Deck Flashing Calculator
Where a deck ledger bolts to a wood-framed house, the IRC requires corrosion-resistant flashing so water can't get behind the ledger and rot the band joist. Enter the ledger length, the flashing piece length and the lap. The calculator counts the pieces with the laps and end extensions, the self-adhering tape for the ledger face, and the cost.
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Bolt the ledger with the deck ledger calculator.
Ledger flashing by ledger length
10 ft pieces, 4 in laps, 2 in past each end, 11 in rim tape on the ledger face, $17.49 a piece.
| Ledger | Pieces | Laps | Tape rolls | Flashing cost |
|---|---|---|---|---|
| 8 ft | 1 | 0 | 1 | $17.49 |
| 12 ft | 2 | 1 | 1 | $34.98 |
| 16 ft | 2 | 1 | 1 | $34.98 |
| 20 ft | 3 | 2 | 1 | $52.47 |
| 24 ft | 3 | 2 | 1 | $52.47 |
| 30 ft | 4 | 3 | 1 | $69.96 |
| 40 ft | 5 | 4 | 1 | $87.45 |
What the code and DCA 6 require
The American Wood Council's DCA 6 deck guide requires approved corrosion-resistant flashing at any ledger connection to a wood-framed wall, citing IRC R703.4. Metal flashing must be at least 0.019 in nominal thick (or an approved non-metallic material), and aluminium should not be used in direct contact with preservative-treated lumber because the copper in the treatment corrodes it. The siding comes off where the ledger goes, and the flashing runs continuously along it — DCA 6's Figure 14 shows the detail.
Where the ledger is longer than one piece, lap the pieces at least 4 in (Decks.com) and seal any shorter lap. Tape doesn't replace the metal flashing, but butyl tape on the ledger face and over seams adds a second line of defence. Bolt the ledger with the deck ledger calculator, tape the rest of the frame with the joist tape calculator, and price the frame with the deck framing calculator.
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Frequently asked questions
- Is flashing required on a deck ledger?
- Yes. The IRC (R703.4) requires flashing where a deck attaches to a wood-framed wall, and the AWC's DCA 6 deck guide calls for approved corrosion-resistant flashing at any ledger board connection to a wall of wood framed construction, run continuously along the ledger.
- How much ledger flashing do I need?
- The ledger length plus about 2 in past each end, with at least 4 in of lap wherever two pieces meet. A 16 ft ledger needs 196 in of run: one 10 ft piece covers 120 in and the second, lapped 4 in, covers the rest — two pieces.
- What kind of flashing should I use with treated lumber?
- Galvanized or stainless steel, copper, or a non-metallic flashing. DCA 6 says aluminium should not be in direct contact with preservative-treated lumber, because the copper in the treatment corrodes it. Metal flashing must be at least 0.019 in nominal thick.
- How much should deck flashing overlap?
- At least 4 in per Decks.com's ledger flashing guide; Trex's guide uses 3 in. Seal any joint that laps less than 4 in with sealant, and run the flashing at least 2 in past each end of the ledger.
- Do I need flashing tape as well as Z-flashing?
- Tape is not a substitute for the metal flashing, but self-adhering butyl tape on the ledger face and over flashing seams adds protection. Trex Protect's 11 in × 50 ft rim tape is sold for rim joists and the ledger; one roll covers a ledger up to about 49 ft with the end runs.
