· Updated

How to Fix a Sagging or Bouncy Floor: Sistering Joists, Posts, and Crawl Space Repairs

A sagging or bouncy floor is a structural problem, but it is often a straightforward DIY fix using sistering joists, adjustable posts, or a combination of both.

A floor that sags, dips, or bounces underfoot is unsettling, but it is usually a fixable structural problem rather than a sign of imminent collapse. Most sagging floors in older homes are caused by one of three issues: a weakened or damaged floor joist, an undersized or failed support post, or a main beam that has deflected over decades. This guide covers how to diagnose the problem and execute the most common DIY repairs.

What You Need

Gather your materials before entering the crawl space. You will not want to make multiple trips.

Step 1: Diagnose the Problem from Above

Before going under the house, map the problem from the living space.

Find the exact location of the low spot. Walk the floor slowly and mark any soft or low areas with painter’s tape. Check whether the sag is a single point, a line (indicating one joist), or a broader area (indicating a beam problem). Use a 6-foot level laid on the floor to measure the amount of sag — most DIY-appropriate sags are 1/2 inch to 1-1/2 inches over 6–8 feet.

Check adjacent areas: do doors stick? Are there diagonal cracks in drywall above the sagging area? These are signs the settlement is ongoing or more significant than it appears.

Step 2: Access the Crawl Space and Inspect

Enter the crawl space with your headlamp, respirator, and protective gear. Bring a bright work light — you need to see clearly.

Navigate to the area below the low spot. Look for:

Damaged joists. A joist that is cracked, split, notched, or has a large knot that has failed will be visibly bowed downward compared to adjacent joists. Run your hand along the bottom edge — a damaged joist often has a sag that is easy to feel.

Rot or pest damage. Soft, punky wood that compresses when you press it, or wood that has a hollow sound when tapped, indicates rot. Termite damage looks like tunnels or galleries running through the wood grain.

Failed or missing posts. The main beam that the joists rest on is supported by posts at intervals — typically every 6–8 feet. A post may have settled, rotted at the base, or in older homes may simply be a wood stump that has deteriorated. Check that every post is plumb and in firm contact with both the beam above and the footing below.

Undersized joists. Compare the joist dimensions to a span table. A 2x8 joist spanning 14 feet in a 40-psf load floor is undersized — it will bounce even if structurally sound.

Step 3: Set Up Temporary Support

Before any repair work on joists or the main beam, install temporary support.

Set a hydraulic jack on a solid base — a 2x10 or 2x12 pad on the ground works if the crawl space floor is firm. Place a 4x4 post or a wood block on top of the jack to distribute the load. Position the jack under the beam or joist you need to work on, 12–18 inches away from the repair area.

Raise the jack until it just contacts the beam or joist — do not lift the floor yet. The jack is there to prevent the joist from deflecting further while you work, not to raise the floor during initial installation.

Step 4a: Sistering a Damaged Joist

Sistering is the correct fix when one or more joists are cracked, notched, or have localized rot.

Cut the sister joist to length. Measure the full span of the existing joist from the sill plate (or header) on one end to the beam or opposite sill on the other. Cut your new lumber to the same length. Match the depth of the existing joist exactly — if the original is a 2x10, use a 2x10.

Slide the sister into position. Work the new joist in alongside the damaged one. This often requires maneuvering through insulation and around pipes — plan your route in advance. The sister joist should sit flat against the face of the existing joist its entire length.

Raise into position. Using the hydraulic jack, slowly raise the sister (and the existing joist) until the bottom face of the sister is level with the adjacent healthy joists. Go slow — 1/8 inch at a time, checking the floor surface above with a level between adjustments.

Fasten the sister joist. Drive 3-inch structural screws or 16d nails in a staggered pattern — two fasteners every 12 inches along the full length of the sister. The two joists should be pulled tight together with no gap. Install joist hangers where the sister meets the beam or header if the original joist connection used hangers.

Gradually lower the jack. Turn the jack handle slowly to lower — the load transfers to the sistered joist pair. Leave the jack in place for 24 hours, then remove it and re-check the floor level above.

Step 4b: Replacing or Adding a Support Post

When the main beam has sagged because a post has failed, the fix is replacing or supplementing the post.

Install temporary support first. Place your jack under the beam on one side of the failed post. Raise it just enough to take the load off the post — do not raise the floor. Repeat on the other side of the post if the post is badly compromised.

Remove the failed post. If it is a wood post that has rotted, it may twist or pull free easily. If it is a concrete block or masonry pier, use a chisel to chip away the failing material.

Prepare the footing. The post must bear on a concrete footing — a continuous pad at least 8 inches thick and 12x12 inches in plan for a typical residential floor load. If no footing exists, pour a new one: dig down to solid soil, form a 12x12x8-inch hole, and pour 3000 psi concrete. Allow 48 hours to cure before loading it.

Install the new post. Use an adjustable steel lally column for the permanent post. Set the column on the footing, adjust it so its top plate just contacts the beam, and then extend it by turning the adjustment collar until the beam is at the correct height. Lock the collar per the manufacturer’s instructions. Check plumb before locking.

Lower and remove the temporary jacks. Transfer the load to the new column gradually. Check the floor surface above — you should see the sag has reduced or eliminated.

Step 5: Raise the Sag Gradually (If Needed)

If the floor above the repair area is still measurably low after supporting the joists, you can raise it back — but it must happen slowly.

Raise your adjustable column (or leave a screw jack in place for this purpose) by 1/8 inch per week. Each increment puts the floor slightly higher. After each adjustment, check for new cracks in the drywall or sticking doors above — these are signs you are moving too fast.

Most floors reach acceptable level in 4–8 weeks of gradual adjustment. Do not try to recover more than 1 inch total — pulling a floor back to perfectly flat after significant settlement is rarely achievable and risks more damage than it prevents.

Step 6: Address Moisture and Root Causes

If rot caused the problem, fix the moisture source before closing up the crawl space. A single repair will fail again if the underlying moisture problem is not resolved.

Common moisture sources in crawl spaces: missing or incomplete vapor barrier on the ground, inadequate crawl space ventilation, exterior grading that directs water toward the foundation, and plumbing leaks. Install a 6-mil poly vapor barrier on the entire crawl space floor, overlapping seams by 12 inches and running it up the foundation walls several inches.

Free: 10-Point Home Maintenance Checklist

Prevent costly repairs with this seasonal checklist. Save hundreds every year by catching problems early.

Free instant download + weekly home tips. Unsubscribe anytime.
← Back to all guides

Save this for next time.

One repair, cost, or how-to tip every Sunday. The Fixup Digest — no spam, no fluff.

Free weekly tips. Unsubscribe anytime.