Advice for filling over Ply & Aerated Concrete block wall

[Sard]

Hi,

I recently fixed some structural ply to an aerated concrete block wall to allow the fixing of a heavy radiator. Unfortunately most of plasterboard had to be removed in the process (see attached) as it had lost its strength and rigidity.

With the ply, I don't have enough recess to lay over a sheet of plasterboard (even with 9mm sheets), so I'm thinking of going with a 2 part coat. I'm thinking of using easi-fill as my top coat which I have loads of at the moment and will paint as my final finish, but unsure which product I should use in my initial coat. I've read that Thistle Hardwall is best for aerated concrete blocks but I also have primed ply wood to fill which the Hardwall may struggle to adhere to give the low suction of ply.

Any advice would be greatly appreciated

 

[dontbelieveawordofit]

what’s the difference in depth between the ply and original board that’s left?

 

[Sard]

what’s the difference in depth between the ply and original board that’s left?

It wasnt level to begin but it goes from 2-3mm upto about 10-12mm. The distance between the block wall from the remaining plasterboard ranges between 20-30mm.

 

[dontbelieveawordofit]

it’s 18mm ply?

 

[Sard]

it’s 18mm ply?

Ah forgot to mention that. It is 18mm ply indeed.

 

[dontbelieveawordofit]

you didnt need to ply the whole area . a short section would have done top and bottom, with the bottom section resting on the floor and fixed to the wall to take the weight of the rad.
anyway , the problem i think is the ply is too thick. you could swap it for 12mm with a 6mm hardiebacker screwed over.
then fill the remaining thermal block with hardwall , pva and skim with easifill or multifinish.

 

[Sard]

you didnt need to ply the whole area . a short section would have done top and bottom, with the bottom section resting on the floor and fixed to the wall to take the weight of the rad.
anyway , the problem i think is the ply is too thick. you could swap it for 12mm with a 6mm hardiebacker screwed over.
then fill the remaining thermal block with hardwall , pva and skim with easifill or multifinish.

Problem for me was the aerated block wall was absolute ****e for securing anything to. I tried all types of plugs and anchors and they all failed. So I went for a long piece of ply that I could secure in multiple places using expanding shield anchors and to distribute the load more broadly. Taking it off isn't an option for me.

Is there an alternative to the above?

 

[dontbelieveawordofit]

yes, aerated block is a joke. 4mm backer boards are available. (try topps tiles.)
you could cover the whole area by screwing the backer on to the ply and bonding the rest to the block with pb adhesive. then skim it in ,
or just backer over the ply , hardwall the block and skim.

 

[Sard]

yes, aerated block is a joke. 4mm backer boards are available. (try topps tiles.)
you could cover the whole area by screwing the backer on to the ply and bonding the rest to the block with pb adhesive. then skim it in ,
or just backer over the ply , hardwall the block and skim.

Thank you, I really appreciate the advice. I'll look into backer boards. Just to cover all bases, in a worse case scenario could I hardwall the concrete upto the ply, mesh tape the joints and then skim the whole lot from that point onwards?

Also just to add, this wall has historically been a cold spot, so I wouldn't mind packing it out and not leaving gaps between wall and plasterboard

 

[dontbelieveawordofit]

a degree of bodging will take place.
I have skimmed over ply several times but not behind a radiator where there are such variations in heat.
if you end up plastering over the ply i’d neat pva, then thin coat and key it with s2 tile adhesive , pva again then skim.
and yes scrim and pva all substrates and joins