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How to Diagnose and Fix Sagging Floors

Carol J Alexander

Published on February 13, 2023


How to Diagnose and Fix Sagging Floors

If you can’t drop a pencil without it rolling away, you need our complete guide to sagging floors, what causes them, and how to get your home on the level again.

To provide you with the most accurate and up-to-date information, we consult a number of sources when producing each article, including licensed contractors and industry experts.

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Have trouble walking a straight line across your house? You may not be tripping over your own two feet; you may be tripping over your floor. Sagging floors in older homes can cause uneven floorboards or cracked and protruding tiles. Keep reading to learn what causes them and what to do about it.

Find out how much it costs to replace the floors in your home

Signs of a sagging floor

Your kids' Hot Wheels roll to the center of the room without the slightest provocation, and your Jenga towers always fall in the same direction, but that’s not quite enough to convince your general contractor that you have sagging floors. So, to uphold their professional integrity, they’ll look for the following signs.

Cracks in the walls of the house

When a floor sags, it pulls the entire wall with it. If you see hairline cracks running across the plaster or drywall in your home, the floor in that room is quite uneven.

Windows and doors problems

It may not be humidity causing your doors and windows to stick. Sagging floors sometimes shift the wall, making windows and doorframes go out of square. Ask yourself if the stickiness is seasonal or happens all the time. You may also notice cracks in the wall running from the corner of a window toward the floor or ceiling.

Sloping floors

If the fish in your aquarium appear to swim uphill, your floors are out-of-level. No fish? Does the top of your entertainment system line up with the ceiling? Use a level on the floor to verify your suspicions if anything looks askew.

Bouncy or squeaky floors

Some floors have a bit of a give when you walk across the room. For example, if items atop your tables seem to rattle when you walk by, it indicates a lack of support underneath.

Cracked grout or tiles

When floors shift because the support underneath gives way, tile floors won’t withstand the pressure. Look for cracks in your grout or the tiles themselves as a tell-tale sign that your floor joists need a hoist.

Uneven space under baseboards

You should see minimal spacing between the baseboards’ bottom and the finished floor. And the spacing should be consistent around the room. If you notice the gap widening in one corner, that’s a sure indication that the floor is sagging.

The parts of a floor

Do you know the song about how the knee bone’s connected to the leg bone? Well, the same is true for your home. Floor joists connect to the foundation, the wall studs, and the subflooring. What impacts one area impacts those connected to it right down the line. The following construction elements affect your floor’s performance in one way or another.

  • Sill plate–The bottom-most horizontal member of the wall that rests on the top of the foundation wall and supports the vertical members.
  • Floor joists–Horizontal structural members that span open spaces between beams. They support the subfloor.
  • Header joist–Also called a rim joist or bandboard, the header bolts to the foundation wall, frames the opening, and holds most of the weight.
  • Subfloor–Typically plywood or OSB, the subfloor covers the floor joists and provides a solid surface for the floor covering.
  • Underlayment–Typically felt or synthetic, underlayment lays between the subfloor and the floor covering. It provides soundproofing and insulating factors.
  • Finished flooring–That’s what you walk on–hardwood, laminate, vinyl, or carpet.
  • Wall studs–Included because, you know, the leg bone/knee bone thing. Wall studs are the vertical members that frame a wall and are covered with drywall or plaster. 

Most common causes of a sagging floor

Sagging floors, particularly in older homes, are more common than you’d think – and the causes are varied. Here, we look at why your floors may sag, list, or otherwise not perform as they should.

Poor construction techniques

Building codes improve with time as technology and materials change. Therefore, older structures may need more integral support than modern ones do. For instance, an older home may have floor joists spaced further apart across a wider span than current building codes require.

Also, some homeowners do more harm than good when DIYing a home remodel, like retrofitting an HVAC system. For example, cutting into floor joists to run piping or wires must be avoided in some locations. Otherwise, you weaken the joists. If you see penetrations like the following in your joists, you probably discovered the cause of your sagging floors.

  • Holes greater in diameter than 1/3 the depth of the joist.
  • Holes within 2 inches of another hole or notch or at the top or bottom of the joist edge. 
  • Square cuts, even in notches. Angle cuts and circles are fine.
  • Any cuts or holes along the bottom of a joist or in its middle third, where the bending forces are greatest.
  • Any notch deeper than 1/6, or end notch greater than 1/4, the joist depth.
  • Notches that exceed 1/3 the joist depth.
  • Notches in the top of a beam greater than 4 inches thick, unless at the ends.

Wood-eating insects

Insects like carpenter ants, powderpost beetles, and termites will make Swiss cheese of the wood in your home before you know what’s happening. If they get to supporting elements like the sill plate or floor joists, those things could be weakened and unable to carry their full load. Also, if insects are your perpetrators, look for damage in your subfloor and hardwood flooring. You may find yourself making more extensive floor repairs due to insect damage.

Soil movement

Over time, the soil under a home settles. However, expansion and contraction, especially if the earth isn’t adequately compacted, cause the ground to shift, putting pressure on the foundation.

Wood rot caused by moisture

Wet basements and crawl spaces cause all kinds of problems. Wood rot is just one of them. Moisture under a home can come from run-off that collects near the foundation because clogged gutters and downspouts aren’t doing their job diverting the water away. If you think moisture is the cause of your sagging floor, employ basement waterproofing measures to address the problem first. Then, with a dry space, you can more expertly fix foundation problems and level your floor.

Tree roots

Tree roots grow in length two to four times the diameter of the crown of the tree. Therefore, trees planted too close to a house can grow into the foundation, causing it to shift. Look for tree root activity 6-24 inches from the soil line.

Why you should fix uneven floors

Sagging floors is a symptom of an underlying problem that leads to even more significant issues. Moisture and insects, left unchecked, will continue to wreak havoc on your home. In other words, you’ll save time and money if you fix the problems now rather than put them off.

Four ways to fix sagging floors in your old house

There are several ways to fix floors that sag. Which method you choose depends on the building’s age and construction and the cause of the problem. Here are the four most common methods used to repair sagging floors.

Sister joists and blocks

For a sagging floor with a minor deflection that involves only one or two weak joists, you can solve the problem by attaching “sister joists” to the original ones. However, call for backup if you find more than a few sagging floor joists. There may be a more significant problem that requires professional repair.

To sister a joist, cut a piece of wood the same dimension as the original joist and at least six feet long. Press it against the underside of the subfloor and hold it in place with vertical supports. Use structural screws to attach the sister to the original joist. Install the screws in pairs about every 8 inches along the length of the sister joist. Fastening the new piece to the old one makes it stronger.

Beam replacement

Always replace damaged beams. Whether rotten from water damage, chewed by insects, or cracked from too many ill-placed penetrations, damaged floor joists can no longer carry the load of your home on their shoulders. 

Support beam replacement is something best left to a professional foundation contractor. They most likely will replace the wooden beam with a steel one.

Jacks and pillars

Remember, as a kid, how you would swing around and round the metal pole in the basement? Well, that pole had a purpose. It helped to support the weight of the house. But sometimes, the weight gets to be too much for that support column, and the floors start to bounce or sag. That’s where jacks and pillars come in.

Maybe that lone column wasn’t enough for the span of the floor. Or, perhaps something happened to damage that post. In any case, you need to either replace it or add a few more support posts to distribute the weight and level the floors evenly.

When using a jack to even the floor, it’s important to note that you cannot simply jack your house up a couple of inches and call it a day. The process must be done slowly for a couple of weeks to avoid other structural issues. Ideally, you want someone with experience, preferably an engineer, to oversee the process.

Piers and shims

In a crawl space, you may find concrete piers with shims on top under long expanses to help support the joists. However, those piers move when the soil shifts due to expansion and contraction. Or, over time, the wooden shims may start to flatten out. Adding additional piers and shims is a straightforward fix in cases like these. However, if the joists, sill plate, or other structural elements show any signs of damage, address that first.

Special circumstances

A home built on a concrete foundation can also have uneven floors. Shifting soil caused by improper water drainage, sinkholes, and tree roots can all cause a concrete slab to sink slowly in one area. 

The tell-tale signs are the same. For example, if you notice baseboards higher on one side of the room than the other, cracks in your plaster, or an excessively damp room, your slab may be sagging. 

However, the fix for a sagging concrete slab is different because you don’t have a basement or crawl space. The most common method to repair a slab is to lift it by injecting the void with polyurethane foam to raise the floor. However, this method won’t work for exceptionally soft soil because the soil will continue to settle and shift. In that case, piers must be installed deep in the ground until they reach load-bearing strata. 

Can I DIY my sagging floors?

Every home improvement job requires a different level of expertise, and fixing sagging floors is not for beginners or the faint of heart. Unlike some projects that, if done wrong, have only cosmetic consequences, shoring up the foundation of your home involves the safety and integrity of the structure. If you don’t know what you’re doing, it’s wise to call in a pro. 

Ideally, you should consult with a structural engineer, whether you do the work yourself or hire out. An engineer knows how to not only diagnose the problem and can advise on how to fix it. Once they’ve studied the situation, they’ll draft an action plan that includes drawings and recommendations for both temporary support while the work is being done and how to carry it out.

You can then follow the plan yourself or pass it off to a contractor specializing in foundation work.

Fix your sagging floors today

Are you tired of losing your marbles over uneven, sloping, or sagging floors? Then it’s time to find out what’s happening with your home's foundation. After checking things out, if what you find intimidates you, call a foundation repair professional to get things done.

Get free quotes from professional residential structural engineers near you

Written by

Carol J Alexander Content Specialist and Subject Matter Expert

Carol J Alexander is a home remodeling industry expert for Fixr.com. For more than 15 years as a journalist and content marketer, her in-depth research, interviewing skills, and technical insight have ensured she provides the most accurate and current information on a given topic. Before joining the Fixr team, her personal clients included leaders in the building materials market like Behr Paint Company, CertainTeed, and Chicago Faucet, and national publications like This Old House and Real Homes.