Deck Ledger Flashing: How to Keep Water Out of Your Wall

Yes, a deck ledger needs flashing, and it is the detail that decides whether an attached deck lasts decades or quietly rots your house frame. The ledger is the board that bolts the deck to the house. Flashing is the metal or membrane layer that sheds water over that joint instead of letting it run behind the board. In Ontario, flashing is not optional: the Ontario Building Code requires ledger connections that keep precipitation from getting behind the ledger, as municipal code guidance explains.

This guide covers what the flashing does, the three places it goes, which materials work, the mistakes we see most, and what to check on an older Ottawa-area home before you build or renovate.

TL;DR

• Flash above, behind and below the ledger. Each layer must lap over the one below it, like shingles, so water always exits away from the house.
• Never bolt through crushed or pinched flashing, and seal every fastener hole. Both are common causes of hidden rot.
• Fasten the ledger to the house framing, never to brick veneer. Attached decks need a permit in Ottawa.
• Older homes often hide damaged sheathing, mold or lead paint behind the old ledger. Test before you cut or demolish.
• If the wall behind your existing deck is soft, stained or smells musty, have it opened and assessed before you add new flashing over it.

Table of Contents

• What ledger flashing does
• The three layers: above, behind and below
• Materials: what holds up in Ottawa winters
• The mistakes that cause hidden rot
• Step by step: flashing a new ledger
• Permits and code in Ottawa
• Our perspective: what we find behind old ledgers
• Signs your ledger is already leaking
• What this costs in the Ottawa area
• Working with us
• FAQ
• Sources

What ledger flashing does

A ledger board sits tight against the house wall. Rain runs down the siding, hits the top of the ledger and has two options: shed off, or wick into the gap between the board and the wall. Without flashing, it takes the gap. Pressure-treated lumber stays wet in that gap, the sheathing and rim joist behind it stay wet too, and the rot starts where no one can see it.

Research from the National Research Council of Canada points to this exact spot. Balcony and ledger connections are a recurring failure point because they are often built before the wall’s second line of defence, the house wrap or sheathing membrane, is properly completed, which makes a continuous seal very hard to achieve. The same source notes that leaks can stay invisible from the outside for years while rot advances inside the wall.

That is why flashing is a wall-assembly detail, not an accessory. It has to connect to the wall’s own water barrier.

The three layers: above, behind and below

Most problems come from treating flashing as one piece of metal. A proper ledger detail uses three layers working together.

Above the ledger

A metal cap flashing runs the full length of the ledger. It tucks up behind the siding (or behind the wall’s weather barrier where the cladding has been lifted) and folds over the top face of the board. Its job is to catch the water coming down the wall and turn it outward.

The top edge needs to be lapped under the house wrap or membrane above it. If the metal sits on top of the wrap instead, water runs behind the metal and into the stud cavity.

Behind the ledger

A self-adhering membrane goes on the sheathing where the ledger will sit. It wraps the fastener holes in the wall and gives a second barrier if water ever gets past the cap. Many Ontario installers also lift the siding and fasten the ledger directly through the sheathing to the rim joist, so the board does not trap cladding behind it.

The membrane must be lapped properly with the wall’s weather barrier, upper layers over lower layers. NRC’s guidance describes this as shingle-style lapping. A frequent error in older renovations is a reverse lap, where the membrane ends up under the flashing in a way that directs water into the wall cavity rather than out of it. The rot then begins before anything shows on the siding.

Below the ledger

The bottom of the ledger needs a way for water to leave. Two details work:

1. A drip flashing that kicks water out past the bottom edge of the board.
2. A notch in the cladding at the bottom of the ledger so water can drain freely.

What does not work is a ledger that sits tight against the wall with nothing to break the water’s path at the bottom, or flashing that ends flush with the board.

Materials: what holds up in Ottawa winters

Ottawa winters mean repeated freeze-thaw cycles, road-salt air near driveways and heavy snow loading against the wall. Your flashing has to survive all of it.

General-purpose galvanized flashing is easy to find at big-box stores. For example, Home Depot Canada lists a 29-gauge galvanized steel deck flashing designed to direct water away from wood. Stock profiles work on many walls. On a wall with uneven cladding, trim boards or an old repair, a profile bent to the actual wall gives a more continuous result. That is why many professionals fabricate custom pieces.

On corrosion compatibility, check the manufacturer’s data for your specific lumber treatment and fasteners. The Illustrated User’s Guide to NBC 2020 Part 9 covers connectors and fasteners in contact with preservative-treated wood, and it is a good starting point for your inspector or contractor conversation.

The mistakes that cause hidden rot

Crushed flashing

The most common failure is not missing flashing. It is flashing that was installed and then crushed. When the ledger is bolted hard over the flashing, the metal is pinched flat against the sheathing. Water can no longer drain between the layers, so it stays trapped. NRC’s research describes this as a hidden rot trap inside the wall. The fix is to keep the cap flashing above the board, not pinned under it, and to leave a drainage path.

Reverse laps

Covered above, but worth repeating because it is invisible after the siding goes back. Every upper layer must sit over the lower layer. Check this at each joint before you close up the wall, and photograph it.

Unsealed fastener holes

Ledgers are fastened with lag screws or structural screws, often 12.7 mm (1/2 inch) in diameter. Each one punches through the flashing and the sheathing. If the hole is not sealed, the screw shank becomes a path for water into the house frame, bypassing the flashing entirely. Seal membrane around each penetration before the board goes up.

Fastening to brick

Municipal building officials in Ontario are consistent on this. A ledger cannot rely on masonry veneer for support, and direct attachment to veneer is generally prohibited unless a professional has designed it. The board must go to the structural framing. Brick also traps moisture behind it, so a brick-clad house usually points toward a freestanding deck with its own posts and beam against the wall.

No access later

Think about maintenance when you plan the deck. Municipal design guidance advises that the deck should not block access to wall components such as gas lines or furnace vents, and the same logic applies to the flashing itself. If you can never inspect the ledger area, small leaks become big repairs.

Step by step: flashing a new ledger

This is the order a careful crew follows. It is a guide to what good work looks like, so you can check it. It is not a substitute for your permit drawings or an inspector’s instructions.

1. Confirm the wall. Find the rim joist and confirm the framing is sound. Probe for soft wood and look for old water stains.
2. Open the cladding. Cut back the siding along the ledger line so the ledger can bear on the sheathing and the flashing can tuck under the cladding above.
3. Install membrane. Apply self-adhering membrane to the sheathing, lapped under the wall’s weather barrier above and over any barrier below.
4. Hang the ledger. Fasten through the sheathing into the rim joist with the specified lag or structural screws. Seal each hole.
5. Fit the cap flashing. Slide it up under the weather barrier and cladding, fold it over the top of the ledger, and keep it out of the fastener path or seal where it is penetrated.
6. Provide drainage at the bottom. Add drip flashing or notch the cladding so water leaves freely.
7. Seal and inspect. Finish with compatible sealant at the cladding edge. Have the inspector see the ledger before it is covered.

Permits and code in Ottawa

The rules sit in the Ontario Building Code, O. Reg. 163/24, which adopts the 2020 National Building Code with Ontario amendments. Municipal guides then spell out the details. One Ontario municipal permit package lists all decks attached to a dwelling among the cases that need a permit. A ledger attachment makes the deck an attached deck, so plan for one.

For Ottawa specifics, read our guides on deck building permit requirements in Ottawa and whether you need a deck permit in Ottawa. Check with the City before you start, since rules and fees can change.

Our perspective: what we find behind old ledgers

We are an abatement and renovation contractor in Ottawa, and we spend a lot of time inside walls that other people never open. That changes how we look at a deck ledger.

When a homeowner calls about a rotted deck, the deck is rarely the whole story. The ledger area is where water has been getting in for years, so it is also where we find the damage that follows: soft sheathing, stained insulation, and in some older houses mold growing in the cavity. On homes of a certain age, the siding, sheathing paint or wall materials we open can also carry lead or asbestos. That is why we test before we cut, and why an old ledger is not a job to rip off with a pry bar.

Our practical advice:

• Treat the wall as part of the deck. New flashing over wet, damaged sheathing hides the problem, it does not solve it. Open it, dry it, repair it, then flash.
• Ask about the order of work. The membrane should be in place before the ledger goes up, and the inspector should see it.
• Photograph everything before closing. Lapping and sealing are invisible later. A dated set of photos is worth having for resale and for warranty questions.
• Choose freestanding when the wall is a problem. Brick, tricky cladding or a wall you cannot access are all reasons to carry the deck on its own posts.
• Keep the paper trail. Testing results, permit records and photos show a buyer’s inspector that the work was done right.

If you want a sense of what else older houses hide, our guides on hidden renovation hazards in older Canadian homes and renovation red flags in older houses walk through them.

— Jason

Signs your ledger is already leaking

Because leaks can run for years unseen, check these on an existing deck. Do it twice a year; our deck maintenance schedule shows what to look at and when.

• Siding above or beside the ledger is stained, bubbled or has peeling paint.
• The ledger board is dark, spongy or splitting at the lag heads.
• A screwdriver sinks easily into the board or rim joist.
• You see rust trails from fasteners or flashing.
• The basement or crawlspace ceiling beneath the deck wall is stained, or smells musty.
• Joists pull or sag away from the house.

If you find several of these, stop using the deck for gatherings and have a professional assess the connection. A ledger that fails under load is a safety issue.

Deterioration that begins where a ledger meets the wall follows the same logic as any exterior wood problem: water gets in at a joint and stays. For more on how this plays out, exterior wood rot repair is worth a read alongside the checklist above.

What this costs in the Ottawa area

Flashing is a small part of a deck budget, but a rebuild after rot is not. The published range for our pressure-treated deck installation is $45 to $75 per sq ft (CAD). Cedar versus pressure-treated is a separate decision with its own costs, which we compare in cedar vs pressure treated for Canadian decks.

If opening the wall reveals mold, our published range for a single room or bathroom is $2,000 to $6,000, and a small contained area is $500 to $1,500. Testing and lab analysis is $300 to $600. These are published ranges, and a real number comes from an itemized quote after we see the job.

Working with us

If your deck project involves an older house, we can handle the full sequence under one roof: testing, abatement where needed, demolition of the old deck or ledger, permit management and the new build. We give itemized quotes, so you see what each line costs, and estimates come within 24 hours. See our decking and fencing services, or contact us for a free estimate.

FAQ

Does a deck ledger need flashing?

Yes. Ontario’s code requires ledger boards to be flashed so precipitation cannot get behind them. Flashing is part of the connection, not an extra, and an inspector will look for it.

What flashing should I use for a deck ledger?

Use corrosion-resistant metal, typically galvanized steel or aluminum, as a cap over the ledger. Add self-adhering membrane behind the board. Keep aluminum from touching treated lumber directly, and check your fastener and lumber compatibility.

What is the best way to flash a ledger board?

Lap every layer like shingles: membrane under the wall’s weather barrier, cap flashing under the cladding and over the ledger, and a drip or notch at the bottom. Seal every fastener hole and never bolt through crushed flashing.

Can I attach a ledger to brick?

Generally no. Municipal guidance says the ledger cannot depend on masonry veneer and must be fastened to the house framing, unless a professional designs another solution. A freestanding deck is the usual answer for brick walls.

Can I fix flashing without rebuilding the deck?

Sometimes, if the wall is dry and sound. Because the ledger sits against the flashing, repair usually means lifting siding and sometimes releasing the ledger. If the wood is already soft, have the area assessed first.

Sources

• Code Guideline Part 9 Decks, Carleton Place
• NRC moisture management publication, Solplan Review
• O. Reg. 163/24 Building Code, ontario.ca
• Building a Deck permit package
• Peak galvanized deck flashing, The Home Depot Canada
• Illustrated User’s Guide: NBC 2020 Part 9
• Uncovered Deck Design Guide, City of Edmonton

Recommended

• Low clearance decks in Canada: why screws often beat hidden fasteners
• Inspect Twice Yearly: Deck Maintenance Schedule for Ottawa Homeowners
• Make Your Deck Last 50+ Years: PVC vs Composite, Ottawa Installer Rules
• Deck Building Permit Requirements in Ottawa