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I plan to add a subfloor in the unfinished attic, and lay down hardwood on top of it. The attic has large temperature swings, from about 50 to almost 100F (something will have to be done about that later), which makes me concerned about how to properly install the subfloor and then the hardwood planking on top to accommodate thermal expansion.

I will install at the highest temperatures in the range, so contraction is probably what I should properly worry about.

What is done to allow for expansion/contraction? Is there a preferred alignment of subfloor panels to the underlying joists? and is there a preferred alignment for the hardwood slats? I would like to make the hardwood direction match the rest of the house...

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    wood expands 5 times more perpendicular to the grain than along the grain. That said, humidity is a far more important variable for wood expansion and contraction. – dandavis Jun 29 '20 at 20:38
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Upon further research, it was made clear that humidity/temperature need to be stabilized first, temperature excursion would damage a hardwood floor.

After the temperature excursion is addressed through proper heating and cooling, the primary concern for expansion is against the walls, where a 1/2" perimeter is to be left for expansion. If the Drywall is floating above the floor, the hardwood and subfloor can expand under it as needed.

The 8x4 boards of subfloor should be laid down orthogonally to the direction of the rafters.

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