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Unread 01-17-2019, 10:45 AM   #11
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Join Date: Dec 2015
Location: Berkeley, CA
Posts: 1,096
Use kiln dried (KD) lumber, as green lumber will take at least several months to acclimatize.

Rotating is difficult for two reasons:

(1) The diagonal of the cross section is greater than the height of the cross section. This could be addressed by reducing the cross section just over the bearing points (planing would be better than notching).

(2) A bigger issue is that neither the new joists nor the existing joists will be straight, they will be crowned. Joists are typically installed crown up. So if you try to rotate up a piece of lumber whose crown is greater than the existing joist, it will hit in the middle of the span. The existing joists have likely deflected downward somewhat, so their current average crown is probably less than your new lumber.

For working around (2), I'm not sure if installing the new joists crown down would be acceptable or not. In any event, the point of the sistering is to stiffen the joists, not to strengthen them, so it is unnecessary to achieve bearing at the end. It is also unnecessary to attach the existing subfloor to the sisters.

Cheers, Wayne
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