A 4’x15’ roll of 12mm rubber underlay is deceptively heavy.
But a quick trip to the Sound Isolation Company and I was back in business.
I had some leftover 8mm rubber floor from the gym, so i pieced together a complete rubber underlay with old stuff and new stuff.
And I have to admit, I outsmarted myself. The old floor is 8mm. The new stuff comes in 5mm, 10mm, and 12mm. I decided I would use 3/4” OSB for the first layer over the 8mm and then use 5/8mm over a new 10mm underlay, and they would come out close enough. Then put 3/4 over the whole thing for a second layer. But I got them reversed in my mind at some point, and I put down 10mm in the front of the room with 3/4” OSB over the top of it, leaving 5/8” OSB to go over the 8mm rubber - double whammy.
Since I needed an extra roll anyway, I bought 12mm. So 12mm plus 5/8” OSB is pretty darn close to the 10mm plus 3/4” OSB, so that worked out well. But then it steps down from 12mm to 8mm rubber with a bit of a discontinuity.
I’m going to live with it. With the way I am laying the OSB, it occurs at the midpoint of an 8 foot sheet. I feel it bridges the gap just fine. It’s all under the riser, anyway. And if the riser is off by 4 or 5 mm at the back edge I can make up the difference in how I lay the joists.
Besides, I don’t think the framers were that close in some spots.