How to Install Deck Flashing on an Existing Deck

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how to install deck flashing on an existing deck

A deck can greatly increase the value of your home as well as your enjoyment of it. 

With the right design and materials, your deck can become the focal point of your backyard. It can also provide a little more livable space when you have get-togethers or parties. 

However, if you are going to install a new deck, you must do it correctly. 

A deck that has not been properly installed can cause serious issues for your home. This is especially true when it comes to protecting it from rain and other weather elements. 

Flashing is one of the best ways to keep water from damaging your home. Along with your gutter system, flashing protects the foundation and prevents mold, rot, and mildew. 

In my experience in construction, I have noticed that many DIY jobs are done without attention to these things. These kinds of precautions will end up saving you a lot of money in the long run. 

In this article, we have laid out how to install deck flashing on an existing deck. Taking the time to add this important component will protect your home and keep you safe from mold. 

The steps of installing flashing on an existing deck include: 

  • Choose the right type of flashing 
  • Remove the deck boards next to the house
  • Install your flashing 
  • Seal the joints and reinstall the deck boards

Why Install Deck Flashing on an Existing Deck?

why install deck flashing on an exisiting deck

Deck flashing is a piece of thin metal that pushes water away from the house. 

Much like your gutter system, deck flashing directs water and protects the foundation from damage. Without these essential components, you could end up with serious problems. 

When water is allowed to pool around your home’s base, deterioration can happen. 

This is especially true with a deck that has not been sealed around the edges. 

Without metal flashing, water will soak into your siding and foundation. This can cause the siding to deteriorate as well as the deck

Even if your deck is already built, you can install flashing and protect it from water damage, swelling, and mold. 

Choose the Correct Flashing 

One of the most important steps in the process is to make sure you choose the right flashing. 

To do this, you will have to know what kind of wood your deck is made of. Not all wood types are compatible with every type of deck flashing. 

For example, if your deck is made of pressure-treated wood, you can not use traditional flashing. Galvanized metal or aluminum flashing will deteriorate due to the chemicals used to treat the wood. 

If you have a pressure-treated deck, it is best to use copper flashing. This will hold up well on the wood and will last for many years in the rain, ice, and snow. 

Remove the Deck Boards

You will have to remove the deck boards that are closest to the house to install the flashing. 

Depending on how your deck was built, this may be a more or less difficult project. 

If the boards on your deck are very long with no breaks, it may be easier to remove them in sections. However, if you can keep them together, this will make the process much easier. 

The ledger board is the board that is attached to the house and the joists. It runs parallel with the wall. 

Remove the deck boards that are on top of the ledger so you can have access to it. 

Install the Flashing 

install the flashing

Once you have access to the ledger board, you can install the flashing. 

The flashing will go underneath the siding on the wall and run over the ledger board. This means that any water that runs against the side of the house will not get stuck between them.

By adding the metal, waterproof flashing, you can get the most protection and avoid mold and rot. 

If your home has traditional siding, the flashing will be easy to install. If it has stucco or other types, you will have to seal the joints and ensure that no water gets behind it. 

Caulk the Joints and Reinstall the Boards

Once you have nailed the flashing to the ledger board and under the siding, it is a good idea to add caulk. 

Using silicone caulk to completely seal up the joints will give you the highest level of protection. 

Put caulk on all the joints and let it cure completely. Silicone caulk is waterproof and will not rot away when rain or other moisture makes its way to the joints. 

Once the caulk has dried, you can reinstall the deck boards and enjoy a waterproof deck that will last for years. 

Frequently Asked Questions 

faq how to install deck flashing on an existing deck

Is deck flashing necessary?

Deck flashing is incredibly important, especially if you live in an area that gets a lot of rain. Not having it will increase your chances of things like mold, rot, and mildew in your crawlspace or basement. 

The only time flashing may not be necessary is if you live in an incredibly dry area that rarely gets rain. 

Does flashing have to go under siding?

Flashing doesn’t always have to go under your home’s siding to be effective. However, this is one of the ways you can make it look seamless and give it a cleaner aesthetic. 

If you don’t put the flashing under the siding, you at least need to add caulk for a tight seal at the joints. 

How do you stop flashing from leaking?

If your flashing is leaking, this is a sign that it has deteriorated in some way. Oftentimes, it simply means that the caulking that seals the joints has started to come apart. 

Check every inch of the flashing to make sure that it is holding together and look for any cracks. Then, you can repair the affected area or replace the piece of flashing altogether. 


A deck for your home is a fantastic investment and it is incredibly important that you protect it. By making sure everything is properly installed, you can be confident that your home and deck will be safe. 

Following the above steps will allow you to install the correct equipment, even if your deck is already installed. 

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Michael J. O’Connor is a writer and marketing specialist from the Bay Area of California. A graduate of Sonoma State’s Creative Writing program, he spent many years as a contractor and carpet layer, learning the ins and outs of flooring and general contracting. When he’s not typing away at his desk, he enjoys hiking with his dogs, woodworking and collecting rare books. See full biography here.

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