Solid Hardwood, Engineered and Laminate Flooring - Hardwood Gaps - See the subfloor

Our 3 year old house has VERY bad hardwood floors throughout the first floor. The large gaps are 1/8" and run a few feet long. We use our central air unit in the summer, and these sizeable gaps have not improved during the summer months. Our house is typically between 70-72 degrees year-round. The relative humidity is anywhere between 30-45% (about 32% w/o a humidifier downstairs and 30-45% with 2 in the upstairs bedrooms). It looks as though the "wood floor guy" put in a filler when the house was built b/c you can see pieces of material between the wood where it has separated. I have also vacuumed up pieces as they have broken off. I have read every post regarding gaps - wood fillers seem to break off. Someone used wood caulk b/c it is flexible and another person used Bondo with concrete colorant. Any suggestions or feedback on these methods???? So frustrated with these unsightly gaps that collect dirt, kitty litter granules, and ANY other liquid spilled on the floor!!! :wall:

If this were an older house I'd ask if gaps occur at some regular interval. It's a common problem where the subfloor is planks run the same direction as the flooring. But presumably you have a stable modern subfloor, so I guess the flooring was relatively wet when put on. Tight schedules = bad logistics.

Apparently the flooring was seen to shrink soon after Floor Guy got paid and the contractor opted to fill the gaps... which continued to enlarge.

Anyway, to solutions:

That the filler is crumbling out is good, because you're going to remove it. With awl (etc.) and vacuum. Scrape the gaps out 'till you reach wood.

Fill with splines of hardwood veneer. I'm positive you can find the matching species. Different thicknesses are available, 1mm and up. You'll probably use a variety of thicknesses to get the best fit. Easiest to use "unglued edge banding" which is just veneer sold as ribbons. A straight edge, which goes to the bottom of a gap, is there already. Maybe use 5/8" wide banding, you slice in two roughly down the middle, for a 1/4" deep gap. The idea is to let the splines protrude very slightly, then sand them flush.

Glue these in with the same varnish your floor is topped with. Just run a brush along the joints so varnish seeps in. Needless to say, you'll want to tackle this in sections, and kneel on dropcloths + pillows lest the splines crack and split down below the flooring.

80 grit paper, hand-sanded with the grain, will bring this flush. Maybe ~50 grit if the splines are poking out a lot. You'll have a floor in need of light refinishing. So more hand sanding with finer grits, or rent a floor polisher, your call. Then stain if appropriate, and varnish.

Thanks for the feedback. It's nice to see a solution to gaps other than getting your humidity levels in check (which they are). I really appreciate the details in your reply!

My father-in-law and I attempted our first hardwood floor installation in my dining and living room five years ago. During the cold season the gaps get as big as 3/16" and close up when it warms up. In the larger gaps I can see filling them in with strips, but will this have any affect when the floor expands? Some of my concern is buckling.
Plus, we are planning on refinishing the floor this summer and are also concerned that when we are ready to start the refinishing, the gaps would have already closed. Do we have to move ahead our plans?

I had a similar problem in my last place. I used about 4 times the normal amount of moisture cured poly, and poured it on so it filled the gaps, then brushed level. By the end of three coats, the gaps, while visible were no longer a problem. They never closed up again, but it didn't seem to cause any additional distortion of the boards