How do I keep a basement floor from shifting?

A basement floor shifts when the ground under it or beside it changes, so the way to keep it still is to keep the soil's moisture steady and send water away from the house. Most of that is maintenance you can do from ground level: grading, gutters, downspouts, the sump pump and a record of any cracks.

Why a basement floor moves

Some soils change volume with moisture. Fairfax County's guide to its marine clay soils says the clays shrink during dry periods of the year and swell during wet periods,^[1] and that damage to floor slabs occurs when underlying clays shrink and swell, sometimes requiring replacement of the floor slab.^[1] Not every Virginia lot has clay like that, but water pooling at a foundation, or soil drying out under it, causes trouble nearly anywhere. The Northern Virginia Soil and Water Conservation District says most wet basements are a result of water problems outside of the house.^[2]

The aim is therefore a steady moisture level around and under the house. The Fairfax guide describes its maintenance steps as maintaining good drainage around the house and keeping the soils under the footings and in the basement wall backfill at stable moisture levels.^[1]

Start outside, at the grade

Backfill next to basement walls normally settles after several years, leaving depressional areas for water to collect.^[1] The district's advice: "Land around your house should slope away from foundation walls. Ideally, a 5% grade should be maintained for a distance of ten feet around your home."^[3] Fill any low spots against the foundation with soil, not mulch, and keep the slope pointed away from the wall. In Fairfax County's clay areas, the guide specifically calls for silty or clayey soil rather than sand, gravel, or mulch to build the grade up.^[1]

Gutters and downspouts

A house sheds a lot of water. The district notes that when it rains hard, an average sized home can experience hundreds of gallons of runoff coming from the roof,^[3] and that if downspouts and sump pumps are not set up correctly, it can cause foundation problems, erosion or wet spots in the yard.^[3] It says gutters should be cleaned free of debris at least once a year,^[3] and more often with trees close to the house. Downspout or sump pump extensions (non-perforated corrugated plastic pipes) can be attached to the downspouts to direct flow further from the house.^[3] If your gutters are high, have them cleaned by someone with the right ladder and experience.

Sump pump and stairwell drains

The district's list of actions for a wet basement includes these: make sure your sump pump is turned on and functioning, and clear basement stairwell drain grates of leaf litter and debris.^[2] The Fairfax guide adds that basement sump pumps, if required to maintain a dry basement, should be in good working condition.^[1]

Trees, pools and watering

Roots pull water out of the soil. The Fairfax guide says large trees should be at least 20 feet away from the house.^[1] It also says that if you have a swimming pool, you should make sure that there are no leaks,^[1] since heavy leakage into the soil can weaken it. Do not try to fix a dry spell by soaking the ground: the guide says "We do not generally recommend this practice, however, since uncontrolled addition of water can create more problems than it solves."^[1]

Keep a record of cracks and level

Slabs and walls often crack and stay cracked, and a crack by itself is not proof of a problem. What matters is whether it changes. The Fairfax guide says to measure and keep a record of the width and extent of the cracks and the time of the year that you made the recordings.^[1] Add a photo of each crack with a ruler beside it. Note, too, whether a floor that slopes now sloped last year. The guide observes that foundations that have settled during dry periods will often return to near the original position after rainfall replenishes the soil moisture,^[1] so the season belongs in your notes.

Stop and call a pro if

• The floor is rising, sinking or cracking in new places. Movement that continues after you have fixed the drainage needs a diagnosis. Fairfax County says correctly identifying problems usually requires a thorough investigation of the soil and structural conditions by a qualified professional.^[1]
• Walls are cracked or bowing. The district says wet basements and wall or foundation cracking can have serious consequences for your home's structural integrity and your health, and that they should be addressed right away.^[2]
• You suspect a gas pipe or meter is being pushed on. Call the gas company immediately if you think that your natural gas pipe or gas meter is under pressure from a damaged wall, since an explosion could occur from a leaking pipe.^[1]
• The basement stays wet after you have done the outside work. The district's advice for that case is: "If the problem remains, call a professional."^[2]
• Anything needs cutting, jacking or digging at the footing. That is foundation repair, not maintenance.

Getting help in Virginia

If the floor is already uneven, read why floors in a house go uneven and, if you only need it flat for new flooring, how to level a floor before installing it. Browse more flooring answers or the flooring guide. When you are ready, describe the problem and licensed local contractors quote it.