How to Install Deck Flashing

When planning a new deck, it’s fun to spend time choosing colors and styles for your deck boards and railings. It’s fun to dream of what your deck could be. But as you think about what you’ll see day to day on your beautiful new outdoor space, it’s essential to think now about what you won’t see: the wood that supports your deck.

Wood that gets wet again and again will weaken and show damage over time. Stop the rot by being proactive during deck construction.

Flashing provides the secret superpower your deck needs to fight water damage and rot. 

With proper flashing, you can enjoy quality time in the fresh air for decades.

Trex RainEscape® Wall Flash minds the gap between your first deck board and your house — where water damage can start. The first deck board is generally mounted slightly away from the house, leaving a small gap where water and ice can become trapped. Over time, this trapped water will cause your wood to become rotten.

Trex RainEscape Wall Flash seals the space between your house and the deck to keep water out of the gap. This easy-to-install, self-adhesive wall flash comes with a 25-year warranty from Trex – the world’s #1 deck company

Step-by-Step Instructions

Trex RainEscape® Wall Flash is one piece of the overall flashing system used to keep your wood safe when you add a new deck to your home or replace an existing deck. 

Especially, if you’re cutting vinyl siding to install a deck ledger board, you must be careful to ensure moisture cannot touch the wood where your deck and home meet. The first deck board is generally mounted slightly away from your house, leaving a small gap where water and ice can become trapped. Over time, this trapped water will cause your wood to rot.

If you live in an area with a freeze/thaw cycle, this protection is even more critical. The wall flash prevents ice build-up that can expand, contract, and move the deck board over time.

While metal (such as aluminum flashing) and extra layers of roofing felt (tar paper) were often used in the past, today, vinyl Z flashing and rolls of vinyl-back flashing provide better protection. In fact, the house’s sheathing should be triple protected from moisture — with building paper or roofing felt stapled to the sheathing, then with back flashing; and then the Z flashing. This method preserves the ledger board.

The following highlights from this Decks.com article, provide a step-by-step look at how to DIY flashing. See the full article for detailed instructions and photos. Also, while these instructions are a good general guide, check your local codes to ensure compliance with regional building laws.  

1. Cut a piece of flashing with tin snips. Tin snips are the fastest and safest tool to cut vinyl flashing. You may also use self-adhering flashing, especially if your local inspector prefers it.
2. Score with a utility knife. Do not cut all the way through. Be sure to clamp a straight edge to the surface to ensure a straight line. This is just a score, not a cut. Cutting halfway through will allow it to bend very easily.
3. Bend flashing with a block and framing square for a 90-degree bend to fit inside the house corner wall. Mark for cutting to length with a square and pencil, and cut with tin snips. To make a bend at an inside corner, clamp a straight edge onto the piece and score with a knife, and bend carefully.
4. Measure carefully to cut around the door.
5. Cut flashing.  
6. Slip flashing under the siding.  
7. Slip the siding up and around the door. Slip the back flashing under the siding at least 3″ at the top and the sides. You may need to pry out the siding first.  
8. Attach the ledger board. 
9. Nail the Z flashing over the top of the ledger. When nailing your vinyl Z flashing to the house, be sure to nail every 12″ along the house with roofing nails.  
10. Slip the flashing behind the trim. Install flashing that is made for ledgers. It should be vinyl and have a “Z” shape that wraps around the top of the ledger.  
11. Overlap the flashing in the corner. At an inside corner, slide the first piece in so its bottom lip slips between the two boards.
12. Be sure to caulk the flashing. Squirt a bit of gutter caulk or silicone sealer to seal any joints that don’t overlap 4″ or more. At this point, you can also caulk the inside corner of the ledger.
13. Carefully slip in the siding. Slip the bottom piece of siding under the next-to-bottom piece. You may need to cut notches for obstacles, and you may need to pull out nails so you can slip the piece up far enough.
14. Use 1-1/4″ thick board to hold the siding at the proper height.

To keep water and ice from building up between the first deck board and the flashing, use an EPDM foam, such as Trex RainEscape Wall Flash.  Click here to see how easy it is to install Trex RainEscape Wall Flash.

If doing DIY flashing or deck construction sounds like too much, find a quality deck builder in your area.

Extra Waterproofing

Along with making sure you waterproof between the house and deck, it’s good to protect your substructure during your new deck building. The substructure includes the beams and deck joists that support your deck boards. 

Choose a joist and beam tape such as Trex Protect® to cover the wood and shield it from moisture. Trex Protect is a non-skid, self-adhesive deck flashing tape that shields the tops of joists, rim joists, beams, and the top of the ledger board from moisture that can lead to the development of rot and wood decay.

Other benefits of flashing tape:

• it can act as a barrier between wood and galvanized metal such as joist hangers
• helps seal deck fasteners and prevents moisture penetration
• eliminates splitting from freeze and thaw 
• provides a non-skid surface making it safer during installation

Protecting your deck — the ledger board, substructure, and all the wood in your deck — at the time of construction is key to fighting wood’s biggest enemy: Water. A small investment in flashing will pay off for years to come even if you don’t see it.