Change the mortar width. This actually changes the sloped edges of the tiles as well as the mortar width. I don't like this - but it's just the way the tile node works.

After we learn all about this shader, I'll show you how to build a proper tile behavior from scratch, using only math nodes and the U and V nodes. Then we won't have this problem.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)