Questioning the real weight load of plaster

[Grazza]

There are many blogs out there showing a table of maximum weight certain substrate - for example:

https://www.**********.co.uk/knowledge-base/tiling-background-weight-limits

...and they all say 20kg for gypsum-based plaster - so I'll assume we are talking about products such as multinish in most cases

So I have been looking into this as I wasnt sure if this refers to:

• fresh skim on new plasterboard
• reskim + PVA on previously skimmed plasterboard
• reskim + PVA on previously skimmed block wall
• fresh skim on Thistle Hardwall on block wall
• fresh skim on sand and cement undercoat on block wall

you see where this is going....

The plaster will have a different weight limits in each case!

Here is what I have found out.

Mapei say that you need to use a plaster primer to stop the adhesive sulphates reacting with the plaster and making the tiles come away from the plaster. See here:

Primer G 1kg | Tile Mountain

Prepare your surfaces for tiling with Primer G 1kg. Essential for a professional finish. Simplify your projects – order now for efficient surface preparation!

www.tilemountain.co.uk

So I thought this was why the 20 kg limit.

But then I found something on British Gypsums website top of page 3:

https://www.british-gypsum.com/documents/product-data-sheet-pds/british-gypsum-pds-thistle-multifinish.pdf

...which says:
"You can apply tiles up to 20kg per m2 directly to Thistle
finishing plasters, except where the system includes a
bonding agent. As the total weight of tiles and plaster
applied over a bonding agent is limited to 20kg per m2,
consider tiling directly to the background instead."


So, is the 20 kg limit because of the PVA used by the plasterer?

Does anyone have experience with large format porcelain on multifinish on a block wall?

 

[Trigger]

Hi,

My take on it is that a finishing plaster can take upto 20kg of weight per m2 if lets say the plaster was applied directly to plasterboard.

However if the plaster was applied over a bonding agent such as PVA then they are suggesting that the PVA can hold upto 20kg per m2, so you would have to factor in the weight of the plaster itself aswell as the adhesive/tiles/grout. I guess in this scenario they are suggesting the weakest part of the system is the PVA.

With regards the block wall, if you already have a block wall that has been plastered it will not have multifinish applied directly to the block wall, there will either be a S&C or Hardwall backing coat applied to the blocks prior to the multifinish. I am pretty sure that all gypsum plasters (Bonding/hardwall/multifinish) can only hold 20kg per m2, so if you had a hardwall backing coat you would have to factor in the weight of the multifinish, tile adhesive, tiles, grout. So I would avoid large format tiles.

I have never found figures for S&C weight limits.

BG arrive at these figures by testing the pull weight on the backgrounds.

 

[Trigger]

Just an additional comment. If tiling onto plasterboard that has been plastered you need to be aware of two weight limits. The plasterboard can hold upto 32kg, so you need to factor in the weight of the plaster, adhesive,tiles, grout. Aswell as being aware that the tiles, adhesive, grout do not exceed the 20kg weight limit of the multifinish.

If that makes sense🙃

 

[Grazza]

Thanks for your take on it Trigger.

Yes I read it too that its the PVA that as a weak point but one of my walls is aerated block+hardwall+multifinish but NO PVA.
When I look at this wall its rock solid and I just cannot imagine how this would fail.

But, I am supposed to use a plaster primer. I have used BAL SBR before and currently testing dilution of Mapei Primer G., so this ultra thin film now becomes the weak point I suppose?

I've had London Tile company tell me that I can get away with lighter ceramic on primed plaster, and Tile Giant have kind of laughed it off and said it will be fine with porcelain. Plus a couple of old general builders have said "just do it".

Just dont want to board it with hundreds of fixings in my Block walls as I feel this will destroy the wall when removed in future renovations.

Aggghhhhh

 

[Dav3yboi]

Why would you skim over the plasterboard and then tile it?

Just tile straight onto the board… better yet, get rid of plasterboard from any tiling job especially in bathrooms and use a backer board…

But weight limit in the tiling game is 20kg/m2 on plaster (skim)
32kg/m2 on plasterboard

100kg/m2 on Bal board/hardie backer

Those weights include the tile, adhesive and grout etc…

And someone mentioned PVA… this is a NO NO and any tiler worth his salt will say this.. anyone that says it’s ok doesn’t know what they’re doing..