Joined: July 2004
||Posted: 04 Oct. 2019,09:20
If you use the VDW 815, that's designed for 3-5mm joints, so ideal for the sort of joint you are considering.
The 30mm depth requirement is "up for discussion", shall we say. There does need to be a minimum depth, and that is probably at least 20mm. Then there's a depth at which possible failure is pretty unlikely, and that's probably closer to 25mm. And finally, there's a depth at which failure almost never happens and the supplier gets to flog lots of lovely mortar. This all gets a little complicated by the fact that the 815 is designed for narrow joints, so, while the VDW 850 at 10mm width may well last for decades at 20mm depth, with just 5mm or so of the 815, some additional depth is necessary to give the material enough 'bant' to survive.
With these planks of yours, I'd lay as directed (and be aware that some granites really need to be pre-sealed), then rake through the joints with a big nail to give a bit of additional depth and clear out any accidental debris. This should give you 20-25mm depth, and that would probably be adequate for a residential patio.
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