I’ve just finished laying out roughly 120 sf of 1-1/2” x 3/4” T&G red oak strip flooring and before I begin nailing it down, I wanted to pause for a sanity check based on the following knowns:

House location: Austin, TX

Relative humidity in house (has central a/c): 60%, which I understand to be a bit high but I think is close to average for this 1940s (w/1980s addition) house based on past measurements

Flooring package: pier and beam with beam spans of 4’, 2x6 joists 24 o.c., 1x6 pine subfloor on the diagonal, new 19/32” Exposure 1 plywood underlayment, Aquabar “B” vapor retarder

Crawlspace under house is well-vented (per flood zone requirements), but there is no insulation, vapor barrier, or plastic sheeting over the ground

Length of time of the flooring has been stored in this house: approximately 1 year

Current MC of the flooring: 6% (consistent across all bundles from a lot of 250 sf)

Length of time the plywood underlayment has been in place: approximately 6 months

Current MC of of the plywood underlayment: 9% - 10%

Planned perimeter gap: 3/4”

Moisture meter: Lignomat mini-LIGNO E/D with fresh battery

In the original rooms of the house, there is also 1-1/2” x 3/4” red oak installed directly over the 1x6 subfloor—so no underlayment and no vapor retarder that I’m aware of. The MC readings from the original oak flooring varies from 9% to 11%, with some adjacent boards varying by 1%. Aside from all the physical wear, the floor is in relatively good condition: no buckling or significant gaps throughout.

One possibly concerning point is that in the room where the flooring has been stored for the past year, there is old (at least over 12 years old) plywood underlayment that shows MC readings in the range of 9% - 14% within a space of roughly 150 sf. Interestingly, the hardwood flooring stored (stacked) in this room in the same exact spot for the past year has MC readings of 6% across all sample boards, even those directly in contact with the plywood underlayment. For whatever reason, the underlayment in this room seems to be an outlier compared to other readings through the house. The room in which it’s installed happens to be furthest from the ground (approximately 3 1/2’) because of the property’s grade, though I’m not sure this has any bearing. The original oak flooring is in rooms closer to grade.

My initial feeling is that I should be OK to proceed with the install, but thought I'd post here first to see if there's cause to reconsider.

Many thanks in advance for your valuable thoughts!

It is quite possible that your wood bundles have been affected by the heating season over the winter to lower the moisture content. It may be that the inside of the bundles are still around 9%. I would lay the floor with a shim between the rows every three feet using a metal washer to provide for planned expansion of the flooring in the future. The gap will close as you are sanding the floor and being walked on. An eighth inch thick washer ought to be about right over the 3 foot space between them from my experience.
A 3/4 inch expansion gap is too large in my experience, too. If the floor expands that much the nails would have pulled loose enough to not cause any pressure on the wall to cause a problem before the floor buckles. I think half an inch will be good.
Measure the distance between 20 rows of the existing floor to see if the floor has expanded over time, see how much to shim to remain flat and relieve expansion pressure..

Hi Pete, thanks for sharing your thoughts--but, oh man, you now have me going down an entirely new rabbit hole considering the spacer rows! :D

One thing I didn't mention in my original post: for the layout, I'm starting outward from roughly the center of the room. This would give me, based on your recommendation, two or three spacer rows as the direction I'm laying the boards is perpendicular to the room's 12' length: two if I forego one in the center (i.e., the starting point) or three if I include it.

I had originally planned for the 3/4" perimeter based on the NWFA's (2019 Installation Guidelines) recommendation for a gap that matches the thickness of the installed flooring. And now, rereading the publication, I do see their mention of additional "internal spacing". I think I originally glossed over this because I understood it apply to floor spans over 20 feet (Part IV, D.2)--but, it seems, it could be applied more broadly?

Out of curiosity, I checked the MC on boards in the center of the stack and they all read 6% on the dot--just like all of the others.

Given that I'm seeing consistent MC readings, and that I'm installing from the center outwards, would you still consider the spacers?

Did you measure across 20 boards to see if the measurement equals to30"?
This is what the boards would be without affects of the humidity in your house.
If the measurement is more than 30", then you may want to use spacers.

Ahh, I had misunderstood the point you were making--makes sense now. I took some measurements in a few spots throughout the existing floor and I'm seeing mostly 30-1/8" with a few measurements closer to 30-1/16", so definitely some historical expansion.

As for the ambient factors--inside house humidity levels coupled with moisture making its way from below the floor (crawlspace)--could one expect some measure of mitigation since the new flooring will be installed with a vapor retarder whereas the old was not? Perhaps the answer is in the fact that it's a "retarder" and that, over time, moisture will eventually pass through.

So, with the layout starting from center, I've got roughly 6' of boards which would equate to about 5/16" of total expansion in each direction into a perimeter gap of 1/2" or 3/4", which sounds reasonable on the face it. Of course, I may be (probably am?) thinking about this the wrong way and there other compounding factors I'm ignorant of.

I should say that I'm in no way trying to reason myself out of your recommendations in order to fit my notion of what I'd "like" to do. This is all very informative (and much appreciated) as I'll be tackling more flooring projects in the near future. My only real concern is ending up with a situation where the gaps never end up closing and trying to provide as much info to get me going in the right direction.

Since there is negligible difference between the old floor and what would be expected when you lay the new floor just lay it without any spacers and you should be OK.

Thanks for taking the time to chat through it with me, Pete. Cheers!