SettingsSettings for this calculationUS
The length of the deck ledger board being flashed.
The ledger's length — and then order flashing LONGER than it. The flashing has to turn up behind the cladding above and return past both ends of the ledger, because an end left square is a funnel: water running down the wall arrives at the end of the flashing and goes behind it. Ledger flashings fail at their ends far more often than along their length.
How far the flashing extends past each end of the ledger board.
This kick-out allowance directs water running down the flashing away from the wall past the ends of the ledger, rather than letting it run behind the siding at the joint.
Total flashing length needed
20 ft
They open the calculator with your figures already in it
Deck Ledger Flashing Length Calculator: 20 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Flashing length = ledger board length + 2 x end overhang, providing a kick-out allowance to direct water away from the wall at each end of the ledger
Inputs used
- Ledger Board Length
- 19.5 ft
- End Overhang (Each End)
- 3 in
What this calculation does not cover
- Gives one continuous length, and flashing does not come continuous. Stock pieces run about 10 ft (3 m), so any ledger longer than a single piece has joints, and each joint needs the upper piece lapped several inches over the lower and sealed so water crossing it stays outside. Order the calculated length exactly and every lap comes out of it — the joints end up butted, and a butted joint in ledger flashing aims water straight at the rim joist.
- Says nothing about which metal. Modern pressure-treated lumber carries a copper-based preservative, and aluminum flashing in direct contact with it corrodes — sometimes through, within a few years, in the one place on the deck nobody can inspect. The metal is chosen by the treatment chemistry (heavy hot-dip galvanized, stainless, copper, or a coated product rated for contact with treated wood), not by the length, and the right length in the wrong metal buys a hidden failure.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Part of a bigger job
Or plan the space itself: measure it once, doors and windows included, and this figure comes back worked out on that space, with what goes with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Flashing length = ledger board length + 2 x end overhang, providing a kick-out allowance to direct water away from the wall at each end of the ledger
Cite this page
Your workspace
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Tools and safety for this job
To frame joists and a deck. Generic types, no brands, no prices.
Protection this work requires
- Hardwood dust is a recognised carcinogen: extract at the tool and wear a P2/N95 respirator when cutting or sanding indoors.
- Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
- Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
- Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
Show the 7 tools this job needsHide tools
Essential
Circular saw
Claw hammer
Cordless drill/driver
Spirit level
Tape measure
Recommended
Impact driver
String line and pins
Also needed as materials: circular saw blades, drill and driver bits, impact driver bits.
what each carpentry tool is for, and the spec that decides which to buy where one does.