A homeowner called me last spring because a marble would roll clear across her dining room. She figured she needed the whole floor torn out. We crawled under the house, jacked one tired beam back up over two weeks, sistered three cracked joists, and walked away for about $2,800. She had been quoted $14,000 by someone who never went into the crawl space.
That gap is the whole story with sagging floors. The repair can be small or it can be serious, and the only way to know is to find out why the floor moved in the first place. A bouncy floor is rarely the floor itself. It is almost always something underneath it that gave up.
So let me walk you through the pieces, what each one costs, and where I see people quietly overpay. I have priced thousands of these, and the math is more predictable than most folks expect once you know what you are looking at.
What "sagging" actually means under the house
When a floor sags, sinks, or bounces, the structure carrying it has lost support. That structure is usually some combination of joists (the parallel boards your subfloor sits on), beams (the bigger members the joists rest on), and posts or piers (the vertical supports holding the beams off the ground).
A myth I hear constantly: "the floor is rotting." Sometimes it is. More often the floor is fine and a post has settled, a beam has split, or a joist has been notched to death by a plumber forty years ago. The sag you feel up top is the symptom, not the disease.
The single most expensive mistake is paying to "level the floor" without finding the cause. If you jack a floor flat without fixing the failed support, it sags right back, and now you have cracked drywall too.
The usual suspects
- Undersized or damaged joists that flex (the classic bouncy floor).
- A settled or rotted support post sitting on dirt instead of a real footing.
- A cracked or sagging center beam spanning too far between posts.
- Moisture and rot, often the leftover damage from a plumbing leak or a wet crawl space.
Jacks and shims: the cheapest fix
If a single post has settled and the wood is still sound, the repair is often just lifting that spot and adding proper support. We set a temporary jack, raise the beam slowly (a little each day, never all at once), then install a permanent adjustable steel post on a poured concrete footing.
This is the lowest tier of work. A single new support post with a footing typically runs roughly $400 to $1,200 installed, depending on how much digging and concrete is involved. Add an adjustable jack post on top and you are still usually under $1,500 for one location.
Here is where money quietly disappears: some crews "fix" a sag by hammering wood shims on top of a rotting post and calling it done. It looks level on the day they leave. Six months later you are back to square one. Pay for the footing.
A floor that took fifteen years to sag should not be lifted in one afternoon. Raising more than about an eighth of an inch a day risks cracking plaster, popping tile, and jamming doors. A patient lift is a cheaper lift, because you are not also paying to repair the upstairs.
Sistering joists for a bouncy floor
That trampoline feeling when you walk across a room usually means the joists are too small for their span or have been weakened. The fix is "sistering," where we bolt a new joist alongside the old one so they share the load.
Per joist, sistering tends to run roughly $200 to $600 depending on length, wood versus engineered lumber, and how cramped the crawl space is. A bouncy room often needs several joists done, so figure a real-world range of around $1,000 to $3,500 for a typical floor section.
What moves that number? Access, mostly. A walk-in basement is cheap to work in. A 24-inch crawl space full of ductwork doubles my labor because everything is done lying on your back, passing boards through gaps. Tearing out a finished ceiling below to reach the joists adds drywall and paint costs on top.
Beam repair and bigger structural work
When the main carrying beam has split, rotted, or sagged between posts, you are into the upper tier. We may add intermediate posts to shorten the span, sister the beam with new lumber or steel, or replace the beam entirely.
Adding a post or two to support a sagging beam might land around $1,500 to $4,000. A full beam replacement, with temporary shoring of the whole house section while we swap it, commonly runs roughly $5,000 to $12,000. Steel I-beams and long spans push toward the top of that.
If the sag traces back to a foundation problem (a settling pier, a cracked footing, water undermining the soil), that is its own project and its own budget. Same story I told about a flooded basement repair: the visible damage upstairs is cheap compared to fixing the source down below.
Cost breakdown by repair type
| Repair | Typical cost range (USD) | What it covers |
|---|---|---|
| New support post + footing | $400 - $1,200 | Dig, pour footing, set adjustable steel post |
| Sister one joist | $200 - $600 | Labor, lumber or LVL, fasteners |
| Sister a bouncy room | $1,000 - $3,500 | Several joists, crawl space labor |
| Add posts under a beam | $1,500 - $4,000 | Shoring, footings, new posts |
| Full beam replacement | $5,000 - $12,000 | Temporary shoring, new beam, reset |
| Rot remediation (per area) | $1,500 - $6,000 | Remove rot, replace wood, address moisture |
Where labor, materials, and permits land
For most jobs I price, labor is the heavyweight at roughly 60 to 70 percent of the total. Materials (lumber, steel posts, concrete) are surprisingly modest, often a few hundred dollars even on a mid-size repair. Permits for structural work vary by town but commonly run $150 to $700, and skipping them to save that money is a great way to fail an inspection when you sell.
What pushes the price up or down
Two identical sags can cost wildly different amounts. The drivers I see over and over:
- Access. Tight crawl space versus a stand-up basement can double labor.
- Moisture. A wet crawl space means rot keeps coming back. Budget for a vapor barrier or drainage, or you pay twice.
- Finishes above. Tile, hardwood, and plaster crack during lifts and add repair cost.
- Span and load. A floor under a kitchen island or a stone hearth carries more weight and needs beefier support.
It is the same logic as any home repair where the easy part is visible and the cost hides in the prep. The actual garage door spring swap is quick, but the diagnosis is what you are really paying for. And if your crawl space rot came from old, leaking systems, sometimes the smart move is to fix everything at once while it is open, the way people bundle a whole-house rewire when the walls are already torn up.
If a contractor quotes a five-figure foundation job for a marble that rolls, ask them to show you the failed member in the crawl space. A good one will. A sales-driven one will talk about "settlement" and hand you a financing brochure.
How to keep the bill honest
Get at least two contractors who will physically go under the house, not just walk the floor with a level. Ask each one to name the failed component (which post, which joist, which beam) in writing. When the diagnosis matches, the price usually does too.
And fix the why. If water put your post into the dirt, a $9,000 lift over a wet crawl space is money set on fire. Dry it out first, then support it.
Can I just live with a slightly sloped floor?
A small, stable slope in an old house is often cosmetic and safe to leave. The concern is movement that is getting worse, soft spots, or visible cracking in the structure below. If the sag is active, it will not stop on its own.
Does homeowners insurance cover sagging floor repair?
Usually not for normal settling, age, or long-term moisture, which insurers treat as maintenance. A sudden covered event, like a burst pipe that rotted a joist, may be partially covered. Read the cause-of-loss section before you assume.
Is sistering joists a job I can DIY?
A single accessible joist in a stand-up basement is doable for a confident DIYer. Anything involving jacking a beam, working in a tight crawl space, or load-bearing members is worth a pro. Lift wrong and you crack the house above.
Sagging floors look scary and price like a mystery, but they follow rules. Find the failed piece, support it properly on a real footing, and fix whatever soaked or settled it. Do that and most of these jobs land in the low four figures, not the nightmare number the first salesman threw at you. Crawl the crawl space, or hire someone who will.
