The deck ledger is the beam bolted to the side of a house that carries the entire load of a deck. It is also the one framing member most likely to fail silently, because the joint where it meets the wall is a perfect water trap. Rain hits the siding, runs down, and finds every gap between the ledger and the house. Over a few seasons, water works behind the board, rots the rim joist, and turns a solid deck into a safety hazard. Proper flashing keeps that water moving away from the house, and the sequence is simple once you understand how the layers fit together.
The ledger connection demands the same engineering respect whether you are attaching a deck ledger to a water table foundation or to a framed wall with siding. Water follows gravity in both cases, so the flashing rules stay the same: membrane goes behind, metal goes in front, and every layer sheds water onto the layer below it.
Why the House-to-Deck Joint Collects Water
Building a deck is not a difficult project, but the house connection is where most premature failures start. The intersection of deck and wall creates a ledge that catches runoff, and the ledger itself bridges the warm house interior and the cold exterior, which invites condensation. If water gets trapped behind the ledger, it has no way to dry, and rot progresses out of sight for years.
The flashing approach changes when you attach a deck ledger to brick veneer, because you cannot tuck membrane behind house wrap on masonry. The principle still holds, though: interrupt the water path before it reaches the wood. Understand the wall type first, then choose the detail.
How Water Travels Through the Joint
- Rain hits the siding and runs down the face of the wall.
- At the top of the ledger, water meets the seam between siding and deck.
- Without flashing, it wicks behind the ledger and into the rim joist.
- Trapped moisture rots the ledger and the joist ends from the inside out.
- With a drip cap, water hits the metal and is pushed out and away from the house.
The best way to visualize a correct install is to follow one drop of water from the siding down to the ground. It should hit the siding, reach the house wrap, then land on a drip cap that curls it away from the house. If the drop can find a path behind the ledger, the detail is wrong. The International Residential Code requires flashing at the ledger-to-wall connection, and inspectors look for the metal layer, not just caulk.
The Cost of Getting It Wrong
A failed ledger connection does not announce itself. The first signs are often subtle: water stains on the rim joist in the basement, soft spots near the door, or spongy decking. By the time rot is visible, the fix usually means removing decking and joists, cutting out the damaged rim joist, and rebuilding the connection. That work routinely costs several times what a roll of flashing membrane and an afternoon of careful installation would have.
Flashing Materials and Tools
You can watch the full sequence of how to install and flash a deck ledger from start to finish at Fine Homebuilding, which shows the same layering order described here. The material list is short and most of it is available at any lumberyard.
- Self-adhering membrane (peel-and-stick ice and water shield).
- Metal drip cap flashing or z-flashing, pre-bent to match the wall.
- Galvanized or stainless carriage bolts with large flat washers.
- Exterior silicone or polyurethane caulk.
- Copper flashing stock and a brake, if the wall is masonry.
- Circular saw with a carbide blade for cutting the flashing notches.
Choosing Between Peel-and-Stick and Metal
Membrane and metal do different jobs, so most correct details use both. The membrane seals the face of the house behind the ledger and reaches up the wall to catch runoff. The metal drip cap sits above the ledger and physically pushes water away from the wall. Copper is the premium choice for masonry because it bends into a mortar joint and lasts decades without corroding.
Material Comparison
| Material | Where it goes | What it does |
|---|---|---|
| Self-adhering membrane | Behind the ledger, up the wall 12 in. or more | Seals the sheathing and blocks water that runs behind the ledger |
| Z-flashing | At the bottom edge of the ledger | Redirects water that would pool on top of the ledger |
| Drip cap flashing | Above the ledger, under the house wrap | Pushes water out and away from the wall |
| Copper flashing | In a masonry mortar joint | Bridges brick or block where membrane cannot be used |
| Bolts and washers | Through the ledger into the rim joist | Carry the load and space the ledger off the wall for drying |
Step-by-Step Flashing Installation
This sequence follows the layering order in an expert guide to flashing deck ledgers, which keeps the installation code-compliant from the first cut to the last bolt.
The Layering Sequence
- Strip the siding in the ledger zone so the membrane can reach the sheathing.
- Install the self-adhering membrane behind the ledger location, extending at least 12 inches up the wall and tucked behind the house wrap.
- Mark the bottom of the ledger and install a short piece of z-flashing at that line.
- Set the ledger and bolt it to the house, placing 3 or 4 flat washers between the board and the wall as spacers.
- Seal around every bolt head with exterior caulk as a second line of defense.
- Install the metal drip cap above the ledger, cutting small notches so joist tops stay flush with the ledger top.
- Pull the house wrap down over the top of the drip cap so water sheds onto the metal, not behind it.
- Install the decking and let the siding above terminate slightly above the deck surface so air can move through the joint.
Bolts Beat Lag Screws
Lag screws look convenient, but they lose their grip when they only bite an inch into the rim joist, which is exactly the situation at a ledger. Through-bolts with washers distribute the load across the full thickness of the rim joist and clamp the ledger firmly. The washers between ledger and wall are not an accident: they create a drainage gap so moisture escapes instead of pooling behind the board. Keep the bolt pattern to code spacing, typically two rows staggered, and torque everything before the decking goes down.
Flashing Ledgers on Brick and Other Wall Types
On sheathed walls with continuous insulation, you may need to mount a deck ledger over ZIP System R-sheathing or rigid foam, which changes fastener length and adds standoff requirements. The flashing sequence stays the same, but the bolts must reach the framing through the extra thickness, and the membrane has to bridge the foam without tearing.
Brick Veneer Detail
- Grind out the mortar along one row of bricks to create a clean slot.
- Fabricate copper flashing with a small hem on the end that fits into the joint, acting as a wedge.
- Bend a standard hem on the lower exposed edge so water drips clear of the wall.
- Insert the flashing into the mortar joint until it seats tightly.
- Seal the joint with a high-quality flexible caulk and let it cure before loading the deck.
The copper wedge detail works because the flashing becomes part of the masonry, not an afterthought glued to the surface. Mortar joints are the natural flashing plane on brick, and copper is the traditional material because it bends precisely and shrugs off the alkaline chemistry of mortar.
Continuous Insulation and Rigid Foam
Homes built with exterior rigid foam raise the ledger off the structure, so the bolts must be longer and the membrane must extend over the foam to the sheathing behind it. If the foam compresses under the ledger, the connection loses stiffness and the flashing plane shifts. Set the ledger with a spacer system that keeps the foam intact, then flash exactly as you would on a flat wall.
Code Requirements and Common Mistakes
Some manufacturers offer a flashing-free deck ledger attachment for T1-11 siding and similar panels, but understand what those systems do and do not protect. They address the connection, not the water path, so verify that the siding-to-ledger joint is still flashed and that the product is rated for your wall assembly and local code.
- Skipping the membrane behind the ledger.
- Relying on caulk alone at the top edge.
- Leaving siding trapped behind the ledger where it wicks water.
- Installing the drip cap below the house wrap instead of under it.
- Forgetting the spacer washers, so the ledger sits tight against damp siding.
- Using lag screws that barely reach the rim joist.
Code sets the floor: flashing at the ledger, bolts sized for the joist depth, and a ledger that is notched or shimmed to sit flat. Most jurisdictions inspect the ledger connection before the decking goes down, so the flashing detail is one of the few things you cannot fix after the fact without tearing the deck apart.
Signs an Existing Ledger Was Flashed Wrong
When you inspect an existing deck, look beyond the flashing itself: joist hanger gaps and ledger attachment code concerns often show up together, because the same rushed crew skipped both. A few minutes under the deck can reveal a decade of hidden water.
- Water stains or white mineral deposits on the rim joist below the ledger.
- Rust streaks running down the siding from the bolt heads.
- Caulk that is cracked, peeled, or was clearly the only water barrier.
- Decking that feels spongy near the house.
- Siding rot at the ledger line, visible from the side of the house.
- Joist hangers with missing nails or gaps between the hanger and the ledger.
If you find any of these, plan the repair like the original install: pull the decking near the house, strip the siding, replace rotted framing, and rebuild the flashing in the correct order. The layers are what keep the water out, and the layers are the same whether you are fixing an old deck or building a new one. Take the time to do the joint right and the deck will outlast the siding it is attached to.