Hi all, Looking for recommendations on screw size for laying sp101 over old floorboards. [email protected](5.5) and using the equation of 18mm floorboards +6mm sp101 gives me 24mm rounded down to 20mm so I don't go bursting any pipes or making the electrics spark makes sense. My overall concern is there is a mix of old and new(ish) floorboards and the house being over a hundred years old i'm worried about pushing my luck with 20mm 3.5 spax screws, could I drop down to 16mm 3.5 spax or are these going to be a little short. Any advice will be much appreciated. By the way the area is 13 sq metres so 5 sheets @8x4 feather joints then tackifier and Karndean looselay.