Evening all,just looking for others opinions on this subject. So today I turned up at a job to tile a kitchen floor only to find it has been covered in 6mm ply after I recommend cement board or backer board be put down,it's screwed down every 6 inches with drywall screws and they have clearly not taken as they all just spin when I put a drill on them,tiles are 60x 60 porcelain and no way I'm taking the chance so walked away,I was just wondering though if it was all screwed down properly would it be ok to put down some matting then tile onto or just rip up the lot and start again, any info on this would be much appreciated. Cheers.