between each joist (open-web joists 24" on center) the floor sags between 1/8 and 1/4 inch. The subfloor is plywood, but im not sure on the thickness. I plan to install Bruce engineered floors and plan to float them rather than glue. I'm trying to figure out what is best to use to fill these dips between the joists. I've done a bit of reading here and have gathered that strips of felt paper might be my best option??? I can't imagine using any kind of filler because the dips are many and I'd have to rent a cement mixer or something . Any other thoughts, suggestions, etc.?

Thanks in advance,
Jeff

I recall installing a 3/4" nail down floor 5 yrs. ago over a 3/4" plywood subfloor with the trusses spaced 24" o.c. There were the same "dips" between every truss like you're encountering. When I asked the builder about it, he said that is what the architech specified and that it met code. Man, was that subfloor bouncy between the trusses. Anyway, back to your problem. Use a long straight edge to accurately flatten the floor. Try using cheap 3 tab composition roofing shingles or roll roofing to build up those low spots. You can staple it in place with a good hamer tacker. I'd put the mineral coated side down on the subfloor. 30 lb. felt will work but since it is thinner than roofing, it'll take more layers to build up. If you have the clearence, you could add a layer of 1/2 CDX to stiffen things up after flattening the suibfloor with the roofing. Good luck.

Hi Gary - I'm curious, would you still recommend adding a layer of 1/2" plywood if I were to install 3/4" nail down floors opposed to the engineered floating floors?? My baseboards are coming out so the height wouldn't necessarily be an issue, but at the same time I don't want the added cost/time/labor if it isn't really necessary.

If your existing plywood subfloor deflects (moves up and down) alot in between the 24" oc floor joists, it would be a good idea to add more plywood to stiffen things up. Or add blocking from below if you have the room to work. The reason? While 3/4" flooring will add stiffness to an existing subfloor, if that subfloor has considerable deflection, the flooring will move up and down as well. This can contribute to gaps between boards, loose fasteners, sqweeks and an uneven floor. Besides, how can one flatten a floor when that floor is moving up and down? It will be a judgement call on your part. Good luck!