While there are many ways to tackle this type of window, here's one possible sequence that will deliver a highly water-resistant assembly. It's a complex installation. Unfortunately, choosing to build this type of architectural feature is an intentional choice that means taking on an involved series of steps. So if you're going to make that choice, do it right. (All drawings courtesy IBACOS.)
FOR ALL FOLLOWING STEPS + INSTRUCTIONS:
— Use butyl- or acrylic-based flashing tapes. No asphalt.
— Use sealant that meets ASTM C920 Class 25.
— Use a roller or burnisher to firmly seal all self-adhesive flashing in place.
— IBACOS recommends using housewraps that don't rely on mechanical perforations to achieve permeability.
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