hi all, looking for some advice. have an upstairs bathroom floor to tile approx. 4m x 2.5m. Existing floor is 20mm t&g chipboard fixed at 400mm centres with no noggins along theentire lenghts of joists. Not in any way ideal to start with! After screwing, there is still some deflection but not as much as i expected. the customer wants u.floor heating which i will lay on 10mm marmox, slc over top, then tiles as normal.
however, this has left me with a quandry, do i:

1 - overboard the chipboard with 12mm or 18mm ply, then marmox etc.
2 - fix 20mm marmox straight to the chipboard, then ufh etc.
3 - take up all the chipboard, noggin at 280mm centres, relay 18mm ply, then marmox etc.

i know that ideally option 3 would be the way to go, however, there is so little deflection in the chipboard now that it is screwed, i wonder if overboarding with 12mm ply would stiffen the floor up to a satisfactory standard, and save me a lot of time and mess.

any advice would be greatly appreciated, as i have just about pulled out what little hair i have left trying to decide! cheers, jim

If it were my floor, I would go with option 3. Putting the marmox on the floor will not stiffen it in any way....

To do the job properly you need to get rid of that deflection, as Stewart says, option 3. It may be more costly and time consuming but it'll cost less than having to do it twice and you've got a lot of moneys worth of materials going in to that floor. Do it right, do it once

I have to agree with the gents above Im afraid.

I think you'll find that the majority on here will advise on option 3. We've all come across similar dilemmas and you'll sleep better at night knowing that you've done it properly, and with your reputation in tact.

It just needs to be minimal deflection... if you can say it is that minimal ,then over boarding with min 10mm Marmox will suffice.. but only you can decide if it is minimal..

thanks guys for all your advice - i think it will have to be option 3 - like you say, peace of mind and no problems later on. cheers, jim