What type of screws to fix 12mm plyboard over floor boards

[Mutley56]

Hi all,

Ongoing saga with damn bathroom floor tiling.

From the useful answers to my previous posts realting to the issue of cracked grout on my porcelain tiled bathroom flooring, I now believe the plyboard is moving / flexing. The contractor has agreed to relay the flooring, and I want to tell him what screws to use to fix the 12mm plyboard to the floor boards. Please tell me what size and type of screws should be use and what material they should be made out of.
I will specify that the screws will be used every 20cm through the 12mm WBP plyboard sheets. The plyboard sheets will join over the centre of the joists, BAL flexible powedered adhesive and flexible powdered grouting will be used for the porcelain tiles.

Many thanks for taking the time to view and hopefully reply to my post.

Mutley56

 

[lancenotalot]

http://www.screwfix.com/prods/10249...less-Steel-Countersunk-3-5-x-25mm-Pack-of-200

 

[Richard C]

I would never rely on the original floorboard fixings by just screwing over boarding onto the top of them. I always screw THROUGH the floor boards INTO the joists below which will also help stiffen the floor up; over boards just screwed onto the existing floor may still flex, curl & give you problems. Your contractor will obviously need to establish the location of any pipe work if the joists have been notched; & cables, although they should never be run in notched joists. You can use stainless steels screws but they are expensive & not really necessary; I usually use deck screws.

Having another look at your previous thread, you need to establish what was allowing the previous floor installation to flex! What’s your contractor going to do differently this time around? I know some of the tiles weren’t bedded correctly but I believe you currently have 12.5mm over boarding, was that incorrectly laid & how was it was fixed? You need to be careful you don’t just replicate what you currently have; why would it be any more successful this time around! I can’t tell from here obviously but if the original problems weren’t caused by movement it may be that there is just too much flex in the floor joists in which case you need another solution.

 

[Mutley56]

lancenotalot & Richard C, thanks for replying tp my post.

I understand that the original plyboard overboarding was screwed down into the joists (and not the floor boards as I said in my last post), although I won't know until the contractor lifts the floor up! Then I can see where the screws were placed, where the joints between the plyboard sheets were placed, i.e. over the centre of the joists or not and whether the exisitng floor joists are sound enough to take the plyboarding and tiling.

As you mention Richard C, I don't just want a repetition of the original failed installation by the plumber who was used by the contractor who renovated my house. Ideally I would prefer to bring in a different tradesman to do the job (I've met one with excellent recommendations from neighbours since I moved into this house a few months ago). However, as I still owe a bit of money to the original contractor I feel that he will expect to be given the chance to rectify the problem so that he can claim the final balance owing - (I won't be paying up the final amount owing until the flooring issue is resolved).

Despite what the contractor said, I am concerned that he may decide to only lift a few tiles in the area where the grout has cracked and then put more screws in and lay a few new tiles. In that instance he will not have investigated the underlying reason for the failed flooring. I would then feel that I would then be in a position to tell him to stop work and leave the job. I would then tell him that I will use the money owing to pay for my prefered tradesman to do a complete re-flooring job. Is that a reasonable course of action and if so can you give me any tips on how to tell the contractor that this is what I am going to do?

Thanks again, and I will update you on the outcome of this saga.

Cheers,

Mutley56