The International Residential Code requires that deck ledgers be flashed so water does not contact the band joist, and it specifies the materials and minimum corrosion resistance for flashing and related work[1][2].
Code basics you need to know
The IRC explicitly requires ledger flashing to prevent water from contacting the house band joist; see the ledger flashing provisions in IRC R507.* and the general flashing requirements in R703.4[1][2].
Flashing must be corrosion-resistant metal with nominal thickness not less than 0.019 in (0.48 mm) or an approved nonmetallic material compatible with the structure and decking[1].
The 2024 IRC adds that when attaching a ledger to an existing wall without a water-resistive barrier (WRB), a WRB must be installed behind the ledger and ledger flashing and must extend to the top of the ledger flashing vertical leg and at least 1/2 in beyond the sides and bottom of the ledger (IRC R507.9.1.7)[2].
Acceptable ledger attachment methods and fasteners
The AWC DCA-6 guide provides the prescriptive ledger-attachment tables that the IRC references; consult DCA-6 Table 5 for exact fastener types, sizes, and spacing for your ledger connection[3].
Prescriptive and tested attachment methods include through-bolts, 1/2-inch lag screws, or tested/listed structural screws. Ordinary nails or nonstructural screws are not acceptable for a ledger attachment under the prescriptive guidance[3][4].
Manufacturers publish ICC-ES- or manufacturer-listed structural screws (for example Simpson Strong-Tie SDWS) as acceptable alternatives to bolts or lag screws; always follow the fastener manufacturer's ledger tables or the DCA-6 tables for spacing and allowable loads[4][5].
For contact with pressure-treated lumber or exterior exposure, use corrosion-resistant fasteners (hot-dip galvanized to the applicable ASTM or stainless steel); manufacturers also provide stainless options for severe environments[5].
Accepted flashing approaches
A commonly accepted approach is a redundant two-layer system: a self-adhering membrane installed against the sheathing or WRB plus a metal cap or Z-flashing over it to divert water away from the ledger[1][9][10].
Do not place bare aluminum flashing in direct contact with modern alkaline pressure-treated lumber without a durable barrier (bituminous membrane or adhesive flashing). Pressure-treated (PT) chemicals can cause corrosion; use stainless, coated galvanized, or compatible nonmetallic membranes where appropriate[1][7][11].
Follow the flashing product manufacturer's published installation sequence, lap requirements, and wall-extension guidance to maintain product compatibility and warranty (manufacturers such as Vycor, Trex, and others publish those instructions)[6][11].
Inspection: what to look for
Visually confirm continuous flashing behind siding and that a metal cap or Z-flashing covers the top of the ledger. Look for staining on siding or fascia above the ledger — staining commonly indicates water entry[7][8].
Probe the ledger and adjacent band joist with a screwdriver or awl where safe and permitted; soft or spongy areas indicate decay and concealed damage[7].
Check interior walls adjacent to the ledger (behind doors, in basements) for stains, mold, or softened sheathing that signal water entry from failed ledger flashing[7][8].
Inspect fasteners for corrosion, gaps, missing or incorrect types. The presence of random nails or ordinary deck screws instead of through-bolts, 1/2-in lags, or listed structural screws indicates a non-compliant connection[3][4].
If flashing, WRB, or fasteners look suspect, be prepared to remove deck boards and possibly the ledger to fully inspect the band joist, sheathing, and WRB; many failures are hidden under siding and nail flanges[7][8][10].
Typical repair sequence used by trades
Trades commonly follow this sequence: remove obstructing deck boards and framing as needed; remove and replace any rotted band-joist or rim framing with pressure-treated or decay-resistant material; restore or install a continuous WRB behind the ledger; install a self-adhering flashing membrane up behind the WRB or to the sheathing per the product instructions; install a corrosion-resistant metal cap/Z-flashing over the membrane; reattach the ledger with approved fasteners using DCA-6 or manufacturer tables; then reinstall decking[7][10][6][3].
When making repairs, follow local code (IRC and DCA-6), the flashing product manufacturer's instructions, and the fastener manufacturer's ledger tables to ensure compliance and longevity[1][3][6].
Practical checklist for a field inspection
Do you see continuous flashing behind the siding and a metal cap over the ledger? If not, mark for further work and testing[7].
Are fasteners the correct type and spacing per DCA-6 or the fastener manufacturer's table (no random nails or ordinary deck screws)? If not, record locations and fastener type[3][4].
Are there stains or soft sheathing on the interior or exterior adjacent to the ledger? If yes, plan to open the wall or remove the ledger for a full inspection[7][8].
Is flashing metal compatible with pressure-treated lumber or is there a durable membrane between them? If bare aluminum is in direct contact with modern PT lumber, note replacement with stainless, coated galvanized, or a bituminous membrane[1][11].
Confirm gutters and downspouts are functioning and directing roof water away from the ledger; poor roof drainage increases ledger exposure to moisture (see our gutter maintenance guidance for upkeep) check gutters and downspouts[12].
When to call a pro and what to ask
Call a contractor when you find soft framing, interior staining, or when flashing or ledger attachment cannot be verified without removing deck components. Ask the contractor to:
Show ledger fastener calculations or DCA-6/manufacturer table references for the proposed fasteners and spacing[3][4].
Show the flashing product's installation instructions and confirm the proposed sequence (self-adhering membrane beneath a metal cap is a common, redundant approach)[6][9].
Specify corrosion-resistant fasteners compatible with pressure-treated wood and the local environment[5].
Costs and next steps
Repair costs vary widely by extent of damage and local labor; consumer cost references can give examples of deck repair ranges for homeowner expectations, but get site-specific estimates and contractor quotes before budgeting[12].
After repairs and before reattaching finish decking, test the repaired area: re-pressurize (if plumbing was involved) and inspect for leaks, and visually monitor the interior for staining over the following rainy events. Explicitly test for leaks after re-pressurizing or restoring finishes to avoid concealed failures[7][10].