Priming back of ply for over boarding

[IHB]

Hi

Can someone explain the reason for priming the back of ply used as over boarding and whether it's necessary when used to overboard T&G floorboards?

Reason I ask is that I've already fixed some ply without priming the backs. Not tiled yet so I can lift, prime and refix but I was wondering if it was necessary.

Tiles will be large format Limestone or Marble. T&G floor is old, dry, well supported 20mm pine boards. No underfloor heating.

TIA
Ian (Keen DIY - and it's my own floor)

 

[DHTiling]

Hi Ian.. it is basicly to prevent moisture damage through the back and edges of the ply but it really much better these days to use a cement board for over laying IMO..

 

[IHB]

Thanks Dave

Do you think I'm likely to get enough moisture through the suspended T&G & ply to trouble the tiles?

I'll probably lift and prime to be on the safe side but it seems a bit OTT to me as a layman - I'd have thought if there was enough moisture to cause the tiles a problem then I'd have other (bigger) problems as well.

Ian

 

[DHTiling]

IMO Ian.. if water was to damage then it would come in through the face side as well..

If you have some SBR then you could prime the face with that and it will stop moisture that way..

IMO now Ian ply is out dated and also a lot of it is imported from china etc and is full of resins that stop adhesive from adhering well.. and can also leach into the grout joints causing them to stain..

 

[IHB]

If I used a non ply over board then would it add the same strength to the floor as ply?

I can see it helping with lateral expansion/contraction movement but will would it prevent vertical deflection as well as ply?

 

[DHTiling]

Ply only helps if you use a certain thickness.. min 15mm is bs standard.

I use hardi backer and this is put down with an adhesive bed and screwed and i think it gives a very solid floor but as with any over boarding the sub floor must be prepped right anyway ..

 

[IHB]

Thanks for the advice Dave,

Would you be happy to use a backer board (6mm?) on to a well supported, suspended timber ground floor?

Do you think it's comparable in strength to 15mm ply?

Sorry for all the questions, I've not used backer board before.

Ian

 

[DHTiling]

Thanks for the advice Dave,

Would you be happy to use a backer board (6mm?) on to a well supported, suspended timber ground floor?

Do you think it's comparable in strength to 15mm ply?

Sorry for all the questions, I've not used backer board before.

Ian

I always do now.. as long as it is solid then i have no probs wot so ever in doing it..

Some of the other lads on here use No more ply but not one i have tried yet.

As for strength.. i know it does not state that hardi is stronger for deflection than ply but in my usage of it i would say it is very strong once laid on a bed of adhesive to a good floor..

 

[IHB]

Thanks for the help.

Think you might have me switching to a backer board.

Ian

 

[DHTiling]

No probs.. I think once you try it Ian.. you will not use ply again.. Hardi does not swell like ply can with heat and is totally moisture stable and no leaching of resins to blow the tiles off.. and best of all.. low installation heights for the customers.