Helayna from Toronto, Ontario wrote that a few of her hardwood floorboards simply bend down when you step on them in one area. She is afraid that at some point they might just break through. The ceiling below is finished, so she can't check it out from below. She had a contractor who suggested drilling holes and filling the space with expansive foam, but he said that he wasn't sure how much foam to put in. What to do?
There must be problems with the subfloor below the hardwood, and if the problem is widespread, she will have to open the floor from either the top or the bottom to find out if there is a rot problem or something else compromising the structural integrity of the floor--maybe even a missing section of the subfloor. And I wouldn't go shooting in expansive foam for two reasons: first it is not strong enough to resist being compressed when you walk over it, and second it would probably push the ceiling below right off the joists.
If you would like to try to fix a problem like this one with the least disruption to the house possible, I would suggest drilling a 3/8 hole through the hardwood, but not through the subfloor. You can then poke something through the hole to be sure that there is another layer of something there, even though it is obviously not very sturdy. If there is no subfloor at all, it is renovation time. If there is some kind of weak subfloor, use PL Premium construction adhesive (this is a polyurethane adhesive that will not shrink and will dry rock hard) and force it into the hole so that it will spread out between the hardwood floor on the top and whatever there is as a subfloor. Put in as much as you can force in, working slowly to give it a chance to flow sidewise. Put a wooden plug into the hole before the PL Premium dries. Then be careful not to walk on it for two or three days to let it really cure. This should create a rigid mass out of the moving boards -- and nothing will ever move again in this area.