Sagging or Bouncy Floors Over a Crawl Space

What this actually feels like

Homeowners describe this a few different ways - bouncy, springy, spongy, soft, or a floor that "gives" when you walk across it - and the words people use are actually a useful first clue. A genuinely bouncy or springy floor moves and returns to shape, often across a wider area rather than one exact spot. A soft or spongy floor, especially one confined to a smaller area, is a meaningfully different finding and more likely to involve the framing itself rather than just what is under it.

Insulation or decay, and how to tell them apart

Two different problems produce this symptom, and distinguishing them before calling anyone saves money, because one is inexpensive and common and the other is more serious and less common.

A field test a contractor will use, and that you can understand even if you should not perform it yourself under the house: pressing a screwdriver or awl into suspect wood. Sound wood resists the tip. The USDA Forest Products Laboratory's Wood Handbook is specific that serious decay requires wood moisture content above the fiber saturation point, averaging 30 percent - wood that wet has typically lost enough structural integrity that the tip sinks in with little resistance, which is the practical line between a cleaning-and-insulation job and a framing repair.

Why floors here do this

The local housing stock makes this a common enough finding to have its own page. Citywide in Seattle, 30.1 percent of housing units were built before 1950, per Census ACS estimates, the highest share of the 19 places this site covers (see the full city-by-city data). The census does not record foundations or insulation, but in older houses with crawl spaces, whatever floor insulation was installed has often been in place for decades. Age alone degrades batt insulation's grip and loft even without a moisture problem involved; add sustained winter dampness, with about 75 percent of the year's rain at Seattle-Tacoma International Airport falling from October through March, and the same age that weakens insulation is also the age at which a moisture-driven decay problem has had the most time to develop, if one is present.

What happens if you ignore it

If the cause is insulation, ignoring it mostly costs you comfort and energy - a floor that stays cold and a heating bill that stays higher than it should, without the problem getting structurally worse on its own. If the cause is decay, the calculus is different: untreated decay in contact with ongoing moisture continues to consume the wood's structural capacity over time, and what starts as one soft spot can spread to adjacent framing that is exposed to the same moisture source. The USDA Forest Products Laboratory notes decay is relatively slow below 50°F and above 95°F; a crawl space here spends much of the wet season above that lower figure, so this is not a problem that reliably pauses itself over the winter.

What you can check yourself

• Note whether the bounce is wide and even, or confined to one soft spot.
• Check whether the affected area lines up with a known plumbing line, exterior wall, or any past water issue.
• Look at the visible insulation from the access hatch, if you can safely see it, for sagging, staining, or obvious gaps in coverage.
• Note how long the floor has felt this way, and whether it has gotten noticeably worse recently - a sudden change is a more urgent signal than something that has been present for years.

What this does not include: getting under the house to probe framing, remove insulation, or assess structural capacity yourself. That is a job for someone who does it professionally, and in Washington anyone contracting for it has to be registered with the Department of Labor & Industries, bonded and insured; how to verify a Washington contractor shows where to check.

What a professional evaluation adds

The screwdriver test above is simple in principle but needs to be done systematically across the affected area and the framing around it, by someone who knows what sound Douglas fir framing should feel like versus what compromised framing feels like. A professional inspection is what actually rules decay in or out, rather than guessing from the surface symptom, and it is the only way to know before committing to a scope whether you are buying an insulation job or a framing repair.

The support system, and what repairing it involves

The third cause is the one the comparison above does not reach, and on an older house it can matter as much as the other two: the floor is not decayed and not under-insulated, it is simply not supported the way it needs to be. Bounce is a stiffness problem before it is a strength problem, and stiffness comes from how far the framing spans between supports.

It helps to know the names, because a written scope will use them and a vague one will not. Working downward: the joists carry the floor; they land on a girder (also called a beam) or on the sill plate sitting on the stem wall; the girder is held up by posts; and each post bears on a footing or pier block in the ground. A failure anywhere in that chain reads upstairs as the same soft, moving floor.

In older houses, the usual culprits are near the bottom of it:

• A post that has settled because its pier block was set on soil that has since moved, which is more likely where the ground stays wet through the winter.
• A post rotted at its base where end grain meets damp ground or a pier with no capillary break.
• A girder that has sagged between posts spaced further apart than anyone would space them today.
• Joists notched or drilled for plumbing and heating runs added decades after the house was framed, each notch removing depth exactly where the joist needed it.

The repair vocabulary

• Sistering — fastening a new joist alongside the existing one so the pair carries the load together. It is the cheaper answer, and it works where the original member is sound but split, over-notched or under-strength. A sister has to run the full bearing length and land on the same supports at both ends; one that stops short of the bearing is decorative.
• Replacement — where decay has reached the bearing ends, or gone far enough that there is nothing sound left for a sister to act against. Distinguishing this from the case above is the whole point of the evaluation described in the previous section.
• Adding or resetting posts and footings — shortening the span rather than strengthening the member. For bounce specifically this is frequently the real fix, and it is often the cheapest one: a properly footed post under a sagging girder can do more than a great deal of work on the joists above it.
• Replacing the girder — where the beam itself has deflected rather than its supports having moved.
• Jacking — raising a settled floor back toward level. This is done gradually, over weeks, in small increments. Lifting quickly transfers the movement into the finishes above: cracked plaster and drywall, split tile, doors and windows that stop closing. A quote that offers to level a floor in a single visit is describing a different job from one that intends to leave the rooms upstairs intact, and it is worth asking which you are being sold.

When a structural engineer belongs in it

Most crawl space framing repair is contractor work and does not need an engineer. Four situations change that: when the fix alters how load reaches the ground — removing or relocating a post, replacing a girder, touching anything bearing; when the movement is in the foundation itself rather than in the wood above it; when the proposed repair and the symptom do not obviously connect to each other; and when you want a scope written by someone with no work to win from it, which is the same reasoning behind getting a second opinion on any large crawl space quote. A building department may also want stamped drawings before it will issue a permit for structural alteration, in which case the engineer is not optional.

Permits

Whether a permit is required is decided by the work and by the jurisdiction, not by the label on the estimate. In the City of Seattle, the residential code exempts insulating an existing building and in-kind or similar replacement or repair of deteriorated members, provided the building envelope is not changed, and exempts minor repairs valued at $6,000 or less in any six months as long as no loadbearing support is removed, reduced, altered or relocated. Adding posts, footings or a girder where none was, or changing how the floor is held up, is outside both. Seismic work under the house, such as bolting the house to its foundation, has its own Earthquake Home Retrofit Permit in Seattle. Outside the city, each city sets its own rules, and in unincorporated King County crawl space work is not on the county’s list of work that needs no permit. The home page’s section on crawl space repair in Seattle, neighborhood by neighborhood, covers the city side of that.

Two things to put in writing before work starts: which permits the contractor's price includes, and who is pulling them. Permit fees sit outside every cost range published on this site and are normally billed at cost, so a quote that is silent on them is not a complete number.

Fixing it

Where degraded insulation is the cause, insulation replacement removes the old batt, installs new floor insulation to current code R-value, and air-seals the rim joists - modeled at $2,500 to $5,825 for a Seattle-area crawl space, before sales tax, and itemized in the 2026 Seattle crawl space cost guide. If you are also dealing with cold floors specifically, why your floors are cold covers the R-value table most bids skip.

Where decay is present, or where a post has settled and taken the girder down with it, the framing work is its own scope: crawl space structural repair. A localized repair is modeled at about $375 to $1,350 per location, and a floor system needing several of them is a different conversation. Either way it goes alongside whatever is causing the sustained moisture - usually drainage or waterproofing - because repairing the wood without fixing the water source means doing the repair again. See wood rot in a crawl space for how serious decay actually gets.

A free in-home inspection tells you which of the three you actually have before you pay for any of them, and you get a written, no-obligation quote. A useful one names the cause it found, lists each support or repair location separately, and prices the moisture work alongside the structural repair rather than leaving it out - repairing the wood without the water source means doing it again.