Foundation settling: what's normal and what's a problem

What is foundation settling?

All houses sit on soil, and soil compacts under the weight of a house. If your house settles evenly, you may never know it happened. Our structural contributor, Daniel Mercer, says the houses that end up needing an evaluation usually have differential settling: one corner, one stretch of footing or one interior support goes down more than the rest. That pulls the house out of square and opens cracks between its parts. People often call all of this house settling, but it’s the uneven kind that matters.

Settling isn’t the only way a foundation moves. Heave works the other way: soil that swells or freezes pushes part of the house up. And if you have a crawl space and the floor slopes, it may not be the footing. Daniel often finds a sagging beam, a deteriorating post or wood weakened by moisture instead.

1. Uniform settling: the whole house drops about evenly. Often few or no cracks.
2. Differential settling: one part drops more. The house is pulled out of square and cracks open.
3. Heave: soil that swells or freezes pushes part of the house up.

SoilFootingWhere the house started

Why houses settle

The soil under a house changes for a handful of reasons:

• Fill that wasn’t compacted well. Soil brought in to level a lot can keep compressing for years.
• Water at the foundation. Downspouts that empty next to the wall, a yard that slopes toward the house, a leaking pipe or a sump pump that discharges a few feet away.
• Soil that swells and shrinks. Some soils change volume as they get wet and dry out.
• Frost. Freeze-thaw cycles move shallow soil, which matters more in colder states.
• Voids underground. Sinkholes in limestone areas (karst) and old underground mines can take support away from part of a house.

Water is one of the first things I look into. But finding water doesn’t mean it caused the settling. I separate three ways water can affect a foundation. It can wash soil out from under a footing. Soil can lose strength when it’s saturated. And some soils swell and shrink as they get wet and dry out. Water can also weaken wood supports without the footing moving at all.

I look for water that’s discharging near the foundation: downspouts, how the ground slopes when it rains, the end of the sump pump discharge line, standing water, washed-out soil, buried pipes and signs of moisture inside. I also ask when the cracks first showed up, and whether they get worse after rain or after a leak. A sump pump that discharges a few feet from the foundation can send the same water right back.

1. Loose or poorly compacted fill under a footing
2. A downspout emptying at the wall, with the yard sloping toward the house
3. A leaking pipe washing out or softening the soil
4. Soil that swells when wet and shrinks when it dries
5. Frost depth: shallow footings above it can move
6. An underground void: a sinkhole in limestone (karst) or an old mine

SoilConcrete / masonryWood

Here’s what’s worth checking in the states where Daniel and I work. These conditions show up in some areas, not everywhere:

None of this applies to every lot in a state. Daniel says conditions change more from one lot to the next than from one state to the next. If there’s any sign of an old mine, a void or sinking ground, he wants the soil investigated before anyone decides on piers.

Normal settling vs. a foundation problem

This is how Daniel sorts what he sees:

No single crack width tells you whether a house is safe. Daniel looks at more than that: the pattern, any offset, how the crack changes over time and what’s around it.

Signs of foundation settling

Inside the house:

• Several doors or windows that start to stick or fall out of square at the same time.
• Drywall cracks at the corners of doors and windows that come back after you patch them.
• Gaps between the floor, the wall or the baseboard that keep getting wider.
• A floor slope you can feel, especially one that’s getting worse.

Outside:

• Stair-step cracks in brick or block, especially where the units have shifted.
• Diagonal cracks running from the corners of windows and doors.
• Gaps around window and door frames.

Any one of these on its own may not mean much. When several show up together and keep getting worse, that’s when Daniel gets concerned.

Settling cracks: which ones are normal

Hairline shrinkage cracks that aren’t growing are typical in concrete and usually aren’t a sign of settling. A stair-step crack in a block or brick wall shouldn’t be dismissed as cosmetic, especially if the blocks have shifted. A horizontal crack in a basement wall that bows into the house usually comes from soil pressure, not settling. That’s a different problem, covered in our guide to bowing basement walls.

For how to read each type, see our guide to foundation cracks.

How much settling is acceptable?

There’s no magic number. A 1-inch (25 mm) difference between two points on your floor is just that: a measurement, not a diagnosis. You need to know whether the house was built that way, whether the floor framing is sagging, or whether the foundation itself has moved.

Engineers take relative elevations across the floor with a laser or a level. They set fixed reference points and come back months later to measure again. They also check walls for lean and bowing, measure cracks and inspect the supports.

Some engineering groups have published criteria. A 2007 guideline from the Foundation Performance Association in Houston recommends that tilt across a foundation stay at or under 1%, and that deflection stay within about L/360 (the span length divided by 360). It was written for Texas, though, and says to use it with caution elsewhere. No building code says that a crack this wide means you need piers. The International Residential Code requires footings that carry the house’s loads to suitable soil, and ground that slopes water away from the foundation. In the end, it comes down to whether your house is still moving.

How to track movement

If the signs are mild and nothing looks dangerous, Daniel suggests this:

1. Photograph each crack with a ruler in the shot, the date, and the same angle every time.
2. Mark two or three permanent points and record the width and length.
3. Sketch a simple floor plan with the cracks, the doors that rub and any slopes.
4. Note heavy rain, dry spells, leaks and any digging or construction nearby.
5. Take a first reading, another after a month, then every three months for 6 to 12 months, if a professional thinks it’s safe to watch.

That schedule is a rule of thumb, not a code requirement. If the readings start changing faster, a wall looks like it’s leaning, or a support has lost contact, don’t wait for the next reading. There’s more on measuring cracks in how to monitor a crack.

Foundation settling repair: how it’s fixed

Repair starts with the cause. If you can find it and fix it, that may be most of the job.

When the house keeps settling, the usual fix is to carry the foundation down to firmer soil with piers or, for floor slabs, to lift them:

1. Bracket attached to the footing
2. Steel pier, driven down using the house’s weight to push against
3. Helix plates: the pier is screwed in, and the installation torque is recorded
4. Firmer soil the piers are meant to reach

SoilFirmer soilConcreteSteel pier and bracket

You don’t always have to lift a house back to level. Lifting it can open new cracks and shift walls, pipes and windows. Often the reasonable goal is to stabilize the house and bring it back only part of the way.

For prices, see foundation repair cost.

When to call a structural engineer

A contractor can handle sealing or a simple repair when it’s obvious what needs to be done. When the plan involves piers, underpinning, new supports, straightening a wall, lifting the house or correcting active movement, Daniel wants an independent structural engineer to define the problem and the scope first. If the soil itself is the unknown, a geotechnical engineer should be consulted too.

He wouldn’t automatically avoid a company that both diagnoses and repairs, but he’d keep the conflict of interest in mind. You can’t compare a quote for eight piers with one for four on price alone, until you know what the piers will carry, where they’ll go and how they were designed. More on that in how to compare foundation repair quotes.