Deck levelling pad for a sloped zinc flashing — level on top, water still runs underneath

A deck board has to sit level on a zinc flashing that slopes. A solid shim does that, and creates the problem this one is built to avoid: it dams the runoff, so water stands permanently between the wood and the metal. That puddle matters more than it looks. Deck boards are copper-treated pine (ACQ, copper azole) and copper treatments corrode zinc. Wet, in contact, with no air moving through, the runoff carries copper onto the flashing — the one layer of the whole build you cannot replace withou…

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