Short answer
On a DIY budget, diagnose a sagging floor joist by measuring deflection and finding the root cause (moisture, over-spanning, notches, rot). Then shore the area with temporary supports, carefully jack the floor back toward level in small increments, and reinforce the joist—most commonly by sistering a new, straight joist or LVL alongside the old one with adhesive and structural fasteners. Add blocking/bridging, repair bearings or hangers, and fix the moisture or load issues so the problem doesn’t return.
How to identify the problem
Signs of a sagging joist:
- Bouncy floors, sloped surfaces, or gaps between baseboards and flooring
- Cracked tile/grout, sticking doors, or squeaks centered along one joist bay
- Visible crown-down or notched/damaged joist in the basement/crawlspace
Measure and document:
- Stretch a taut string line or use a laser across the joist span; measure the dip at mid-span.
- Check moisture with a pin moisture meter if crawlspace/basement is damp.
- Look for notches/holes that violate code (e.g., overdrilled for plumbing), insect damage, or rusted joist hangers.
Deflection guideline: L/360 (common comfort minimum)
Example: 12 ft span = 144 in → 144 / 360 ≈ 0.4 in allowable
If you’re seeing 3/4" of sag on a 12 ft span, that’s significant.
Tools and materials
- Temporary support: adjustable steel jack posts or screw jacks, solid cribbing (4x4 or 6x6), and a load-spreading beam (4x6 or double 2x10)
- Jack: 12–20 ton bottle jack (with cribbing; don’t rely on hydraulics alone)
- Lumber: straight 2x8/2x10/2x12 or LVL to match/upgrade existing; pressure-treated where touching masonry
- Fasteners: structural screws (e.g., Simpson Strong‑Tie SDWS, LedgerLOK, or Spax) or 1/2 in carriage bolts with washers; hanger nails for joist hangers
- Adhesive: construction adhesive (polyurethane like PL Premium)
- Hardware: joist hangers, post bases/caps as needed, composite shims
- Tools: circular saw or recip saw, drill/driver, impact driver, clamps, level or laser, string line, tape measure, moisture meter, PPE (gloves, eye/ear protection)
Cost range (DIY): $150–$600 per joist depending on lumber choice and hardware; add $60–$120 per adjustable post. Time: 4–8 hours per joist plus setup.
Step-by-step repair
1) Find the cause first
- Fix leaks, add a vapor barrier in crawlspaces, and improve drainage/ventilation. If the cause is over-spanning, plan to add a mid-span beam/post or upgrade the joist size.
2) Set temporary shoring
- Place a temporary beam perpendicular to the joists, spanning at least 3 joists beyond the target.
- Use adjustable steel posts under the beam on solid footing (thick plywood over concrete; use wider pads on soil).
- Snug the posts to remove bounce—don’t start lifting yet.
3) Lift slowly (if needed)
- Using a bottle jack under the temporary beam, raise the floor in very small increments (1/8 in per day is a safe pace). Rushing can crack finishes or stress framing.
- After each lift, transfer the load to screw posts and let the structure settle.
4) Sister the joist
- Cut a new straight joist (or LVL) to the same length as the span so it bears fully on both supports or sits in hangers. Longer is better—avoid short patches when possible.
- Dry fit, then spread construction adhesive along the side of the existing joist.
- Clamp the new member tight to the old. Fasten with staggered structural screws every 8–12 in, top and bottom thirds, starting at mid-span and working toward the ends. Through-bolts every 16–24 in also work well.
- If full-length sistering isn’t possible, use the longest piece you can and be sure it bears at least 3 in on supports or is supported by new hangers/blocking.
5) Reinforce support and connections
- Replace damaged or undersized joist hangers; use proper hanger nails or structural screws rated for hangers.
- Add solid blocking/bridging at mid-span to distribute loads and reduce bounce.
- If the span is too long, consider adding a budget-friendly mid-span beam (double 2x8 or 2x10 with posts) beneath multiple joists. Set posts on proper footings, not bare soil.
6) Remove shoring, test, and tidy up
- Slowly remove the jack load while the new sister carries the load. Walk-test the floor. Reinstall insulation/vapor barrier where removed.
Safety considerations
- Verify no plumbing, gas, or wiring is in the jacking path; relocate or protect as needed.
- Never rely on a hydraulic jack alone—transfer load to cribbing or screw posts.
- Use adequate pads under posts to prevent punching through slabs or soil.
- Wear PPE; cutting and drilling overhead produces debris and noise.
Tips for best results
- Select the straightest lumber you can find; LVL is stiffer if budget allows.
- Pre-drill for bolts to avoid splitting, especially near ends.
- Use adhesive—it meaningfully reduces squeaks and improves composite action.
- Address moisture: poly sheeting on crawlspace soil, gutters/downspouts extended, dehumidifier if needed.
- Record before/after measurements to confirm improvement.
Common mistakes to avoid
- Using drywall/deck screws for structure—use rated structural screws or bolts.
- Sistering only a short section at mid-span when ends are rotten or unsupported.
- Over-jacking in one session, causing drywall/tile damage.
- Leaving gaps between the old and new joist; clamp tightly before fastening.
- Setting posts directly on soil or thin slab without pads/footings.
When to call a pro
- Sag over about 1 inch, multiple joists affected, or a main beam/girder is cracked or settling.
- Significant rot, termite damage, or a spongy sill plate/rim joist.
- Foundation movement (wall cracks, sloping across entire rooms).
- You need new footings, beam sizing, or permit drawings.
- Utilities or structural walls complicate the load path.
With careful shoring, slow lifting, and a solid sistering plan using structural fasteners and adhesive, most single-joist sags are DIY-fixable over a weekend. Fix the root cause and you’ll get a stiffer, quieter floor that stays that way.