Short answer
Yes—DIYers can safely inspect and stiffen uneven or bouncy floor joists, provided you work methodically, use proper support while jacking, and avoid damaging utilities or the structure. Start by diagnosing the cause (loose subfloor, long spans, damaged joists), then apply the right fix: add screws to the subfloor, install blocking/bridging, sister joists with new lumber and construction adhesive, or add a mid-span beam/adjustable columns. Call a pro for major sag, rot, foundation movement, or if new footings/beams are needed.
What causes bouncy or uneven floors
Common culprits:
- Loose or under-fastened subfloor sheathing
- Over-spanned or undersized joists
- Lack of blocking/bridging
- Notched/over-drilled or cracked joists
- Moisture damage or insect activity
- Settled supports or beams
Typical performance targets:
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Floor deflection targets:
- L/360 minimum for most floors
- L/480 or stiffer preferred for tile/stone
Bored holes/notches in sawn joists:
- Holes ≤ 1/3 joist depth, ≥ 2" from top/bottom edges
- No notches in middle third of span
- End notches ≤ 1/4 joist depth
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Tools and materials
- PPE: safety glasses, hearing protection, dust mask, gloves
- Lighting: work light or headlamp
- Measuring: tape, 6–8 ft level or laser level, string line, straightedge
- Fastening: impact driver, drill, structural screws (e.g., GRK/Spax), 2–1/2" subfloor screws, construction adhesive
- Lumber: same-depth dimensional lumber for sisters/blocking; LVL if adding beams
- Metal: joist hangers, hanger nails, Simpson Strong-Tie connectors
- Jacks/support: hydraulic bottle jack or screw jack, adjustable columns (lally posts), temporary beam (doubled 2x10/2x12), cribbing blocks
- Sawzall/circular saw, pry bar, stud finder, moisture meter
Estimated costs: $50–$150 for screws/adhesive; $100–$300 for blocking; $200–$600 to sister a few joists; $80–$150 per adjustable column; LVL beams $200–$600+. Time: a weekend for 1–3 joists; longer for beams/columns.
Step-by-step: inspect first
- Map the problem from above
- Walk the area and mark bouncy spots. Check for squeaks. Snap a chalk line where floors dip.
- Inspect from below (basement or crawlspace)
- Look for cracked, notched, over-drilled, or sagging joists; dark/soft wood; insect tunnels. Probe with an awl.
- Verify bearing: joist ends fully supported and tight in hangers.
- Identify plumbing/electrical/HVAC before drilling or jacking.
- Measure elevations
- Set a laser across the space or pull a tight string line. Measure to joist bottoms to find sags and highs.
Fixes that often solve bounce
Tighten the subfloor
- From above (best): Drive 2–1/2" screws every 6–8" along joists; add construction adhesive at seams if accessible during flooring work.
- From below: Where you can, add screws up through joists into subfloor (pre-locate joists to avoid missing).
- Effect: reduces squeaks and minor bounce. Low cost, quick win.
Add blocking or bridging
- Install solid blocking (same depth as joist) or metal cross-bridging every 6–8 ft along the span, aligned in a row and fastened.
- Keep services clear. Pre-cut tight, use adhesive and structural screws or nails.
- Effect: distributes loads and reduces vibration; won’t fix major sag alone.
Sister joists for stiffness or damage
- Choose straight, crowned lumber matching the joist depth. Orient crown up.
- Jack first: Place a temporary beam under the sag (two 2x10s with a spacer) and a jack on solid cribbing. Lift slowly—no more than 1/8–1/4" per day—to relieve load and correct sag gradually.
- Apply construction adhesive to the existing joist, clamp the sister in place, and fasten: staggered structural screws every 6–8" along both top and bottom thirds. Through-bolts (3/8") every 16–24" add more bite.
- Length: Full-span is best. Minimum: extend past the damaged/sag area, bearing at least 3–4 ft beyond on each side. Bearing on supports is ideal.
- Recheck level, then release the jack carefully.
- Effect: major stiffness improvement and damage repair.
Add a mid-span beam or support columns (for long spans)
- Build a beam (LVL or doubled/tripled dimensional lumber) under joists at mid-span; support with adjustable columns set on proper footings. Follow local code for footing size/depth.
- Use joist hangers or bearing blocks to tie joists to the new beam if needed.
- Effect: cuts the span in half; dramatic stiffness improvement.
Finish leveling the surface (if needed)
- After structure is stiff, correct small height variations: plane high joists or add tapered shims/underlayment. Self-leveling compound works on the subfloor, not directly on joists.
Safety and best practices
- De-energize circuits in the work area where possible. Protect/relocate pipes and wires before drilling or jacking.
- Spread jack loads with a thick plate/cribbing; never jack against drywall or a single joist.
- Lift slowly to avoid cracking finishes above. Watch for movement, creaks, and binding doors.
- Use code-compliant fasteners for connectors. Hanger nails or structural screws only—no drywall screws.
- Control moisture: fix leaks and improve ventilation before repairs, or new lumber will suffer the same fate.
Common mistakes
- Over-jacking too fast causing drywall cracks or lifted partitions
- Short sisters that don’t span the problem area or lack adequate fastening/adhesive
- Cutting big holes near the bottom/top of joists or not respecting hole/notch rules
- Trying to fix bounce with leveling compound alone
- Ignoring beam/column settlement or moisture/termite damage
When to call a pro
- Sag over about 1" across a room, or visible beam/post movement
- Rot, mold, termite damage, or multiple cracked joists
- Foundation settlement, not just bouncy joists
- Adding beams that require new footings or altering load paths
- Heavy finishes like stone/tile needing L/480–L/720 performance
- Permit or engineering sign-off required by your jurisdiction
Tackle the low-risk fixes first—subfloor screws and blocking—then move to sistering or new supports if needed. With careful setup and patient jacking, DIYers can achieve a flatter, quieter, and significantly stiffer floor.