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Ask the pros-May 2011

Drip edge on stone wall
By Reggie Bullard
I found your site on line and was happy.  I have a wall where the 
traditional stucco is sloughing off from an exterior property dividing block wall. The wall is capped in flat stone and trimmed with cultured  stone.  It runs 50' long and stands 6' in height and was put up over  diamond mesh and felt with what look to be 3/4" mason nails.  My opinion is that water is getting behind and freeze thaw is popping the  nails.  The stone overhang is inconsistent, flush out to 3/4"  depending on the stone cap coverage.  My question is can you recommend  a drip edge to shed water and provide a caulk seam between stone  and flashing or another better solution for the rebuild.  Also I have  another section with a gentle arch under cultured stone that  is failing.  Water seems to be getting in at the cultured stone mortar  joint spacing where it meets the stucco flush.
Thank you for your time and any help you can give.

Obviously, the overhang on the cap wasn't enough. It should extend out at least 3/4" past the finished stucco.
It is a good idea to have the edge angled down, or a drip groove cut in at least 3/4" away from the wall. These mortar joints always leak. I guess there isn't a material that is perfect for a cap, except maybe metal, which wouldn't look right on this wall.
I don't see why people just don't put mortar directly on the block. I have found failure in old walls where the mortar was failing on areas where there was lath, but bonded fine to areas that had no lath. It seems the lath interfered with putting pressure on the block for a good bond.
Nails sometimes don't hold good on block, either. They probably were spaced too far apart, or just broke the block when they were put on.
Regardless, if you get water in the wall, between the block and the stucco it will fail.
If you remove all the stucco, you may try just putting the mortar directly on the block, wetting it first. If you have
any doubt, an acrylic bonder mixed in the scratch coat will be insurance against failure.
The life of this wall is the cap, and the way it is put on, allowing or a good drip edge.

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