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Overboarding chipboard with ply - best practice?

I would like to know the best practice for securing the tiling substrate/subfloor when overboarding chipboard with ply in an en-suite bathroom.

My builders have laid 22mm T&G chipboard over the joists. Whilst they have glued the joins, I don’t think they have glued the boards to the joists. The builder will now lay 12mm ply onto the chipboard and they intend on securing the ply using a long screw which will go through the chipboard and into the joist.

The tiler will then apply a decoupling membrane like ditra may to tile on to.

My question is, should the chipboard be secured with screws first, before securing the ply, or is one long scre sufficient to secure both layers.

Additionally, should the ply be glued and screwed, or just screwed.

I saw on a forum somewhere that the ply should only be secured to the layer below, but it’s unclear whether that applies when the chipboard subfloor is not secured with screws. I don’t want to have any movement later down the line, so just wondering if someone can tell me the best bractice. Overboard with one screw, screw both, screw then glue and screw etc.

3 Answers from MyBuilder Carpenters & Joiners

Best Answer

The chipboard should not be glued to the joists but should be glued together as they have done, however the chipboard should be screwed to the joists independently of the plywood, approx every 200mm. It is absolutely fine to use plywood but ensure that they use treated hardwood ply and not softwood as it does not warp and is made to be used outdoors as well as internally. this should be glued to the chipboard as well as screwed to the chipboard again at 200mm between fixings. If the Ditra mat is installed correctly then this will create a completely watertight seal ensuring that no moisture will penetrate through to the subfloor.

Regards Ashley
A.Carter Carpentry ltd


Answered 4th Feb 2019

I have always used a 3 mm hard board then ply and screwed down , no need to glue but as they are using toung and groove the glue makes sense.
Never had movement problems personally with that aproach.
Cheers , Dan.


Answered 1st Feb 2019

Hi, I would not recommend ply wood. It is a matter of time before deterioration. Use a cement board such as Hardiebacker. Much easier to work with than ply and will last.
Hope that helps.


Answered 1st Feb 2019

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