25mm or 30mm granite for ventilated facade cladding?
This is not a decision the stone supplier can make for you, but he can tell you two things: how much weight and how much cost the two thicknesses differ by. The rest is the designer's calculation against wind load and the fixing system.
How much weight do the two thicknesses differ by?
At a granite density of about 2700 kg/m³, 25mm weighs roughly 67.5 kg per square metre and 30mm about 81 kg, a difference of 13.5 kg. On a facade of several thousand square metres that accumulates into tens of tonnes on the brackets and the structure. So thicker is not simply safer — it is also adding load.
Why should the supplier not decide this?
Because thickness is set by three things: local basic wind pressure, building height and shape, and the fixing system you choose — kerf, undercut anchor or continuous groove. All three live in the design drawings, which the stone factory never sees. A supplier saying we always do 30 only means 30 is what he stocks, not that your project needs it.
What else does cladding demand from the slab?
Thickness tolerance, flatness, and intact edges and corners. Uneven thickness loads the brackets unevenly, and a chipped corner becomes a crack starter at the kerf or the drilled hole. So for cladding the quotation must state tolerance and no chipped edges in addition to thickness — those two affect safety more than 5mm does.
What are the real benefits of thinner slabs?
Three: less weight, lower cost, less load on the substrate. On very tall buildings, or refurbishments where strengthening the wall is constrained, those three can be decisive. The price is higher demand on processing accuracy and bracket quality — thin slabs have less tolerance for error, and chipping or hidden cracks have more serious consequences.
How do you pin this down at purchasing stage?
Get one sentence from the designer: the design thickness, the permitted tolerance, and the fixing type. Then copy that sentence verbatim into the order and the specification column of the quotation. Do not accept according to standard practice — that is the supplier's standard, not your drawing.
Once thickness is set, what else must be settled?
Two things easily missed: the position and size of kerfs or holes, and who cuts them, factory or site; and whether the back face is sealed and how many faces in total. Factory cutting is more accurate than site cutting, but if the dimensions do not match the brackets on site, the whole batch is scrap. Settling these two is worth more than arguing 25 against 30.