05:21PM | 02/09/01
Member Since: 02/06/01
4 lifetime posts
I have a 3rd floor condo which has 4 inches of concrete pored on top of the plywood for fire protection. It is a very cheap concrete which is very dusty (almost like it has no sand in it.) The floor had many cracks and you can feel feel some motion in certain places. You can even hear the plywood squeak in certain places. In a couple of places, the concrete looked like two plates that were 1/4 of an inch apart in height. I wanted to put hardwood on the floor so first I tried to fix the cement by pouring some self leveling cement on it. This helped but some of the cracks came back. A few are just hairline cracks, some a bit bigger. My goal is to put Mirage engineered red oak which is a 3/8 inch floor designed to be glued down directly onto concrete or cork or plywood. You can also staple it into plywood. I have been trying to decide on the best way to do this.

Some contractors have said I can just glue the wood right onto the concrete. Others have said that I'll have problems with the floor and need a subfloor. But this means putting lots of screws into the concrete which could make the floor worse.

My question is this: I am trying to understand worst case scenerio's. Suppose I glue the wood directly unto the concrete and the concrete cracks some more or the current cracks get bigger, what will exactly happen?

Suppose I put a 1/2 inch plywood subfloor, and the concrete again moves and the cracks get bigger. How will my subfloor help? If the concrete moves, won't the subfloor move with it? Should I use liquid nail to adhere the plywood to the cement or screws or both and how far apart should the screws be? What is the best way to level the plywood? I read somewhere where someone suggested sanding. Can you really sand plywood? I thought it was glued together from many sheets so I don't see how you could sand it.

What I really am trying to understand is the physics of this. The cement moves and then what? What happens to the wood above it and how can a subfloor stop this? Any info would be appreciated.


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