Plywood underlay to subfloor - size screw?
I disagree, 3/4" screws will not work, not enough bite.
Please double check the thickness of your subfloor, and try to read the markings from underneath too. No way should any subfloor be just 1/2" Is it possible another layer was removed because of re decorating in the past?
In any case, if it is only 1/2 thick, adding another 1/2" may still not be enough? Years ago many homes where built with a 2 layer sub system consisting of 1/2 + 5/8". That is the minimum I'd recommend.
Then you say you have 2x6 joists, YIKES! What is the unsupported span of those joists? Measure from face to face of the supports, accurately.
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