Seattle’s long wet season is not especially violent — it is persistent. For flat and low-slope roofs, that persistence makes drainage the single most consequential design and maintenance factor.
Ponding Is a Symptom, Not a Style
Calling a roof “flat” does not mean it should hold water indefinitely. Standing water accelerates membrane aging, finds every seam defect and adds structural load. Ponding evidence is one of the first things a low-slope evaluation documents.
Drains, Scuppers and the Things That Block Them
Most Northwest drainage failures are mundane: debris over drains, clogged scuppers, displaced ballast, growth in slow-drying corners. Scheduled roof maintenance with documented photos catches these before the interior does.
Design-Level Fixes
Where geometry itself traps water, corrections happen at replacement: tapered insulation, added drains or scuppers, reworked crickets around equipment. Commercial roofing replacement planning should treat slope-to-drain as scope, not as an accident of the existing roof.