I have a question regarding underlayment. I'm about install the underlayment for 3/4" hardwood flooring. The under layment I bought is 3/4" T & G plywood. It's CD Exposure 1. The current subfloor is 3/4" wood planks about 5" wide. Would it be a problem if I install the 3/4" plywood over the 3/4" subfloor planks? Would this be too heavy for the floor or is it too thick (3/4" + 3/4") to install the hardwood floor? I got the 3/4" plywood for the underlayment thinking that the thicker the underlayment the better. Is this true?

The only thing you may run into is a height issue. You may need to cut doors and may trap the appliances if going in kitchen.

be sure to do any leveling before you put the new subfloor down. It will make a nice soloid subfloor by beefing it up that much. The only concern is height issues that may come from appliances and doors.

Thanks for the advice.

So I shouldn't worry about the weight from the 3/4" board as opposed to 1/2". Part of the floor is sagging down over 1" in a 3' radius. That's where the refrigerator was placed.

If it's sagging that drastically check for rot. besides that the weight is not a problem. I put 3/4 ply ove planking all the time.

I looked at one of the joist that was exposed and didn't notice any rots by the lowest point. I don't think its rot. The whole back half of the floor is lower than the front half, not by 1 inch but around 1/2 inch. The front half of the floor is on top of the garage and that may be why it's higer.

I'm planning on putting 3/4 " plywood underlayment for the back half of the floor and 1/2" plywood for the front half and use 30# felt paper on the 1" low spot. This should even out the floor a bit. Please let me know if this is a good or bad idea. Also should I use subfloor construction adhesive when installing the plywood underlayment to the subfloor plus dry wall screws or just use dry wall screws?