Crawl Space Repair: Sagging Floors, Rotten Wood, Water and Mold — What Each Fix Involves and Costs (2026)

Read the Symptom First

Almost everything that goes wrong in a crawl space traces to one root: moisture. Damp ground breathes vapor into the space; the vapor condenses on cool wood; the wood feeds mold and rot; the rot softens joists; the floors above start to bounce and sag. So the repair menu below has an order — water, then wood, then insulation and sealing — and any quote that replaces wood without naming the moisture fix is selling you the same repair twice.

The Structural Repairs

Supports and jacks ($1,500–$5,000 typical section). Sagging floors are commonly re-supported with adjustable steel columns on proper footings, or new piers where old ones settled or tilted. Done right, the load path goes to a footing sized for the soil — not to a stack of blocks on dirt, which is the shortcut that re-sags. Severe sags are raised gradually over weeks to avoid cracking finishes upstairs.

Joists ($150–$700 per joist). Rot-weakened joists are usually sistered — a new joist fastened alongside the old across its span — or replaced when too far gone. The moisture meter decides which; wood above about 20% moisture content will keep rotting, so drying precedes carpentry.

Sill plates and band joists ($1,000–$8,000). The wood sitting on the foundation is the worst place for rot because it is closest to masonry moisture. Repair means temporary support, cutting out the rot, and splicing treated lumber — labor-heavy because everything above rests on it. Access and extent set the price.

Piers and footings. Tilting or sinking masonry piers get rebuilt or supplemented; widespread settlement is a foundation conversation, not a crawl space one.

The Moisture Repairs (Do These First)

• Exterior water management — gutters, downspout extensions, grading. The least expensive line with the biggest effect; standing water often ends here.
• Drainage and sump ($1,500–$5,000) — where water still collects: perimeter drain to a sump with a discharge line.
• Vapor barrier ($500–$2,000) — sealed ground cover, installed to spec, not loose plastic.
• Mold remediation ($1,500–$5,000) — where framing shows growth: cleaning the wood and the space properly before it is sealed in.
• Dehumidification / encapsulation ($3,000–$15,000) — for humid-climate spaces that grow mold on repeat: what encapsulation costs and what it actually fixes.

Urgency, Honestly

Same week: standing water, sewage, active leaks, floors sagging enough to feel unsafe, or a support visibly leaning. Same season: confirmed rot, mold on framing, failed barrier in a humid climate — these grow. Watchlist: minor bounce with dry readings and no smell — measure it, date it, and re-check; not everything under a house is an emergency, and a contractor who says otherwise before taking a moisture reading is selling.

Getting It Quoted Right

Have any bidder walk you through: the moisture readings they took and the source they identified; which wood is repaired versus monitored, by location; where new loads land (footings, not soil); what happens to the insulation; and what on the quote prevents recurrence. Get a structural engineer's opinion (a few hundred dollars) when sag is widespread, spans look undersized, or bids disagree wildly — cheap relative to steel placed wrong. And if the trigger was a sudden plumbing leak, loop in the insurance process before demolition: sudden causes are generally covered, gradual ones are not, and the documentation decides which story your crawl space tells.