The four causes, and how each one looks from above
What we found
A sagging floor has four possible causes and they differ in price by more than 5x. Sistering joists is the one quoted most often and the right answer least often.
Sistering runs $200 to $700 a joist and is easy to price by the unit, which is exactly why it gets quoted by the unit. But in the pre-1940 housing that dominates intown Atlanta, Decatur, East Point and Historic Norcross, the member that actually failed is usually the pier, the girder or the sill. A quote for twelve sistered joists over a settled pier is twelve joists of work landing on the same failed bearing point, and the floor moves again.
Method: Comparison of the four failure modes against our own line-item pricing model for Metro Atlanta, assembled August 2026 and published in full on our cost pages.
| What failed | How the floor behaves | Typical cause | Typical cost |
|---|---|---|---|
| Floor joist | Localized soft spot or dip, often near a bathroom or exterior door | Rot from a plumbing leak, or a joist notched for a pipe | $200 to $700 per joist sistered; $350 to $1,100 replaced |
| Girder | A dip running in a straight line through the middle of the house | Undersized girder, or a splice that lost bearing | $900 to $2,600 per section |
| Sill plate | Sag along an exterior wall, often with a gap at the baseboard | Moisture wicking from masonry with no capillary break | $45 to $110 per linear foot |
| Pier | Dip centered on one point, worst under a renovated room | Settlement into clay, rotated brick, washed-out mortar | $550 to $1,400 per pier, plus $400 to $1,100 per steel post |
Step 1: Map the floor before anyone quotes you
- Get a 4-foot level or a laser level. A marble works for finding direction but not for measuring amount.
- Take readings on a grid across the affected room, roughly every 4 feet, and write them down.
- Mark the lowest point. This is the single most useful piece of information about your floor.
- Note the shape of the low area. A line means a girder or a beam. A point means a pier. A localized soft patch means a joist.
- Note what is above and near the low point. A dip under the newest bathroom or a relocated kitchen almost always means added load on framing sized for less.
The pattern that saves you money
A sag that follows a straight line through the house is a girder or the piers under it. A sag centered on one spot is a single bearing point. In both cases sistering joists is the wrong repair, and joist sistering is what gets quoted most often because it is easy to price by the unit. Establishing the shape of the sag first is the most useful thirty minutes you can spend.
Step 2: Go under the house and look at the bearing points
In the crawl space, follow the low point down. Look at the pier directly under it and check whether it is plumb, whether the mortar joints are intact, and whether there is a visible gap between the top of the pier and the girder it should be carrying. A gap means the pier settled and the girder is now spanning further than it was designed to.
Then check the joist ends where they bear on the sill plate along the exterior walls. Press a screwdriver into the wood. If it sinks in easily, that wood is compromised. In Metro Atlanta this is most common in the pre-1940 pier-and-curtain-wall stock through intown Atlanta, Decatur, East Point and Historic Norcross, where heart pine sills sit directly on masonry with no capillary break and have been wicking moisture for a century.
Step 3: Match the repair to the failure
Sistering a joist
Sistering bolts or screws a new joist alongside the failed one, transferring load to sound material. It runs $200 to $700 per joist installed. It is the right repair when individual joists are cracked, rotted at the end, or notched, and the bearing points are sound. It is the wrong repair when a pier has settled, because the new joist lands on the same failed bearing point as the old one.
Adding a bearing point
An adjustable steel post on a poured concrete footing pad runs $400 to $1,100 installed and is the standard solution for a mid-span sag on a floor system that is otherwise sound. It is also the standard fix for bounce, technically deflection, where the floor is carrying its load but flexing more than it should because joists span too far.
Rebuilding the pier
A pier rebuild including a new footing runs $550 to $1,400. This is the most common actual repair on pre-1940 Metro Atlanta homes, and the most commonly skipped one, because it requires excavation and forming inside a crawl space rather than fastening lumber.
Step 4: Lift slowly
The single biggest mistake in sagging floor repair is lifting too fast. Temporary shoring goes in, the failed member is repaired or replaced, and then the floor is raised gradually, roughly an eighth of an inch per day on framing older than about 1950. A house that has been settled for eighty years has plaster, tile, trim and door frames that have all adapted to that shape. Forcing it back in an afternoon cracks all of them.
This also means a realistic conversation about the end state. A century-old floor generally cannot be brought perfectly level without damage. The honest goal is structural soundness with the sag substantially reduced, and the expected final elevation should be in writing before work starts.
What this requires legally in Metro Atlanta
Joist, girder, sill plate and pier work is structural and requires a residential building permit and a framing inspection in every Metro Atlanta jurisdiction. Permit and inspection typically add $120 to $450 to a project. Georgia requires contractor licensing for construction work above $2,500 in value through the Secretary of State's licensing board, and several cities require a licensed contractor on record for structural residential alterations.
Frequently Asked Questions
Who fixes sagging floors and floor joists?
Structural crawl space and foundation contractors, working from underneath under a building permit. The right specialist depends on the cause: a rotted joist end from a bathroom leak is carpentry, while a settled pier is foundation work. That is why the diagnosis has to come before the contractor selection, not after. A flooring contractor can level the surface, which conceals rather than corrects a bearing failure.
How much does it cost to fix a sagging floor?
Sistering a joist runs $200 to $700 installed. Full joist replacement runs $350 to $1,100. A pier rebuild with a new footing runs $550 to $1,400, and an adjustable steel post on a poured pad runs $400 to $1,100. A typical Metro Atlanta scope of one or two failed bearing points plus limited joist work lands between $3,200 and $7,000 including the permit and framing inspection.
Can I sister floor joists myself?
Physically it is achievable in a tall crawl space with sound bearing points. Two things argue against it. It is permitted structural work in Metro Atlanta, and an owner-performed structural repair without a permit becomes a documented problem when you sell. And if the actual failure is a pier rather than a joist, you will have done real work and fixed nothing. Do the diagnosis first regardless of who does the repair.
My floors bounce but do not sag. Is that a problem?
It is a serviceability issue rather than a structural failure, and it is usually cheaper to fix than a sag. Bounce means the floor carries its load but deflects more than it should, common where 2x8 joists span too far on 24-inch centers, which describes most pre-1940 Metro Atlanta framing. Adding a mid-span beam and adjustable posts typically resolves it for $1,200 to $3,000.
Will a sagging floor get worse?
It depends entirely on whether the cause is active. A pier settling into clay that shrinks every August is an ongoing process and will continue. A joist that rotted from a plumbing leak fixed ten years ago has already done all the damage it is going to do. This is another reason the diagnosis matters more than the symptom: it tells you whether you are looking at a repair you can schedule or one you should not defer.
What you end up with
A structurally sound floor with the sag substantially reduced, documented with before and after elevation readings.
Sources
Primary sources only. This site never cites a home-services marketplace, cost aggregator or competing contractor, because those businesses compete with this one and their figures rarely trace to anything.
- Georgia Department of Community Affairs: IRC 2012 to 2018 Significant Changes. Georgia's adopted residential code and its amendments, including under-floor space provisions.
- Georgia Secretary of State: Professional Licensing Verification. Public lookup for Georgia contractor licences.
- Georgia Structural Pest Control Commission Rules, Chapter 620-6: Wood Destroying Organisms. Conditions conducive to infestation include cellulosic debris under a building, wood-to-soil contact, and under-floor ventilation not meeting the residential code.
- USDA Natural Resources Conservation Service: Web Soil Survey. Parcel-level soil series data, including the Cecil, Pacolet, Madison and Appling series across the Georgia Piedmont.
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