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Hello,
I wouldn't go much more than 1 1/2" on the lip as it begins to become a trip hazard, especially if they are only 12" deep treads.
One trick, learned from others here, is the use of a angle grinder and diamond grinding cup. For all my capping and stair tread projects on SRW's, I use the grinder on a regular basis to help smooth out any imperfections on the top of the block or make up for any variances in the block tops due to slight deviations that arise as a wall is built.
For blue stone especially, it is important that when you set the tread on the block, it doesn't teeter/rock at all. Although the adhesive may make up for the difference, over time, it may flex a little (as it is suppose to) and the stone can crack. If you have a perfectly flat surface from the get go, then all the glue has to do is hold it in place.....not keep it from rocking.
Many guys just go the mortar route as you can make up for the block deviations....... but I still have my doubts about how well mortared caps/treads hold up over time do to the fact that the wall is flexible.
steve
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