Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It only takes a minute to sign up.
Sign up to join this community
The house I recently bought has hardwood floors, but I've noticed that some spots are spongy when I walk over them. There is visible downward flex in certain spots to an observer of someone walking across the room. I took a coin and tapped across the floor, and the spongy spots have a hollow sound.
What is the cause of this, and is there a remedy without tearing out the whole floor?
The cause is generally that the flooring or subfloor is not securely attached to the joists underneath. Usually this happens because the joist settle or bend over time and the flooring becomes loose or detached in spots.
If you can get underneath the floor and find the soft spots (say from the basement or crawlspace?), you'll likely see that there is a gap between the flooring and the joist. Just get a chunk of 2x4 and sister it to the joist so that the flooring sits on top of the 2x4 with no gap between them.
If that doesn't stop the spongyness, you might want to use some clips or small angle brackets with short nails or screws to attach the flooring to the 2x4 from the bottom, or toe-nail through the 2x4 and into the bottom of the flooring (but be careful not to go all the way through it of course)
I suspect the previous answer will rarely apply to the problem as reported by OP. Most likely the floor isn't "solid" hardwood (ie - it's probably not 2-3cm thick floorboards laid directly over joists that have sagged).
I've often seen what OP describes on "click-lock" flooring, where interlocking 6-10mm thick panels are used to "tile" the area. The panels are usually laminate rather than "real wood", but according to that Wikipedia link it's often called floating wood tile in the US, and some artificial composites are so good you might think they're real wood anyway. Whatever - the problem can apply (though less commonly) even with real "wood strip flooring", as it's often called in the UK.
The American name itself suggests where the problem lies. These "secondary" floor coverings are supposed to float, meaning they should not actually be fixed to the flat surface underneath. In the UK, most modern building have 8' x 4' chipboard sheets screwed into the joists, and the "floating" floor lays directly on these sheets.
Actual floorboards (generally older buildings in the UK) are usually lengths of softwood about 6" wide spanning the entire room. If a floating floor is laid on these, they're normally covered first with a layer of thin (4-6mm) plywood sheets, to get the required flat surface for the floating floor.
With these types of flooring, it's essential the entire top covering is actually able to float - whether it's real wood or a composite material, temperature and humidity changes over time are likely to cause expansion. The hollow/spongy patches aren't caused by sagging. The floor is trying to expand, but is being pinched at the edges because there's not an adequate expansion gap where the floor meets the walls, so it buckles upwards.
Because "click-lock" flooring looks easy, people often do it themselves - or employ an incompetent handyman who does a job that looks good until he's been paid, but causes problems later.
In every case I've seen, the solution involves taking off the skirting boards (apparently called base or mop boards in US/Canada). The floating floor is supposed to go under the bottom of the skirting, but still have about a 1cm gap between the flooring panels and the actual wall. If there's no gap, chisel one out. If the flooring panels were actually butting hard against the front edge of the skirting, saw/chisel the bottom of the skirting so it can go under. Then just re-fix the skirting.
Sometimes the "pinch points" may not actually be at the skirting/wall. You may have a fitted kitchen, for example, where the base units are solidly fixed to the underlying "subfloor". But the same basic principle applies. There will probably be quadrant (scotia) strips covering the vertical/horizontal join. Prise/chisel these off to check whether the floating floor is being trapped against something under there - if so, just adapt the remedy outlined above.
It can also be caused by a floor or subfloor that is not level. High and low spots that are barely detectable by the eye can cause the floor boards that are spanning over the low spots to sag. It might only be 1/16" or less but it is enough that you will notice it as you walk over it. If you watch someone walking over those areas, you will see the boards moving slightly.
Spongy engineered floor over concrete slab is likely due to unlevel surface of slab that should have been addressed prior to installing mat and flooring. Likely not a new installation whereooriginal floor was carpeting over mat(more forgiving).
