One of my favorite consultants and I had lunch in Davis the other day. The subject came up about managing moisture intrusion around installation of trim pieces like casing bead or control joints.
These two pictures are xj-15 control joints, the left defines a plaster panel, the right is decorative and will probably get tile inserted in the middle. Trims with two wings like the xj-15 when installed vertically help manage bulk moisture during a rainstorm by sluicing water on a wall more quickly than water on the face of the plaster.
One of the reasons that the ASTM C-1063 Lathing Standard insists on installing the vertical trim continuously and breaking the horizontal pieces to fit in between is that they create sluices for bulk moisture to pull water quickly off the wall.
So what happens if your horizontal trim mitered joint is installed like the one seen above? The moisture running along the bottom flange of the trim finds a hole like this one and dumps moisture that should be going down the wall and out, into the plaster panels. Then you end up with something that looks like this:
When unpainted, unsealed, floated and heavy textured stucco is applied with poorly mitered joints and no sealant bed, nothing bad happens. When sealed, painted, elastomeric coated, or even smooth troweled plaster is applied, bad things can happen. If you paint or use an acrylic finish, bed the trim in sealant.
If you have questions about this condition or other situations, call or email:
Bell Construction Consulting
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