You notice it first near the tub, or in the spot in front of the fridge. The floor gives a little when you step on it. Over time you start walking around it without thinking, and your guests notice before you do.
Sagging floor repair is done from the crawl space or basement, by strengthening what holds the floor up. New flooring laid over a weak joist won't help. We go underneath and find out what's actually failing.
The short answer
Sagging floor repair fixes the joists and supports below a dipping floor, usually from the crawl space or basement. Common fixes are sistering new joists onto old ones, adding a support beam with steel jacks and replacing rotted wood. Most jobs take 1–3 days and leave your flooring alone.
Why sagging floors happen
Sagging floors happen when the framing can't carry the load anymore, and it's usually one of these reasons:
- Undersized joists. Older homes were sometimes framed lighter than they should have been.
- Notches and holes. A plumber cuts a notch in a joist for a drain, and decades later it shows.
- Long spans. Joists running too far between supports bend under normal use.
- Rot. Damp crawl spaces soften the wood from the ends in.
- Termites. They hollow out joists while the outside still looks fine.
- Sunken posts. The post sinks, the beam follows and so does your floor.
- Heavy loads. Cast iron tubs, big aquariums and stone kitchen islands weigh a lot.
Sagging vs. uneven floor vs. sloping floors
A sag is local, while sloping floors usually affect a larger part of the house. If you've got one dip in one room, that's almost always joists or a post. An uneven floor can be either one, depending on how far it runs.
If the whole house tilts one way, joist repair won't solve it, since the structure has moved. That calls for getting your floors back to level, which is a different job with a different price.
How we fix a sagging floor from the crawl space
We fix a sagging floor from below by adding strength where it's missing. If you need to fix sagging floor crawl space problems, these are the three moves we use, usually in some combination. Floor joist repair is rarely one single thing.
Sistering floor joists
Sistering floor joists means bolting a new joist tight against the old one, after lifting it back up with a jack. The pair then carries the load together. We use glue and carriage bolts, not just nails, and the sister runs as long as the space allows.
- A weak joist sags, and the floor dips with it.
- A jack lifts it slowly back up.
- A new joist goes up tight against the old one.
- Bolts tie them together, and the pair carries the floor.
Adding a support beam and steel jacks
If joists run too far, shorten the span. We add a new beam under the sag and hold it up with crawl space support jacks on concrete footings. One beam can fix several joists at once.
Replacing rotted joists and subfloor
Rotted wood doesn't get reinforced, it gets replaced. Sagging floor joist repair at this stage means cutting out the soft section and fixing the moisture that caused it. We'll show you photos first.
How much floor sag is too much?
A common rule of thumb is L/360, which means the sag shouldn't be more than the span divided by 360. For a joist spanning 15 feet, more than 1/2 inch of sag is worth a closer look. Try it with your own span:
Rule-of-thumb limit (L/360): about 1/2 inch of sag.
A rough guide only. Older homes and heavy loads can call for a closer look.
Measure by stretching a string across the low spot, or lay a long straightedge on the floor and measure the gap. It's a guideline, not a building code ruling. Older homes often sit past it without any danger, but a number that's growing is a different story.
How to fix a sagging floor: what's DIY and what isn't
How to fix a sagging floor depends a lot on what's causing it, and some of it's fair game for a handy homeowner. Finding the problem and measuring it are both safe for anyone comfortable in a crawl space.
Sistering one accessible joist is doable with care, as long as you lift slowly and nothing heavy sits above it. What we wouldn't do is replace main beams or jack up a floor with load-bearing walls on it. Those can bring down more than they fix.
Buying a house with sagging floors?
Should I buy a house with sagging floors? It depends on the cause. One tired joist is a modest fix and a fair thing to negotiate. Termites, widespread rot or a house-wide tilt can get expensive fast.
Get it inspected before you close, and don't just rely on the general inspector's note. Get a written number, and then use it to negotiate.
What it usually runs
Sagging floor work usually runs $1,000–$10,000. Sistering runs about $100–$400 per joist. Those are typical ranges, and your exact price depends on what we find at the inspection. See what a sagging floor costs to fix for a breakdown by job type.
Low crawl spaces and plumbing in the way add time. A free inspection gets you photos and a written number.
Sagging floor repair questions
Can a sagging floor collapse?
It's rare, but it can happen if rot or termite damage has gone far enough. Most sagging floors get worse slowly and give plenty of warning. If you see soft spots, a sudden new dip or a cracked joist, keep heavy items off that area and get it checked soon.
How long does sistering joists take?
Sistering a few joists usually takes a day. A larger area can take 2–3 days, especially when plumbing runs through the space or the crawl space is low. The new lumber has to be cut, fitted and bolted, so access has more impact on time than the number of joists.
Will you need to tear up my floors?
Almost never. We work from the crawl space or basement, so your flooring stays put. The main exception is a rotted subfloor, where a section has to be replaced from above. If we think that's needed, we'll show you why and put it in your estimate.
Can you fix sagging floors in a basement home?
Yes, and access is usually easier. We can sister joists, add a beam and set adjustable steel posts from the basement. If the ceiling is finished, we open only what we need to and leave it ready for patching, which usually isn't included unless your estimate says so.
Does sagging mean foundation problems?
Not usually. A local dip tends to be tired joists or a sunken post. If the whole house slopes one way, or you've got wall cracks too, the foundation may be involved. Readings across every room will show which one you're dealing with.
How do you sister a floor joist?
You lift the sagging joist slightly with a jack, then bolt a new joist tight against it along its length. Glue and carriage bolts tie them together so they carry the load as one. The new lumber should run as long as possible and bear on its supports.
How do you fix uneven floors?
Find out why they're uneven first. Local dips usually get fixed with joist work or new supports from below. Whole-house slopes usually mean leveling. Filling with self-leveling compound only makes sense for small flaws on a sound floor, and only after any movement has stopped.
How do you level a subfloor?
Fix the framing underneath before you touch the subfloor. After that, you can shim low joists, plane high ones or use a patching compound for minor dips. Plywood replacement is the fix if it's soft or delaminated. Doing all that on top of a moving joist won't last.
Run into a term you don't know? This may help: what all these terms mean.