Sagging Floor Joist Repair: How to Fix a Bouncing Crawl-Space Floor

A sagging or bouncing floor over a crawl space is fixable — but a sister joist or a jack only lasts if you first find why it sagged: a rotted beam, a settled pier, an over-spanned joist, or the crawl-space moisture behind all three. This page is the repair. It covers the four jobs that fix a sagging floor — sistering, beam replacement, support jacks, and sill replacement — and, more importantly, the diagnosis that has to come first, because the most expensive mistake in pier and beam work is fixing the symptom and leaving the cause in the ground. If you're still deciding whether your floor is even a problem, start with the sloping-floors diagnosis; if you want the broader system, see the pier and beam pillar.

What Makes a Floor Sag or Bounce

A framed floor over a crawl space is a load chain: subfloor rests on joists, joists rest on beams (girders), beams rest on piers. A failure or weakness anywhere in that chain shows up at the top as a floor that sags, dips, or bounces. The honest first step is not to grab a jack — it's to figure out which link yielded, because the repair is different for each.

Sagging and bouncing "roller-coaster" floors are caused by undersized or over-spanned floor joists (common in homes built before 1970), rotted or weakened beams or joists, settled piers, or compressed or missing shims. The table below maps each cause to the tell that gives it away and the repair it actually calls for.

Two distinctions do most of the diagnostic work. Springy with no dip points at the joists themselves — they're simply spanning farther than their size allows. A defined sag or low spot points at something that failed or moved underneath them: a settled pier, a dropped girder, or rot. Get that distinction right and you've narrowed four possible repairs down to one or two.

Rot: Brown vs White, and Why It Happens

Most "failed beam" and "rotted joist" problems are, at root, a water problem. Wood doesn't decay because it's old; it decays because it's wet. Per the USDA Forest Products Laboratory's Wood Handbook — the standard reference for wood as a structural material — wood-decay fungi need sustained wood moisture above the fiber-saturation point, roughly 28–30%, to initiate decay. Keep the wood drier than that and the fungi can't get started, no matter how old the lumber is.

There are two failure signatures worth recognizing in the crawl space:

• Brown rot breaks wood down across the grain and crumbles it into cube-shaped chunks that powder between your fingers. It's the more dangerous of the two structurally, because it destroys strength quickly.
• White rot leaves the wood spongy and stringy, bleached and fibrous rather than cubed.

Either way, the field test is a screwdriver: sound wood resists the tip, decayed wood gives way. A wood-moisture meter reading sustained above ~20% flags lumber that is conducive to decay and termite activity even before it visibly fails.

Here's the rule that governs everything downstream: rot tracks moisture, so the fix has to start with the water. The wet spots cluster where you'd expect — under a leaking drain or supply line, along a damp sill, beneath a foundation vent that lets humid summer air condense on cool wood. Before any lumber goes back in, the bulk-water source has to be corrected: a plumbing leak under the floor, failed drainage, or a chronically humid crawl that wants encapsulation. ASHRAE Standard 160-2021, which sets the criteria for moisture-control design analysis in buildings, flags surface mold growth at sustained wood moisture around 16% — well below the decay threshold — which is the building-science way of saying a crawl space that grows mold is already trending toward one that rots framing. Replace the wood without fixing the water and the new lumber rots too.

The Repairs: Sistering, Beam Replacement, Jacks

Once the cause is identified and the water is handled, the structural repairs themselves are a short, well-understood menu. All of them, in a damp crawl space, use pressure-treated lumber and galvanized fasteners — the original failure was usually a moisture failure, and the replacement has to outlast it.

Sistering a joist. Sistering means bolting a new, sound joist tightly alongside the damaged one so the two carry the load together. The sister member should overlap the damaged section by a generous bearing length, land on solid support at both ends, and be through-bolted at regular intervals, per the APA — The Engineered Wood Association's joist-reinforcement guidance. It's the right fix for a joist that's cracked, sagging, or under-sized but still bears solidly at its ends. Cracked floor joist repair is almost always a sistering job rather than a replacement, because a crack rarely destroys the bearing the way end-rot does.

Beam or girder replacement and reinforcement. A girder that has sagged or rotted gets reinforced where the damage is local, or fully replaced where it has failed across its length. Replacement means temporarily carrying the joist loads on shoring, removing the failed beam, and setting a new pressure-treated or engineered beam back onto the piers — leveled and shimmed to the corrected elevation.

Support jacks, posts, and added piers. Where a girder sags between widely spaced piers, the cure is often more support, not just a stronger beam: a permanent support jack or post on a new footing, or an added pier, placed under the sagging span to shorten it. That new footing isn't a paver dropped on the dirt — per the International Residential Code (§R403), a footing has to be sized for the load and founded below the local frost line on adequate bearing, which is why an added support point is an engineering call, not a handyman fix. The temporary version of the same idea — hydraulic jacks and cribbing — is what holds the structure during the lift.

Sill replacement. The sill beam, where the perimeter framing meets the masonry, is the single most rot-prone member in the system because it sits closest to the damp foundation. Replacing a rotted sill is its own job, frequently bundled with new perimeter piers, and again done in pressure-treated lumber over a proper capillary break so it doesn't simply rot again.

The lift that ties these together is deliberate. Whether you're jacking a girder to set a new beam or raising a sagged zone before shimming, the structure comes up in small increments — commonly a fraction of an inch per pass — held between lifts so the finishes and plumbing above can adjust. A fast lift is what cracks walls. For the broader staged repair workflow across the whole system, see the pier and beam repair guide.

Symptom vs Cause

This is the spine of the whole page, so it's worth stating plainly. A sagging floor is a symptom: find the cause — rot, a settled pier, an over-spanned joist, or crawl-space moisture — before you sister or jack. The repairs in the section above are the how; this section is the why first.

The reason matters in dollars. Sistering a bouncy joist is a few hundred dollars; if that bounce was actually a girder settling onto a failing pier, the sister joist does nothing for the real problem and the floor keeps moving. Conversely, replacing a "rotted" beam without fixing the drain that soaked it guarantees a repeat. The diagnosis is what assigns your money to the link that actually yielded.

A few practical separations:

• Is it the floor, or the foundation? A springy or sagging floor that traces to joists, beams, or rot is a floor-structure repair you can do from the crawl space without touching the perimeter foundation or the soil. A slope that traces to settled piers and soil movement is a foundation problem. The sloping-floors page walks the slab-versus-pier-and-beam distinction and the marble-test-versus-survey question for deciding which you're looking at.
• Is it one zone, or the whole house? Sistering, beam work, and added jacks fix a defined sagging zone. If an elevation survey shows the whole structure is out of level and needs to be brought back up as a system, that's a whole-house re-leveling job — see pier and beam leveling — not a joist-by-joist repair.
• Is the wood failed, or just under-spec? Probe before you decide. A joist with sound ends gets sistered; a joist rotted through at the bearing gets replaced. Guessing from above the floor is how the wrong repair gets bought.

What It Costs

The numbers below are 2026 national planning ranges, not quotes — access difficulty (a tight 18-inch crawl versus a 4-foot one) is the largest single multiplier, and a sealed engineer's scope is what lets you compare bids honestly.

The cheapest line on this table — joist sistering — is also the one most often sold as the whole answer when the real problem is a settled pier or a wet beam underneath it. The most expensive mistake isn't a high quote; it's a low quote aimed at the wrong link.

FAQ Note

The questions below are what homeowners ask most once they notice a floor that sags or bounces — how the repair is actually done, what sistering is, what it costs, how to tell rot from a simple over-span, and whether jacking will crack the walls. For the is-it-a-problem question that comes before any of this, see the sloping-floors guide; for the whole-system context, the pier and beam pillar; and for the second opinion before you sign, the engineer's report guide.

Get Matched With a Vetted San Antonio Specialist

If your floor sags or bounces and you want it fixed at the cause — not just sistered over — we'll match you with a vetted San Antonio specialist who works from a sealed engineer's scope and fixes the moisture and support before the lumber goes back in. The match is free, the quote is no-obligation, and we don't take a fee from you. We screen for an independent elevation survey on any structural sag, pressure-treated lumber and galvanized fasteners in the crawl, a gradual-lift method, a written plan to correct the water that drove the rot, and a clean Bexar County permit record. If a quote sisters a joist and ignores the wet beam underneath it, we'll tell you. That's the only way an editorial matching service should work.