There’s a reason so few builders and deck contractors properly flash deck ledger boards: It’s a pain. Not only do home buyers rarely inquire about or inspect such a spec, but by the time any latent defects from water and moisture penetration crop up (if they ever do), the builder’s warranty has long since expired.
If that’s your rationale, then good luck. But if you want to build to a higher standard, one that not only protects you from the cost of callbacks but also delivers a quality-built and durable house, then it might be worth a little extra time and training to get this detail right.
The goal is to divert runoff over the top of the ledger board instead of allowing water to get behind it. Like all flashing details, it requires a layered approach designed to shed water away from the home’s structural elements.