Does spacing roof slates introduce a weather-entry problem?
No. The 1.2 mm (< 1/16") gap that SlateSpacer™ introduces is smaller than other gaps that already commonly exist in slate roofs. Joint spaces (the channels between slates) are often up to 4 mm (~3/16") tall and wide, or more, and thickness variances of adjacent slates are commonly 2 mm (~3/32"). Over the centuries, standard overlaps have been well worked out to accommodate these openings. Because it reduces capillary action, SlateSpacer™ actually reduces the most persistent form of water penetration.
