15 lb felt [tar paper ]. or aqua bar B if you can get it .

Thanks , we considered that but had two concerns . First , given that it's a cottage , there will be times when the whole system , heat, water, etc will be shut down , mostly in the winter and we were afraid that the expansion and contraction associated with that could affect a glued down floor also , again because as a cottage , people will be in and out a lot , there could be a fair degree of wear that would mean re-finishing which , as I understand , is a problem with engineered flooring . I was hoping tha there was a new application process for hardwood that I didn't know about such as a sheet underlay with a plywood overlay or something like that . If not , we may have to consider tile throughout .

Yes, I'm not worried about the overall drop, in either direction, just trying to get it as flat as possible (not level). The drop isn't uniform, with dips. For example, along the long wall, the drop toward the middle of the wall is nearly 5/8" lower than the straight line drop. Needs to be very flat along the periphery, so that base molding doesn't have gaps underneath. Other dips are also too much for the floating floor to lie flat, so also need to be fixed. So, I am planning to shim with underlayment (5mm=0.2") in the areas that are dips of that thickness or more. Some areas get a second layer. After shimming with underlayment, can I use floor patch (or some other cement compound) to taper the edges from underlayment to no underlayment, and to flatten other areas? If so, specific type recommendations appropriate for 1/4" thick down to zero thickness would be greatly appreciated. Application tips also. Thanks for the help, Gary

After speaking to the pros they didn't feel double plying and ditra (a water proofing un coupling membrane) would prevent cracking. I had a contractor out to talk about shoring up the flooring to remove deflection. He didn't feel it was really needed unless we laid marble. You need to find new installers. 1-Double ply doesn't do squat for the deflection we're concerned about. There are 2 types of deflection- generally when we speak of deflection, we are speaking of the joist deflection. This is stiffened by adding joists, sistering joists or reducing the joist span. Adding plywood to the surface, or blocking between the joists does nothing to help. There is also between joist deflection. If your joists are 16" on center, 5/8" BC plywood would be the bare minimum when laid with the face grain perpendicular to the joists. 3/4" is better. 2- Natural stone requires two layers of plywood installed in a specific manner we can cover later. This is to decrease the between joist deflection. You still need to meet the L720 joist deflection for natural stone. Sounds like the guys you talked to are confusing the two, or at least don't explain things well enough.