Mongoct, I tried the jumping to check the shimmer! Very low shimmer :) Getting ready to install the floor warming cables so need to get the floor prep finished. Here is the 3/8" ply screwed down 4"oc as Bill V recommended. Now I have just realized something :( ... I have the underlayment installed and I still don't have the last joist on the far left in place yet. When we screwed the new joists to the floor we didn't have the underlayment in place. Now we need to get that last joist in there and the underlayment is in place. I added a link to the old floor discussion below. That links to the "Remodeling" forum. I have posted more detailed information on there. Its about my progress getting the floor to this point and was posted today. Here is the new joist positions, but the far left is missing the new addition: 1) So is it ok to screw through the underlayment into the joist on that last one? 2) Or should we put bridging in between the joist and the plate and toenail these short pieces into the joist and plate? I thought the principle of the layers was to help isolate them from one another. If I screw that last joist to the floor it will go through all of the layers and I will lose the layering effect, right? The reason for the underlayment going in was I had the help and I forgot about that last joist. The plumber isn't done, but he said he didn't care if the joist was there or not. Here is a link that might be useful: Adding joist to existing floor...

The builder asserts that there are 5 extra days labor for attaching the joists to the steel Ibeam. LOL I 've gotta remember that one! Posters above have noted the three common methods of interfacing joists to I-Beams. Toenailing, hanging or simply resting joists on a 14' I-beam doesn't take five days! The most extra man hours it has taken the various carpentry crews I've used to install steel I-Beams is, to be generous, four hours per house. And that's strictly for setting the beams in place as per plans and Code. That includes the time to brace and support the beams, modify the pockets on the foundation walls and shim as necessary and match holes on the end of the beams to the studs in the piers. There may also be a requirement to weld the beams to the plates. Check the engineer's specs against what the builder actually does. Or make a point to ask the building inspector to verify. And a crane for a 14 ft. beam? Must be a two-man framing crew.

Wood subfloors require expansion joints. I have subfloor panels with instructions printed on the boards to leave a 1/8 inch expansion joint. Yet, the idiot contractor butted all joints together. Now I have squeaks and high spots where the floor pops when someone walks over it. The reason for an expansion joint is obvious. Wood expands and contracts relative to the humidity surrounding it. Also, wood floor manufacturers will tell you the same thing for subfloor preparation.

Still looking for answer to question as to how to fasten new installed joist to the 2 layers of existing 3/4 fir. I will be adding a plywood layer of 3/8". I was thinking that I could treat the existing 2 layers as one and just nail or screw through both layers, from the top side, to get the floor anchored the length of the new joist. What about that idea? Here is #1 joist being installed by DH, #2 is waiting for tomorrow: New joist on the far right, it is the same size as the rest, it is just bolder:) : Anyone out there to help me figure out if it is ok to just nail down through both old floors from the top side down into the new joist? Or should I have it screwed down. I will be adding a 3/8" ply underlayment that I will not be fastening to the joist and will not be having the edges join over the joist. Screw down 6"o.c. throughout. Ditra will be my decoupling membrane before the tile goes down.