I'm preparing to install luxury vinyl plank in the living area of the second floor condo I just bought (carpet was installed previously). It's a woodframe condo with a wooden subfloor built in 1985, and there are several places where there are drops and dips in the subfloor. I'm trying to figure out what level of "level" I need to bring the subfloor to and what's the best way to do it. I'm hoping to DIY but understand I may need to hire a professional if it's bad enough.
I'm attaching a photo with a map of the problem areas. The worst is near the entrance to the room where there's a drop of roughly 1" over 4 feet, which from my understanding is quite substantial. The second biggest issue is a local low spot in the middle of the room.
I have had both Henry 549 "feather finish" recommended for filling in the low spots, or self leveling underlayment. Also gotten some recommendations for 1/4 plywood all around to even things out. The low spot in the middle of the room seems like a very straightforward fix with the Henry's 549, but I'm wondering if that 1" drop is over my head.
So far I've had 3 contractors take a look, and every one said that this is the nature of the beast with wood-frame construction and that they wouldn't recommend doing too much since the building will continue to shift over time. They all said everything seemed to be structurally sound (I even took one to the unit below and confirmed that the ceiling looks normal there), so I should only repair to the extent necessary for the LVP install.
I'm wondering if it would it make sense to fill in the local low spot (and other more minor low spots) with Henry's 549 patch, and then add some 1/4" plywood everywhere except for the area marked high, up until say a couple feet out from the drop, then ramp up to the high area using the 549, so that the incline is less extreme? Or if anyone has other suggestions for how to approach this they would be quite welcome! One of the contractors also mentioned using the underlayment that you roll out, but the installation instructions for my LVP mentioned only "APA rated" underlayment which I think is just the plywood.