My floor feels soft and spongy. What causes that?
A floor that gives underfoot is a structural problem below the surface, not a flooring problem. It is usually water damage to the subfloor, joists spanning further than the sheathing thickness supports, or a subfloor that was never fastened properly. Replacing the surface over it does nothing.
Working out which one it is
Soft in one spot, especially near a bathroom, a dishwasher, an exterior door or a water heater, points at water. Particle board subfloor turns to porridge when it gets wet and does not recover.
Springy across a whole room with no water history points at span. Long joists or thin sheathing flex, and the floor feels bouncy rather than soft.
A localised squeak that moves when you step points at fasteners that have let go rather than at damage.
What the repair involves
The finished floor comes up over the affected area so the subfloor can actually be seen, because the damage is almost always larger than the soft patch suggests.
Damaged sheathing is cut back to the middle of a joist, and new sheathing goes in on blocking so every edge is supported. Where a joist itself is damaged or undersized, it gets sistered or reinforced.
Then the subfloor is fastened properly, screwed rather than nailed, and the finished floor goes back or gets replaced.
Find the cause first
A wet subfloor means water arrived from somewhere. A leaking supply line, a failed toilet seal, a shower pan, or moisture coming up through a slab.
We check the source before rebuilding, because the alternative is doing the same repair again in two years. On flooring that has been wet, the readings and photographs also matter if it is going through a claim.
The Work Behind This Answer
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