I've been doing tons of research in preparation for installing our hardwood floors and have come to a couple of questions that I am finding conflicting answers for.

1) Crawlspace prep: I know that NOFMA says a layer of polyethylene(?) is desired. However, from what I see their research is all based on east coast conditions. We live in far east bay California. Summers are dry and windy and fairly warm. Winters are slightly rainy and cool. If we have to install the plastic we will but that will be a huge effort as the crawlspace is very tight and difficult to access. Other than a known drainage issue (which will be remedied) the crawlspace stays quite dry. Is the plastic really necessary for us?

2) Our subfloor is 1 1/8" C D Group 1, Interior, Exterior Glue plywood. We can't tell if it's T&G or not (is there a way to tell?). The joists are possibly 48" OC in some areas, 24" OC in others. I didn't see this type of subfloor listed in the sticky. Do we need an additional underlayment?

We are installing 2 1/4" x 3/4" T&G plain sawn site-finished white oak. We're starting with the kitchen but plan to eventually do almost the whole house (other than bathrooms). House is 1-story L-shaped, 1556 sqft.

Thanks!

1) If you do a moisture check on your subfloor with a wood moisture meter, and it shows 12% or less, you'll probably be OK without the polyethylene vapor retarder. Keep in mind, even slightly damp earth can emit as much moisture vapor as standing water. So unless the earth under your entire house stays completely dry year round, you will need a vapor retarder. The science of moisture vapor migration is complex but basically, if the air in the crawlspace is humid, that humidity is probably coming from the ground. I live if the SF east bay as well and know the area well. In some places, the earth has a high clay content, which retains moisture. Also, if there are any hills near you, the runoff water can travel underground and under your foundation, contributing to a damp and humid crawlspace. If you have any doubts, install the vapor barrier. It is inexpensive insurance against floor failure.

2) This type of subfloor was briefly mentioned in the sticky. I also know these types of systems. They consist of 1 & 1/8" tongue and groove plywood panels installed over 4 x 6 beams 4' OC. There are no traditional joists, just the beams/girders. Sometimes, these type of subfloors exhibit considerable flex in the middle between the 4' span. You should check for that. If it deflects to much, you can do two things. The easiest is to add 1/2" CDX plywood. The other is to add 2 x 6 diagonal blocking between the beams every 2'. In a shallow crawl space, this is difficult. These types of subfloors can also exhibit depressions (low spots) between the beams as well and this is most common. Those low spots would need to be filled and made flat with the higher places. If adding underlayment, it's best to do that prior to installing underlayment. I've done this many times on these types of subfloor systems, which, btw, are no longer used. And haven't been for about 20 years.

Let me guess, you live in Antioch or Pittsburg, yes?

Pittsburg - up on the hill with a great view of the water (and the wind that goes with that view).

We're not entirely positive of the joists. Hubby says they may be the 4 x 6's mentioned in the sticky but isn't positive.

How much is "too much" deflection for wood flooring?

What do we use to fill the low spaces?

Thanks!

Here's the reference to your subfloor in the sticky I wrote:



It's possible the builder, AD Seeno, added 2 x 6 blocking in some areas to support interior walls and cabinets. I know he did in my first home in Pittsburg and I had that same type of subfloor. I can almost guarantee that you have the 1 & 1/8" plywood subfloor with 4 x 6 beams, which are nominally 3.5" x 5.5" . They will be supported every 4 to 6 ft. with 4 x 4 wood posts that sit on precast concrete blocks that are sitting on concrete pier pads. Seeno built these types of homes all over Antioch, Clayton, Pittsburg, Martinez, parts of Concord. Not in WC or PH though. I've lived here all my life and have been a flooring contractor nearly 30 yrs.
I'm this guy: http://www.hardwoodflooringtalk.com/pan ... clontz.htm