Plaster weight limit - what to do

[rivergo1d]

I have 30x60 tiles that weight 20.5Kg per square metre, so with adhesive and grout this will breach the recommended limit of 20 for plastered surfaces.

My wall is sand/cement base with a mixture of plaster finishing coat and easifill (ahem). I have scraped off a lot of the most unsound plaster finish layer, but what remains, covering large areas of the wall, is pretty well bonded and tricky to get off.

My question is - for such a breach of the limit, perhaps 10 - 20%, should I persevere and get all the skim coat off? Or could I do more harm than good in terms of damaging the sand/cement layer, in areas where the finish coat seems very well bonded.

Also, if I end up with any small indented areas of 2 - 3mm - whether through over-vigorous finish coat removal, or because we decided to leave alone with just the loose skim taken off... Should I patch the sand/cement indent with something to make it level, or as it is such a thin layer and the existing sand/cement is such a good substrate, am I best leaving it alone for the tiler to let thicker adhesive areas take up the slack?

Picture attached.

In terms of other solutions - I don't have spare width to put in battens and marmox, although at a push scraping off patches of sand/cement and dotting and dabbing the marmox might leave me enough room. Reluctant to go down that route though, makes it all a bit tight.

I have seen mention of "skim and set plaster" supporting 28kg, but only on forums - this makes me wonder whether previous guidelines used to be less stringent, meaning I could get away with it ....

Untitled

• rivergo1d
• 23 May 2011
• 1

 

[Richard C]

I have seen mention of "skim and set plaster" supporting 28kg, but only on forums - this makes me wonder whether previous guidelines used to be less stringent, meaning I could get away with it ....

Where did you see the 28kg/sqm? Not heard of that one before. Both the plaster manufacturers & the tile Association give the same advice for finish & base plasters which is presumably the result of tests. There is undoubtedly a safety margin in there which I guess could be around 10% but I’ve no idea, depends how lucky you feel; it’ll cost you an awful lot more if it fails.

Even stripping all the finish off will only increase the weight limit if the existing render is in good condition but if it's sound you should be OK. Regards leveling & filling small holes, use cement based tile adhesive to fill/level & you can tile the next day; don’t use base plaster, it’s not suitable & conventional render will take much longer for it to fully cure before you can tile. If you dot & dab the backer boards, you must also use mechanical fixings into the wall behind, you can’t just rely on the strength of the adhesive to hold it all up there. Powder cement based adhesive only with tiles that size, not tub readymix. Use only quality trade adhesive & grout of the correct type for your tiles; cheap DIY products are mostly crap.

Choose your tiler very carefully, some real cowboys around.

 

[rivergo1d]

I have just read that 6mm hardibacker board could carry the weight of these tiles (my instinct would have said I needed 10-12mm cement board).

This is thin enough that perhaps I could mount it on battens on the wall - anyone know what the minimum depth of batten would be?

 

[Richard C]

Who says 6mm is OK for walls? Your instinct is correct; you should only use 12mm backer board (Hardibacker 500) for walls, as per manufacturer’s instructions. As I said before, don’t get hung up about the need for battens, as long as you use mechanical fixings (screws) through the adhesive dabs to secure, it’ll be fine; the adhesive is only used to level the boards & hold them in place initially.