I am installing a 3/4 solid maple harwood floor. My current subfloor is 5/8 plywood under a 3/8 particle board. From what I've read here, I'm tearing out the particle board. My question is what I should replace it with?

also, what type of screws are best to use?

Thanks,
Josh

Unless you need to bring the height of the wood floor up, you may be able to just use the 5/8" plywood subfloor. If I understand correctly, you are going to nail down a 3/4" maple floor. How wide are the boards? If the floor joists are set no further than 16" apart and you lay the maple flooring perpendicular to the joists, you may not need anymore plywood. It's best to refer to the manufacturer's instructions but in general, a 5/8" plywood subfloor over joists 16" oc is the minimum acceptable subfloor for nailing 3/4" flooring to.

Gary,

Thanks for the reply. Yes I'm nailing down a 3/4" solid maple floor. The boards are 4" wide. The joists are 16 OC and I would be laying the planks perpendicular to the joists. Would I have to nail into the joist? It sure would be great if I didn't have to put anything else down. Let me know your thoughts.

Thanks,
Josh

On a 5/8" subfloor with a 3/4" hardwood floor, only approx. 1/4" of the fastener will go into the joists. It can't hurt but may not help much either. If it makes you feel better, go for it. No harm. However, you need to ensure your existing subfloor is sound, flat, and secure. They often need re-nailing and some fussing with. Of course adding more plywood will make everything stiffer and more solid but it is not required as 5/8" plywood is minumum. The manufacturer may require a 3/4" subfloor however, and if they do, I would follow their recommendations by adding more plywood.

Do you really suggest nailing the subfloor? I have had deck screws recommended to me.

ok, so assuming the manufacturers min is 3/4" (I just put a call in), what thickness and material would you intall over my 5/8" ply?

I use the words "renailing" to mean refastening. If you look at the subfloors, over 90% are nailed by the builders. When I build subfloors, I use glue on the joists and a pnuematic framing nailer to shoot down ring shanked 2&1/2" nails. Works great! But when refastening an existing subfloor, you can certainly use screws. Screws work excellent. They cost a little more and take a little longer but they hold extremely well. BTW, nails are required by most local building codes instead of screws. Why? Nails have more shear strength and when severly stressed will give and release a little without shearing off. Screws don't do that. They hold and when their shear strength has reached it's maximum, they break. That's why all sheathing (roofs, walls, floors) are usually nailed instead of screwed. Lately however, a certain type of heavy duty screw has been showing up on the jobs I've been on to hold down the subfloors.
About adding plywood, the least expensive will be 3/8" CDX. This will give you a 1" subfloor and should meet the manufacturer's requirement. You could use 1/4" plywood but it's not available in CDX; only in higher grades and for more money. I install CDX underlayment with staples; a lot of staples. It's fast, cheap and works well. But I'm set up for it with a compressor and a Senco stapler. You can screw down the underlayment but it will take much longer unless you get a auto screw feeder. I don't advise gluing underlayment unless you have to. The reason is if you ever have to remove it for repairs, you will tear the snot out of your subfloors and that's not so great. IMO, stapling is the way to go and what most of the pros do around here.

Gary,

Your giving me a lot of great info. Thanks. What is CDX? Is it the name of just the basic plywood?

One side of the wood is C grade, one side is D grade and the X stands for exterior glue between the plys. The only important thing is the X. you can use ABX (though more expensive) or DDX if you can actually find someone who manufactures it.

It is stamped C-D. No X. I'm assuming I got the wrong stuff??

It won't be stamped with an X. It should say for exteroir exposure. We say X because that's short for exterior exposure. OK?

gary, what gauge staples do you use? and what length should I use with a 3/8 underlayment and 5/8 subfloor?

My Senco stapler ( http://www.senco.com/con_rem/ViewTool.aspx?toolid=550 ) uses #16 gauge staples with a 7/16" crown. You will want to use 1" to 1&1/4" long staples.

Gary, I'm back! So, I'm having trouble finding a 16 gauge stapler. Don't really want to buy one. I have access to an 18 gauge stapler. I called a tool rental place and he said this is what people use for underlayment. I want to get your opinion.

Thanks,
Josh

That 18 gauge stapler is for stapling down 3/8" particle board underlayment which a vinyl floor is going over. Vinyl installers prefer very small staples because then they don't have to fill the staple holes; the glue they're using will do that. Any rental yard should have what's called a general construction stapler. The staple may be a 15 gauge instead of a 16 gauge. The crown could be 1/2" and that's fine as well. I just don't think those little 18 gauge wires work that great for underlayment. Keep looking.