Learn the correct way to properly install deck flashing before attaching the deck ledger board


When building a deck onto your home it is crucial to properly install deck flashing above, behind and below the deck ledger boards. As a home improvement contractor, I’ve personally seen what can happen to a deck that never had deck flashing or improperly installed deck flashing. Deck flashing protects water from seeping behind the deck ledger board and from becoming trapped in between your ledger board and home wall sheathing. If water becomes trapped between your deck ledger board and home sheathing, rot, mildew problems will quickly grow into a big problem in the future. I’ve seen decks that didn’t have flashing or improperly installed flashing begin to detach from the home — that’s not the kind of deck I want to have BBQ party on!

What is deck flashing?

Deck flashing is a thin piece of metal that comes in rolls of length, ranging from 8″ inches by 50′ feet and upto 24″ inches by 50′ feet. The most common types of deck flashing you will find at any local home center is Copper, Aluminum and Vinyl deck flashing.

What kind of deck flashing should you use?

I prefer to use Copper for any type of deck construction I do – you can a also use vinyl flashing. In recent times you could use Aluminum flashing until they changed the way they preserved pressure-treated lumber. The manufactures of pressure-treated decking and lumber used to infuse the wood with a chemical preservative called “chromated copper arsenate (CCA) for protecting the wood. They have since banned that procedure for a much more safer process of wood preserver alkaline copper quat ( ACQ ). Although the new pressure-treated lumber is much safer and less harmful to the environment, the chemicals in the pressure-treated lumber are more corrosive to the hardware.

When building a new deck using

pressure-treated lumber as your framing members and supports, be sure to install copper roll flashing behind the ledger, and also use hot dipped galvanized nails and hardware for your structural fasteners. Hot dipped galvanized joist hangers, nails, lagbolts and post brackets are resistant to the corrosion, and the chemical reaction caused from ACQ.

How to install deck flashing?

It’s actually quite simple to install deck flashing. For example, if you’re building a deck using 2×8 as your ledger board you will want to purchase a roll of copper flashing to the width of 12″ inches. Just follow the steps below.

  • If you’re building a deck to the width of 10′ feet, you want to cut the copper to the length 10′4″ this will allow plenty of room to run past each end of your deck ledger board.
  • Next, remove the first coarse of siding so you can install the deck flashing up and underneath the first coarse of siding.
  • Followed by tacking the roll of deck flashing up with staples or roofing nails, then proceed to install your deck ledger board. Have a look at my photo below for a better idea.

If you look closely, you’ll notice

how I like to take extra precaution when installing deck flashing behind the deck ledger board. You’ll notice how I will use grace ice and water shield protectant to run just below the top of the copper deck flashing. Or let me rephrase that, you’ll notice the copper deck flashing running up and behind the grace ice and water shield ( the black stuff above the copper flashing ).

copper deck flashing detail

Another view from the underside of the deck, to give a better idea of how the deck flashing should run smooth across the underside of your deck

deck flashing

Here is another picture view of how you want to run your deck flashing outwards away from the deck ledger board. This way here, no water can penetrate from the sides of your deck ledger board.

deck flashing

Here’s what the deck frame looked like as I was building it.

deck frame

Here’s what the deck looked like with the pre-primed pine wrapped around the deck! I’ve been constructing all of these decks for this big sub-division, among other projects for this builder.

composite deck wrapped in pine

And finally, the composite deck construction using composite decking, wrapped in pine, with the proper deck flashing ledger board technique.

composite deck



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