My soffits are plywood. I currently have NO ventilation, either soffit or ridge. I'm considering one of these options, especially the 2nd one. I'll do a shallow skilsaw cut to avoid the framing and then screw them into place.
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Shop Air Vent 16" x 8" Mill Undereave Vent at Lowes.com
Shop CMI White Aluminum Face On Vent at Lowes.com
Shop CMI Galvanized Two Way Vent at Lowes.com