharging next to the house can make things worse, so design and discharge location matter.

Irrigation and the other extreme: too little water

In clay soils the problem can be dryness as much as wetness. The same Texas engineering guidelines list a consistent watering program among non-structural measures and warn to avoid excessive watering.^[5] Moisture swings, not wetness itself, drive movement in expansive clay soil.

Plumbing leaks

A leaking supply or drain line under or beside a foundation adds water in one place, softening soil under a slab or swelling clay locally. The ASCE Texas Section guidelines advise testing water, sewer and gas lines after a foundation is lifted and repairing leaks.^[5] Unexplained spikes in water use or damp spots are reasons to check.

What drainage cannot do

Drainage reduces water loading. It does not re-lift a foundation that has already settled, and it does not repair walls already cracked. It is often part of the repair, and the American Concrete Institute makes the same point about slabs: slabjacking levels a slab but does not correct underlying poor soil conditions.^[6]

A seasonal checklist

• Before storm season and again before winter: clear gutters, confirm downspout extensions are in place and discharging away.
• After heavy rain: look for water pooling within about 10 feet of the house, the distance the Colorado Geological Survey gives for downspout discharge.^[3]
• In dry spells: check for soil pulling away from the foundation.
• After any landscaping work: confirm the grade still slopes away.