Deck Flashing and Deck Ledger Boards – How to Flash a Deck Ledger Board

The Installation of Deck Flashing and Deck Ledger Boards is Critical when Building a Deck

By Mark J. Donovan

When building a deck onto your home it is imperative that deck flashing be installed on the deck ledger boards. Without deck flashing, your deck, home and personal wellbeing are at risk.

Deck flashing prevents deck water run-off from seeping behind the deck ledger board and behind the home siding.

If water were to seep in behind the home siding, rot, insect, and mold and mildew problems will quickly arise.

Decks have also been known to detach and fall off homes when deck flashing has not been installed or installed improperly over the deck ledger boards.

What is Deck Flashing?

Deck flashing is a thin strip of metal, which slides up under the first row of clapboard siding that sits just above the surface of the finished deck, and wraps over and partially down the side of the deck’s ledger board.

What’s No Longer the Best Flashing for Deck Ledger Board?

Until January 1, 2004 you could use aluminum or galvanized flashing with pressure treated decks. Up until January 2004, pressure treated wood used a preservative known as chromated copper arsenate (CCA) for protecting the wood. This product is now banned.

New pressure treated lumber uses alkaline copper quat (ACQ) as a preservative, which is less harmful to the environment and to you. The problem with this new pressure treated lumber, however, is that the ACQ product contains much higher levels of copper.

Copper is not a safety issue by itself, however it is more corrosive when put in contact with steel and other metals. 

 Deck floor joists held up by deck ledger board at house.

In time this corrosive action could lead to deck flashing failure if aluminum or galvanized flashing were used with this newly treated lumber.

What is now the Best Flashing for Deck Ledger Board?

Consequently, if you are building a pressure treated deck it is best to use a copper based deck flashing to prevent corrosion. It is also important to use copper nails when securing the copper deck flashing to the deck ledger board and home, again to prevent corrosion.

Installing Deck Flashing

There are a number of ways to install deck flashing, or more specifically how to flash a deck ledger board. 

However my recommendation is to install the flashing such that it slides up 2-3 inches behind the first row of clapboard siding that sits just above the surface of the finished deck, and wraps over and partially down the outside edge (approximately 1 inch) of the deck ledger board.

Make sure when installing the decking boards that you do not punch nails or screws into the deck flashing as this will enable water to seep through the nail holes into the deck flashing and behind the deck ledger board.

Deck Installation Bid Sheet

Use this Deck Installation Bid Sheet to save time and money on your deck construction project.

At deck flashing junctions, overlap the sections of deck flashing by 6-10 inches and apply a bead of silicone caulk.

For help on building a deck, see’s Deck Installation Bid Sheet. The Deck Installation Bid Sheet will teach your how to hire the right deck contractor, and help to ensure that your deck project is completed on time and budget.

Related Information on Proper Deck Flashing and How to Flash a Deck Ledger Board

Additional Deck Building and Design Resources

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