Well, the cabinets are arriving tomorrow and this weekend I have to deal with leveling/flattening our floor.

The kitchen has a tongue and groove subfloor. I installed a support beam underneath to halve the span and stiffen the floor and screwed every plank to every joist to further tighten the subfloor. I am installing a "floating" laminate "hardwood" floor. Using a straight board I seen up to 1/2" gaps over 8'. So I am at the point of having to "flatten" the floor and want some advice.

Manufacturer recommendation is to NOT install under the cabinets, which include perimeter cabinets and a center island. I was planning to put down 3/8" plywood under the cabinets so as to not lose any height and have a smooth transition from the floor to the plywood to move appliances in and out as needed. I was going to do the same with the island, put down a layer of plywood the same thickness the flooring material.

So, my first question is whether I should flatten first and then install plywood, or whether I should install the plywood and only flatten the areas where the floor will be installed?

My second question is how to flatten. I am a little hesitant to use self leveling material because I've heard that it can be a little tricky for a DIYer and there are gaps between the tongue and groove, the material will seep through to the basement. I've been told I can use roofing felt, but 1/2" of roofing felt seems like a lot?? Are there any recommendations?

Thanks!!!!!