Hi, Brad,

As posted, I am a rookie on this but I do understand the principle. That is, put some type of flexible connection between the pool wall and the decking....otherwise, it will crack.

So, since your drysetting, it makes sense to me that your flexible connection will take place automatically because you do not intend to mortar the joints.

I understand what fig is saying by mortaring the first row or coping to the bond beam....that makes sense, too. That would mean to me an expansion joint between the flagstone coping and waterline tile is unnecessary and not workable anyway.

If we're all on the same page, it would then seem logical to me that it is not necessary to use polyvoid since you are using no grout on the remainder of the decking material.

I'm on fairly thin ice and fig understands this better than me. I did a poured concrete decking....as you said..different animal.