Diagnose squeaky and sagging floors — what's actually wrong.
SHORT ANSWER: A bouncy or sagging floor is almost always joist deflection — but the underlying cause matters because the fix is different. Three causes, in order of frequency in our service area: undersized joists for the span (pre-1980 homes), moisture damage from a chronically wet crawl, or pest damage. A vapor barrier won't fix any of these. An engineered jack-post system will. Specialists diagnose by walking the crawl with a flashlight, a moisture meter, and a span-table reference.
The three causes — and the diagnostic difference
1. Undersized joists for the span (most common in pre-1980 homes)
The biggest cause we see in Davis and Sacramento. A 2x8 joist spanning 16 feet was once considered acceptable; today's IRC R301 span tables require 2x10s for the same span. Pre-1980 homes weren't built to today's code — they're not failing, but they're at the edge of capacity and bouncy after 50 years of use.
Diagnostic signs: bounce when someone walks across the room. No moisture damage on the joists themselves. Joists are span-stamped if you can read the lumber, and the stamp confirms the joist is undersized for current code.
Fix: jack-post system installed mid-span. No need to replace joists; the post effectively halves the span.
2. Moisture damage from a chronically wet crawl
Common in older homes that have been sitting on damp dirt crawl spaces for decades. The joists lose cross-section as the wood softens, lose strength, and start to deflect.
Diagnostic signs: visible darkening or punkiness on the bottom of the joists. Moisture meter reads above 18% wood moisture. Crawl space humidity above 65%.
Fix: encapsulation FIRST to stop the moisture, THEN jack-post system to stabilize the joists. Sometimes joist sistering is needed where damage is severe. Skip encapsulation and the jack-post system buys you 5 years before the same joists deflect again.
3. Pest damage — termites or carpenter ants
Less common but more dangerous. A single termite-infested joist can lose 60% of its strength while looking only slightly damaged from below.
Diagnostic signs: small entry holes ("kick-out holes"), frass (fecal pellets) on the dirt below the joist, mud tubes on piers. Specialists tap the joist with a screwdriver — sound joists ring; damaged joists thud.
Fix: pest treatment first (we coordinate with a licensed pest-control specialist), THEN affected joists removed and replaced. Jack-post system added afterward as appropriate.
How to do the diagnostic yourself
Before scheduling an inspection, you can narrow it down:
- Walk the room slowly, in line with the joists (typically perpendicular to the longer dimension of the room). If the bounce is uniform across the floor, joist deflection is likely. If localized, single-joist damage is likely.
- Check crawl humidity if you have access. A $15 hygrometer from the hardware store tells you 80% of what you need. Above 65% RH suggests moisture is part of the cause.
- Check the floor with a ball. Roll a ball across the floor — slowly. If it rolls toward a specific point, you have settlement or deflection there.
The diagnostic on the homepage walks through these questions and gives a preliminary cause and price range before a specialist even shows up. It's not a replacement for the inspection — but it shortcuts the conversation.
When NOT to call yet
If the bounce is only in one spot — for example, a single squeaky board near the stairs — that's often a loose subfloor fastener, not joist deflection. Drive a few finish screws into the squeaky board from above; if the squeak stops, you don't need a contractor. If the squeak persists or the bounce extends across the whole room, then get matched with a specialist.