I have 12″ black poly hanging out the bottom of my OSB sheathing for a wood frame house on a slab foundation. Most of the house is siding, but some is stone veneer and in these areas I have a masonry ledge (lug). The mason wants to leave the black poly in place and lay his mortar bed on top of it … instead of cutting the poly back and laying the first mortar bed directly on top of the concrete lug. He says he likes to keep a separation there to allow for possible movement. This seems goofy to me. Brick lugs without poly have been around a lot longer than those with.
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Leave it unless you have at least a 4" drop (I like 8" drops) to the brick ledge or "lug', as you call it.
The plastic acts as a flashing to keep water away from yur framing. When he runs his felt paper over the sheathing, it will complete the drainage plain down to the plastic. Any water that get through or behind the stone will follow the felt paper and plastic out beneath the stone and not wick back ito your sheathing or framing lumber.