Hi All I very much need some advice. I am building a house right now and I have in my spec an external polystyrene insulation system. It consists of polystyrene (styrofoam to you guys) lath and render. However we want to adhere a thin stone veneer, around 3cm ( 1.2 inches) thick to the styrofoam system. The manufacturers agents advise us that it is OK, but I am having second thoughts about shrinkake and bits of stone falling off the wall. I spoke to a technician at Knauf and he advises not to load up the polystyrene with more than 30kg of weight per square metre (about 65 pounds per 10 square feet) Anyone have any experience of applying stone veneer to styrofoam (or ICU’s) ? My architect and civil engineer both think it will be OK, but I’m very concerned that 10 years from now the whole thing might start to fall down. Many thanks,

Two questions - 1) does the stone have a solid ledge to sit on and 2) can you use tabs that attach the veneer through to something stronger than foam.

hi jonr, thanks for the reply

The stone will sit on the ground, on my 70cm concrete slab. No tabs to go through and fix to the masonry unfortunatly.

What will the metal lath be fixed to?

When you say "manufacturer's agent", is the stone natural or manufactured?

sorry for my late response, the bloody internet has been down for ages over here.

the lath is applied to the styrofoam with a kind of render glue. then plastic screws go in and bolt the styrofoam to the masonry, then another coat of render, then my stone.

the stone is natural but flattened on one side

By far, the majority of the force is downward and applied to the slab. Not much is trying to peel the stone away from the foam.

Okay, so you have a masonry structure, followed by styrofoam, with lath "render glued" to the outside, the whole of which is attached to the masonry with (plastic?) screws. When you say "plastic screws" are you describing metallic screws with threads optimized for holding in plastic or screws actually made out of plastic? When you say "render glue" are you describing a glue made by rendering down animal protein or a cementitious polymer-modified mortar that holds the lath to styrofoam?

What is the height of the assembly? There could be one standard for something that will not topple on someone's head and another for something that could fall from the overhead.

Thanks for all replies

Yes. I have a concrete skeleton with aereated concrete block infill (ytong).

The screws are long and made of plastic but obviously have metal ends to bury into the masonry.

When I say "render glue" yes I am describing a cementitious polymer-modified mortar that holds the lath to styrofoam.

Height is about 9.2 metres at highest point

That's a common system, but I ran into three things doing similar.

1) Does your stone product fall within the parameters stated by the Knauf technician? Typically, that system can be employed using cultured stone due to the controlled, light weight nature of the product. However, some natural thin stone veneers are cut well enough to fall within the parameters, too. I think the limiting factor is the adhesion forces between the render glue and the styrofoam surface.
2) At thirty feet high, you run the risk of stones becoming deadly missiles should that assembly come apart for whatever reason. Earthquakes come to mind as stones won't just fall one by one, but will be flung around with some force. The strength and quantity of the mechanical ties used must be adequate to resist whatever earthquake forces are expected in your jurisdiction.
3) Depending on your weather conditions and the exposure of the assembly, you might want to look into creating an air gap behind the stone for drainage. I used a product called "Driwall Rainscreen" by Keene.