I am looking to lay 15mm thick oak over exising floorboards which are in reasonable nick. I will be laying the floor in the same direction as the existing floorboards as this is the only way cosmetically as the floor will look stupid the other way. I am undecided whether to bother with underlay and just secret nail direct onto floorboards or to float floor and use fibreboard underlay and glue - can someone please advise which would be best and reasons - cheers>>!!