Symptoms
- Floor feels spongy, soft, or gives slightly when stepped on
- Visible dip or low spot in the flooring
- Often near a bathroom, kitchen, or exterior wall
- May be accompanied by a musty smell or staining on the floor above
Likely causes
- CommonWater damage from a plumbing leakLeaking pipes, supply lines, or drain connections can slowly rot the subfloor from below or above.
- CommonLong-term moisture from a failed appliance sealDishwashers, washing machines, and refrigerators with ice makers are common sources of slow leaks.
- PossibleRot from an exterior water intrusion (roof, siding, foundation)Water entering from outside can travel and weaken subfloor sections not obviously near the source.
- Less commonUndersized or damaged floor joistsIn some cases, the softness is structural rather than rot-related, from an inadequate or damaged joist.
What to do
- Identify the exact location and extent of the soft spot by pressing along the floor in a grid pattern.
- Check the room below (basement, crawlspace) directly under the soft spot for staining, dampness, or visible rot on the subfloor or joists.
- Look for and fix any active leak source nearby, such as a supply line, drain, or appliance connection.
- Do not delay this repair; a soft subfloor can worsen and become a fall-through hazard if left unaddressed for a long time.
- For accessible soft spots from below, a section of new plywood can sometimes be sistered or screwed alongside the damaged subfloor to reinforce it.
- For soft spots only accessible from above, the flooring will need to come up to replace the damaged subfloor section.
- Have any affected joists inspected as well, since rot or damage there is a bigger structural concern than the subfloor sheathing alone.
Tools and parts
- Flashlight
- Moisture meter (optional)
- Screwdriver (to probe for soft wood)
Plywood subfloor sheathingSearch: “plywood subfloor sheathing”
When to call a pro
Call a contractor or structural professional if the soft spot is large, near a joist, or you're unsure of the extent of the damage.
Safety
Use caution
Avoid heavy foot traffic or placing heavy furniture directly over a noticeably soft spot until it's inspected, since it may indicate a weakened structural area.