Short answer
Some cracks are normal and cosmetic (especially surface “checks” in solid wood), but others indicate a loss of capacity. Cracks are more likely structural if they are full-depth, run diagonally near supports, occur around notches/bolt holes, show spreading over time, or are accompanied by sagging, bouncy floors, or doors sticking. When in doubt—especially for main support beams—have a structural engineer assess it.
What you’re looking at (context)
“Support beams” in houses are typically:
- Solid-sawn wood (e.g., 6x8, 4x10)
- Engineered wood (LVL, glulam)
- Steel I-beams (with bolted or welded connections)
Each material “cracks” differently:
- Wood: hairline surface checks along the grain are common from drying. Structural problems show as full-depth splits, diagonal shear cracks near supports, or separation in laminations.
- Steel: the beam itself rarely cracks; look for cracked welds, bent/creased flanges, or heavy corrosion thinning the steel.
- Concrete: diagonal shear cracks near supports and wide cracks with spalling can indicate distress.
How to tell cosmetic from structural
Use this quick field guide:
Cosmetic (common/monitor):
- Wood surface checks that are shallow, follow the grain, and don’t run through the beam’s full depth.
- Small end checks that don’t extend far and aren’t growing.
- Hairline shrinkage cracks in concrete <1/32" without displacement.
Structural concern (act now):
- Wood cracks that are full-depth or travel across multiple laminations in LVL/glulam.
- Diagonal (30–45°) cracks near supports or notches—classic shear distress.
- Cracks around bolt holes/notches, or notches deeper than code allows bearing weight.
- Increasing crack width or length over weeks/months.
- Beam sagging, floor bounce, new drywall cracks above, doors/windows sticking, or posts becoming out-of-plumb.
- Steel: cracked welds/bolts at connections, heavy flaking rust, crushed shims/bearing.
- Concrete: wide cracks with spalling, exposed rusty rebar, or displacement.
Field thresholds (rules of thumb)
- Wood hairline checks < 1/32" (0.8 mm): typically cosmetic; monitor.
- 1/32"–1/8" (0.8–3 mm): monitor closely; if growing, evaluate.
- > 1/8" (3 mm), or through-thickness: get a pro.
- Visible sag > 1/2" over 20' span, or new sag/bounce: get a pro.
- Moisture in wood > 19% or soft/crumbly areas: address moisture and evaluate.
Step-by-step inspection
Time: 1–2 hours
1) Identify the beam and load path
- Note the beam type (solid wood, LVL/glulam, steel) and what it supports (joists, a wall above, stair opening, etc.). Heavier loads raise the stakes.
2) Map the cracks
- Mark each crack’s ends with a pencil and date. Note orientation (along grain, across, diagonal), length, and whether it goes full depth.
3) Measure and document
- Measure span, beam dimensions, and any sag using a laser or a tight string line and tape. Photograph with a ruler/coin for scale.
4) Probe and check moisture (wood)
- Use an awl. If it sinks >1/8" easily or the wood is spongy, decay may be present. Check moisture content; >19% is risky for ongoing movement/rot.
5) Check supports and connections
- Look at posts, footings, and bearing points. Crushed wood fibers, split around notches, loose bolts, or cracked welds are red flags.
6) Monitor if safe to do so
- If you believe it’s cosmetic, recheck monthly. A simple crack gauge ($20–$50) or feeler gauges can confirm stability.
Tools and materials
- Bright flashlight/headlamp
- Tape measure; straightedge or laser level/line
- Feeler gauges or a set of coins; calipers (optional)
- Awl/ice pick; moisture meter for wood
- Camera/smartphone; painter’s tape/marker for date marks
- PPE: safety glasses, gloves, dust mask (N95 if mold suspected)
Safety considerations
- Do not remove or loosen posts, shims, or fasteners under a loaded beam.
- Don’t jack a beam rapidly; lifting more than ~1/8" per day can cause damage elsewhere.
- If you observe sudden movement, loud cracking, or bearing failure, evacuate that area and shore with rated screw jacks/temporary posts before further inspection.
Repair paths (examples and costs)
- Sistering a wood beam: Add one or more LVLs alongside with construction adhesive and 1/2" through-bolts or structural screws at 12–16" spacing. Often requires temporary shoring. Materials $150–$600+ depending on length/plies; labor varies. Engineer input recommended.
- Add a post and proper footing: Steel lally column ($80–$150) plus a code-compliant footing (commonly 2'×2'×12" but verify locally). Don’t set posts directly on a thin slab.
- Epoxy for wood checks: Acceptable for sealing/appearance, but don’t rely on it to restore strength without an engineered repair plan.
- Steel: Replace cracked welds or add plates—by a certified welder per engineer details. Address corrosion (clean, prime, paint) once capacity is confirmed.
- Concrete: Epoxy/urethane injection for hairline cracks; structural issues need engineered retrofit and may involve added supports.
Typical professional evaluation by a structural engineer: $300–$800 for a site visit and letter; more if calculations/drawings are required.
Tips for best results
- Control moisture: fix leaks, improve drainage, add a dehumidifier in damp basements/crawlspaces.
- Keep loads reasonable: avoid stacking heavy materials along midspan; distribute loads to bearing walls.
- Photograph with consistent lighting and a scale (ruler) for reliable comparisons.
- If you must jack, pre-load a temporary beam and lift slowly over days.
Common mistakes
- Filling a structural crack with epoxy and calling it “fixed.” The cause must be addressed.
- Over-notching/drilling beams for wiring or plumbing near mid-depth or supports.
- Relying on surface appearance only—ignoring sag, bounce, or connection issues.
- Adding a post on a thin slab without a footing, leading to settlement.
When to call a pro
- Any full-depth or growing crack; diagonal cracks near supports; separation in laminated beams; notable sag/bounce; cracked steel welds; or concrete spalling with rebar.
- If you’re unsure of beam sizing or the load path.
- Before making structural changes (adding/removing posts, sistering, welding). Permits often apply.
If you suspect urgent risk, install temporary shoring under the beam and contact a structural engineer or qualified contractor right away.