Poor workmanship and wayward water are two of the biggest threats to a deck’s longevity. Unfortunately, those dangers often intersect at the ledger board, which anchors the deck to the building. It happens because builders either neglect to install ledger flashing or settle for substandard methods, such as caulking. Without adequate protection against water infiltration, the ledger traps moisture and rots. The rim joist is also in jeopardy of rot. As a result, the risk of catastrophic deck failure increases. It’s a more common occurrence than many people know and very avoidable.
Building Code Requirements for Flashing at the House
The requirements for deck flashing are outlined in section R703.4 of the International Residential Code (IRC). It calls for flashing “where exterior porches, decks, or stairs attach to a wall or floor assembly of wood-frame construction.” The flashing must be “approved corrosion-resistant flashing” that is “applied shingle-fashion in a manner to prevent the entry of water into the wall cavity.” The flashing must also “extend to the surface of the exterior wall finish.” Using this general guidance, it’s up to the builder to select the appropriate deck flashing materials and methods.
Drawbacks to Traditional Flashing
Code-compliant flashing materials include aluminum, galvanized steel, stainless steel, copper, vinyl, and self-adhered membranes that comply with AAMA 711. While copper pairs well with pressure-treated lumber, it can react with (corrode) galvanized steel fasteners and hardware. Stainless steel is an inert (non-reactive) flashing material, but it costs more than most other options like copper. Aluminum, galvanized steel, and vinyl flashing are the least expensive options and the most widely available.
Common metal flashing is typically shaped like an “L” or “Z,” with the wall-side leg tall enough to slip under the building’s water-resistive barrier (WRB)—usually house wrap or felt—and the siding. The horizontal lower leg caps the ledger and, in the case of Z flashing, extends down the ledger face about half an inch. The American Wood Council recommends using “L” flashing that extends beyond the joist hangers. See Figure 14 in “Design for Code Acceptance No. 6 (DCA6), Prescriptive Residential Wood Deck Construction Guide.”
But neither of these flashing materials provides a water-proof seal over the ledger. Water can still find its way to the wood, and moisture can migrate to the building walls through nail holes. With that moisture comes rot and decay. Plus, aluminum and galvanized steel flashing corrode faster when they’re in contact with pressure-treated wood.
While the IRC is silent on accelerated corrosion, some building authorities restrict the use of these metal flashings. The common workaround is to install a membrane barrier between the metal flashing and the pressure-treated wood. But the better solution is to omit the metal flashing and apply a flexible self-adhered membrane instead. It’s an approach that more builders are adopting since new and better flashing membranes have reached the market.
Advantages of Flashing Tape
Traditional flashing is inexpensive and widely available but can never create a waterproof seal. Self-adhered flashing membranes–flashing tapes—do so automatically. They also eliminate mechanical fasteners that create holes and don’t corrode. Membrane flashing might cost a bit more than aluminum or galvanized steel, but it protects the ledger better and is much less expensive than copper and stainless steel.
The primary types of self-adhered membranes used as deck flashing are asphalt-based, acrylic, and butyl-based.
- Asphalt-based membranes. Originally used as an ice and water shield on roofs, this peel-and-stick underlayment is now also sold as deck flashing. It’s fairly inexpensive but tends to ooze excessively in hot weather. It’s also difficult to handle without making a mess and doesn’t adhere well in cold weather. Brand names include WiseWrap ledger flashing from DeckWise and Vycor® Deck Protector from GCP.
- Acrylic membranes. This type of membrane is thin, stays flexible in cold weather, tears by hand, and unsticks easily if you make a mistake. It’s also the most expensive of the self-adhered membranes and isn’t readily available in sizes or formats conducive to ledger flashing. Plus, it needs 24 to 72 hours to be fully set. Nichigo G-Tape from Mitsubishi is one example.
- Butyl-based membranes. This type of membrane is very tacky but doesn’t ooze like bitumen at elevated temperatures. It also adheres better than asphalt membranes and sets faster than acrylic tapes. When applied in cold weather, tack-stapling might be required to hold it in place, but the membrane will self-seal around such penetrations. Butyl-based membranes are available from several manufacturers. Trex Seal aluminum-lined flashing tape is the newest generation of butyl-based membranes. This flashing tape was developed specifically for protecting ledgers and is compatible with all decking materials.
Whatever type of flashing tape you choose, make sure it meets or exceeds the requirements of AAMA 711-13, or it will not be code-compliant.
Hybrid Flashing Tape: Waterproof, Flexible, “Obedient”
Trex Seal is 11 inches wide and has a split release liner (backing) that makes it easy to apply. Its aluminum lining reduces or eliminates the need for stapling because it holds its shape better than unlined tapes and tapes that use a PET lining, such as TimberTech’s PRO-Tac.
Not only does the aluminum lining in Trex Seal minimize flex back and slumping, but it allows builders to pre-bend the tape to fit the inside corner formed by the ledger and building wall. In short, Trex Seal remains tacky and flexible, but the aluminum liner makes it more “obedient” than other flashing tapes.
Even decks exposed to only occasional rain or snow will last longer and stay safer with ledger flashing tape.