Quick answer
Start by finding the cause. Squeaks usually come from movement between the subfloor and joists—fix with screws and construction adhesive from above or below. Sagging points to overstressed, damaged, or rotted joists and often needs structural repair: correct moisture issues, provide temporary shoring, jack slowly if needed, and reinforce with sistered lumber or added beams/posts. Work methodically and safely; call a pro for major rot, engineered joists, or significant releveling.
Diagnose the problem
Common causes:
- Squeaks: loose subfloor fasteners, wood shrinkage, joist/subfloor friction, nails rubbing HVAC/ducts or plumbing contact.
- Sagging/bounce: overspanned joists, inadequate support, rot/termite damage, cut/notched joists, removed bearing walls, wet crawlspace/basement, heavy new loads (stone tile, aquariums).
Inspection steps (60–120 minutes):
- Walk the floor to map squeaks and low spots; mark with tape.
- From below, use a bright flashlight and mirror. Look for cracks, rot, insect galleries, rusted hangers, plumbing leaks, and improper notches/holes.
- Check level with a laser or a tight string line across several joists.
- Probe suspect wood with an awl; soft or flaking wood indicates rot.
- Measure moisture with a meter; address readings above 16–18%.
Tools and materials
- PPE: safety glasses, gloves, hearing protection, dust mask/respirator
- Lighting, moisture meter, awl, stud finder, laser level/string line
- Drill/driver, impact driver, circular saw/recip saw (as needed)
- Structural screws (e.g., GRK RSS, 5/16" × 4–6"), subfloor screws (#8/#9, 2–2½")
- Construction/subfloor adhesive (e.g., PL Premium/PL400)
- Joist hangers, hanger nails or Simpson SDS screws
- Shims (hardwood), blocking material (same species/grade as joists)
- Jack posts/adjustable steel columns, bottle or screw jacks, cribbing lumber
- 2× lumber or LVL for sistering; corrosion-rated post bases if adding posts
Fixing squeaks (1–3 hours)
From above (best under carpet or during remodel)
- For subfloor to joist: drive 2½" subfloor screws every 6–8" along the joist line. Use construction adhesive if the floor is open.
- Over finished floors: use a concealed screw kit (e.g., Squeeeeek No More/Counter-Snap) to locate joists and snap the screw head below the surface.
From below (ceiling open or in basement)
- Apply a bead of subfloor adhesive along the joist/subfloor seam and clamp with temporary screws from below. Add a few 2–2½" screws driven up into the subfloor at squeak locations.
- If there’s a gap, tap in a tapered hardwood shim with adhesive—snug, not forced—then add screws to lock the area.
- Add solid blocking between joists at squeak zones to reduce cross-movement.
Avoid: relying on nails alone, hammering shims dry (they’ll work loose), and overdriving screws that strip out.
Addressing sagging or weakened joists
Stabilize and prep
- Fix moisture first: repair leaks, improve drainage, add vapor barrier in crawlspaces, and ensure ventilation/dehumidification. Wood should be near 8–14% moisture before structural repairs.
- Shore the area: build a temporary support wall or use jack posts with solid cribbing under the beam/joists near the repair zone before altering anything.
Jacking (if correcting sag)
Raise slowly to avoid cracking finishes above.
Jacking guidance:
- Lift no more than 1/8" per day (or every few days) per location.
- Spread load with a beam on top of the jack posts (e.g., doubled 2×10).
- Never jack directly against subfloor; bear on multiple joists.
Reinforcement options
- Sistering a joist: best for cracked, overspanned, or slightly sagged members.
- Use matching lumber (e.g., 2×10 on 2×10) or LVL. Ideally full length with bearing; at minimum, span the middle 2/3 of the joist and extend past the damaged area by 3–4'.
- Apply construction adhesive along the top edge and face, clamp, then fasten.
- Fastener pattern: stagger structural screws every 6–8" along both top and bottom thirds, 2–3" from edges.
- Add a mid-span beam/post line: effective for multiple joists or long spans.
- Use an appropriately sized beam (LVL or built-up 2×) on adjustable steel columns.
- Set posts on proper footings to local code (don’t bear on a thin slab).
- Replace severely rotted sections: cut out only after shoring; scab/sister with full bearing and install proper hangers.
- Improve connections: replace undersized/loose hangers with rated hangers and the manufacturer’s nails or SDS screws. Add missing rim joist blocking.
- Reduce bounce: install solid blocking or X-bracing at 6–8' intervals. This doesn’t fix sag but improves stiffness.
Engineered joists/trusses
Do not cut or notch I-joists or trusses. Repairs require manufacturer-approved methods. This is a call-the-pro situation.
Specs and patterns you can follow
Sistering basics:
- Adhesive: continuous bead along crown and face.
- Screws: 5/16" × 4–6" structural screws, staggered 6–8" o.c.
- Minimum sister length: center 2/3 of span; full-length preferred with bearing.
- End distance: keep screws 2" from ends, 1.5" from edges.
Subfloor fastening:
- Screws every 6–8" along joists; 4" at panel edges.
- Panel glue lines on joist tops for new installs.
Costs and time
- Squeak repair: $10–$60 for screws/adhesive; 1–3 hours.
- Sistering one joist: $50–$200 lumber + $20–$60 fasteners/adhesive; 2–6 hours.
- Jack posts/beam line: $200–$800 in materials; footings add more.
- Pro structural repair: commonly $1,500–$5,000+ depending on scope.
Safety tips
- Wear eye/respiratory protection; old framing can shed debris and rodent dust.
- Verify no wires/plumbing where you’ll cut or screw.
- Shore before cutting or removing any part of a joist.
- Jack slowly; keep people off above during adjustments.
- Use rated connectors and fasteners—no drywall screws in structure.
Common pitfalls
- Trying to “fix” sag with shims alone—address structure.
- Over-jacking in one go, causing drywall cracks or lifted finishes.
- Short sisters that don’t extend past damage.
- Mixing incompatible fasteners in hangers (use the hanger manufacturer’s).
- Ignoring the moisture source; problems will return.
When to call a professional
- Sag greater than ~1" over 10', widespread damage, or visible rot/termite activity.
- Engineered joists/trusses, beam or foundation issues, or removed bearing walls.
- Need for new posts/footings or permits/engineering.
- Persistent moisture, mold, or structural cracking.
With careful diagnosis, proper fastening, and measured reinforcement, most squeaks and moderate sag can be corrected safely and permanently.