I'm sorry for having to ask another subfloor/underlayment question but I couldn't find an answer that was specific enough for my application.

The house was built in the early '70's in NE Oklahoma. It is a crawlspace home, no basement, with the ground being about 2'-3' or a little more from the subfloor. I have a 1 x 8 subfloor installed diagonally across the floor joists (haven't measured the spacing of the joists because I'm too claustrophobic to crawl under the house!). On top of the 1 x 8's is installed 15/16" particle board underlayment. I know that you aren't supposed to nail into particle board so I know I will have to remove this, not to mention that currently the underlayment brings the floor level up to the top of the door thresholds and that definitely will not work.

My question is do I need to put anything down over the 1 x 8's other than #15 roofing felt? The 1 x 8's have a slight gap between them, I'm assuming for expansion, will this be an issue?

Thanks,

Jeff

Assuming your floor joists are 16" OC, one only needs to install the 15 lb. asphalt saturated felt paper over the subfloor. The gaps between those subfloor planks are normal and of no concern. Let me say this. If you are installing a factory finished floor, the subfloor needs to be flat and even for that floor to look it's best. If your subfloor boards are uneven in places ( some are ), that could telescope through to the finished floor. If that's the case, installing 3/8" plywood over those diagonal boards would be a good idea. Also, if your flooring is thinner than 3/4", some plywood may also be warranted. If the flooring is to be sanded and finished in place, having the subfloor perfect is less of a concern.

Thank you for the reply Gary. That's what I was hoping you were going to say . I am headed over there to pull up the particle board today and see what condition the original subfloor is in. Once I get the particle board pulled up I should be able to check the spacing of the joists.

When you say "if your flooring is thinner than 3/4", some plywood may also be warranted." you are talking about the finished floor correct? I will be using 3/4" flooring but was leaning towards some plywood anyway. 3/8" ply would be thick enough?

As I mentioned in my original post, the house is almost 40 years old, and consequently there are some sagging spots in the floor. I have a foundation/floor leveling company who is going to add more piers and beams to level out the floors. The floors right now are very sturdy, they have no "bounce" to them, we just need to get them level. Once that is completed I'll be able to tell exactly what I need to do with the subfloor before installing the finished floor.

Jeff

Yeah, the finished floor. That's because a 3/4" solid wood floor does impart some strength to the subfloor when installed at right angles to the joist system. Thinner wood flooring typically will not add much strength to a subfloor system, so the subfloors should be stiffer. The 3/8" plywood will be enough IF your subfloors are at least 3/4" thick boards and the joists are no more than 20" OC.

Well, I pulled up a good majority of the particle board underlayment. Most of the subfloor is in sound shape. I do have one patch about 8' x 3' made out of several pieces of 3/4" plywood. I also have a spot where the subfloor 1 x 8's are fairly rotten. It is a spot where the kitchen sink used to be prior to a remodel done in 2000-2001. Unfortunately this area is in a corner so it might be difficult to patch.

I am now considering 1/2" or maybe even 3/4" T&G to lay over the top of the current subfloor. Any suggestions/opinions? Also, do I need roofing felt between both layers or just between the plywood and the subfloor?

Thanks,

Jeff

The felt should go between the hardwood and the new plywood.
Have someone check the vapour barrier on the crawl space, making sure it is intact and complete.

Thanks Dennis. The reason I assumed it went under the plywood was that was where it was installed with the particle board (between it and the 1x8's).

I can tell you there is no vapor barrier over the dirt in the crawlspace. I looked through one of the plywood patches today when my foundation guy had it pulled out.

Jeff

Quite often there will be a layer of sand over it.
Better check that out since that could really cause you grief if it isnt there.

One other thing, check for proper amount of cross-ventilation, local building codes vary, so also check your building dept.