Roof underlayment in CZ4c

This is a forum discussion among builders, not a news story, so it won't change what's available to homeowners generally. But it's a useful look at the kind of detail that goes into building a roof that lasts in a wet climate, which matters if you're planning a re-roof or new build in a rainy region.

A homeowner in coastal Oregon, building in a zone with heavy winter rain, described a vented attic roof design meant to avoid ever needing repairs. The assembly includes a dark standing-seam metal roof, a fire-resistant peel-and-stick underlayment with zero permeability (meaning no moisture can pass through it), pressure-treated plywood, cedar strips creating a small air gap to cut down on condensation, a layer of mold-resistant plywood, and attic venting designed so air flows evenly from the soffits to the ridge without over-ventilating. Their open question was whether a second underlayment layer, placed above the plywood, should also block moisture entirely or let some vapor through, since using two zero-permeability layers back to back can trap moisture inside the roof rather than letting it dry out.

One reply pointed out that the metal roofing itself already blocks all moisture movement, which matters for the underlayment choice beneath it. For anyone planning a roof replacement or new build in a wet climate, the takeaway is that layering fully vapor-blocking materials can backfire if the assembly has no path for trapped moisture to escape, and getting that balance right is something worth discussing directly with whoever designs your roof assembly.