The manufacturer (TimberTech) only requires 16" and allows 24" for MAX products.
As for ''flex'' the issue seems to have only arisen for PVC products and it is more about expansion/contraction of the material than ''flex''.
The SS grooved hidden fasteners can't adapt enough for that; so TimberTech has suggested that Cortex plug and screw fasteners be used intermittently or when a known problem arises.
Others have just opted for the CAMO system...
But the largest issue is installers not pre-drilling as directed.
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