4" joists?

**bathrooms** · 27-02-2009

Our fitter has covered a bathroom floor that has existing floorboards, with 9mm ply and tiled using porcelain tiles 300mm x 300mm with flexible adhesive. A bulge has appeared in the floor from the door to the outside wall but the rest of the floor is completly flat. He is worried that it is because the house is approx 150 years old and the joists are 4" deep that the floor has too much deflection. Would thicker ply be required? More additive to make more flexible? Reinforce between joist? Sack fitter?
What would be the best option so as to remidials.

Many Thanks.

wetdec · 27-02-2009

Hi

Shouldnt be a need for remedials, ply should of been fixed at 150 - 200 centres which should of prevented any bulging.

When you say door to the outside wall what do you mean....we cant see it

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**theyomper** · 27-02-2009

Do you know if the joists were sound at the ends going into the outside wall? Sometimes they will rot out and will not be sitting on anything. Putting more additive wont help the problem needs to be sorted first in my opinion.
When you say bulge do you mean upwards, if so what type of ply was used?

**bathrooms** · 27-02-2009

Room is about 12ft x 10ft with door in centre of 12ft wall and outside wall the other 12ft wall. The floor is flat from left wall till door and from other side of door to right hand wall. Almost directly centre of the door the is the apex of the bulge the tile over this is in one piece, but the tiles either side have cracked down the middle, this is why we need to remove and repair. The fitter says he has screws at 300mm centres and that there is no joint in the ply at this point.

Thanks.

**Dave** · 27-02-2009

It sounds like you have tenting of the tile installation....

How tight were the tiles cut to the perimeter of the room..?

BS standards is 300mm spacing for fixings , so the fixer hasn't done owt wrong there,but 4" joists are not exactly ideal...Deflection wise.

But i would check the perimeter first.

wetdec · 27-02-2009

Really didnt realise there was a BS for over boarding with 9mm ply, on a thicker board I can see it as its stronger. I would go for closer for good reason every time under 12mm....................

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**Dave** · 27-02-2009

And if it is under 15 mm then standard flexi isn't no good either is it tony..?? if you want to stick to full BS guidelines.....but if he had used a S2 adhesive then 9mm would be fine wouldn't it..?..if everything else was done right...

wetdec · 27-02-2009

150 - 200 centres / 2 part fexi would be my answer (or C2/S2 to you), you the one with the BS book..............

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was looking at the practicality 9mm is just 3mm over the 6mm that you love so much

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**Sir Ramic** · 27-02-2009

Has the Ply bulged or just the tiles?....sounds like bad ply to me

**pjc** · 27-02-2009

Originally Posted by Dave

And if it is under 15 mm then standard flexi isn't no good either is it tony..?? if you want to stick to full BS guidelines.....but if he had used a S2 adhesive then 9mm would be fine wouldn't it..?..if everything else was done right...

Flexible tiles then boys ? lol

**mikethetile** · 27-02-2009

my immediate thought was ,as dave said ,lack of expansion gap but then i had another thought

what ply is it............is it waterproof

its possible that as the bulge is at the outside wall and the house is 150 yrs old that there is water or damp ingress at this point blowing the ply

untill you lift the tiles at the point its hard to say what the problem is