The Sagging-Floor Fix: Find the Cause Before You Jack the Symptom

A sagging floor over a crawl space is one of the few house problems you can usually fix permanently for hundreds, not thousands — if you diagnose before you jack.

Find the cause first

Get under there with a good light and a screwdriver, and check in this order:

1. Moisture and rot. Probe joists and the beam near the sag. Soft wood means a water problem — fix the source (grading, gutters, missing vapor barrier) or the repair rots too.
2. Failed posts or footings. Piers sinking into soil, shimmed stacks of scrap, posts resting on dirt — the most common cause in older homes.
3. Undersized or over-spanned joists. Bouncy-but-sound floors with long clear spans sag from arithmetic, not damage.
4. Notched or drilled joists from past plumbing work — a 2x10 with a three-inch notch is a 2x7 where it matters.

The repair sequence

Jack slowly. A 20-ton bottle jack under a temporary beam (doubled 2x8s perpendicular to the joists) does the lifting — at roughly an eighth of an inch per day. Wood that bent over years accepts correction over weeks; forcing it in an afternoon cracks plaster, binds doors, and can split joists.

Then make the fix permanent:

• Sister damaged joists with same-depth lumber, glued and through-bolted, bearing on the same supports — a sister that stops short of bearing is decoration.
• Replace improvised posts with steel adjustable columns on proper footings — poured concrete, not a paver on soil.
• Add a mid-span beam where joists are over-spanned; halving the span stiffens the floor by a factor of eight, which is why this is the highest-value structural dollar in the house.

Know when it’s bigger than you

A sag paired with cracked foundation walls, doors racking out of square upstairs, or rot that crosses multiple joists into the sill is engineer territory. And any jack work near gas lines or old cast-iron drains deserves real caution — lifting a house an eighth-inch also lifts everything plumbed through it.