Uneven floorboards

hi best to plane or sand down high spots ,fill low spots with flexi self levelling compound and then ply over using min 6mm ply. all a thick underlay will do is cause the joints in the laminate to split.

As above But I would use a HT7 edge sander with 24 grit papers to remove high spots in the floor boards.

Hello, You are best to sand down the high spots in a diagonal direction to the floorboards ( if using a drum sander) and then ply over the top, if their is still a discrepancy in levels you could then use a suitable self levelling compound to correct this.

First use a straightedge or spirit level isolate high and low spots, this will avoid sanding in areas that are flat. Sand high spots. Fill low spots. On major low spots you could pry up and effected board and place a shim over the joist( in the case of a joist being higher or lower causing a rise or low spot) and refix board. ensure floor is flat checking with a straightedge. Then fit 6mm ply over floor. your then ready to begin your flooring install.