Keeping flagstone patio from telegraphing expansion joints

[mfinley919]

What would the pros recommend? If I install a flagstone patio I want to pour a concrete pad underneath and then mortar the flagstone to the pad. If the pad is 20x20 it should have expansion joints. How do you prevent the expansion joints below from telegraphing their cracks into the mortar bed and the flag stone?

Thanks

[Rich]

I've heard of using an anti-fracture mat (I think that's what it's called). The one I saw was orange and had little squares to hold the mortar.

[Dano]

A couple years back..did a 30k sq ft build out at the Ronald Reagan Building in DC using a light weight concrete in fill on top of substrate slab. We used poly to create the seperate the two to eliminate mirroring of any substrate cracks. One product out on the market is "Visqueen" which if I recall is not much differant than poly.