Why did my balcony rot where it meets the house, and what is flashing?

What flashing does

Water on a wall runs down. When it reaches a horizontal interruption, such as the top of a ledger board, a windowsill, or a trim band, it has to go somewhere. Flashing is a shaped piece of sheet metal or self-adhered membrane that catches that water and sends it out and over the face of the obstruction instead of behind it. It works by lapping: everything above drains onto everything below, like shingles.

At a balcony, the critical flashing sits on top of the ledger, tucks up behind the wall's weather-resistive barrier and stucco or siding, and kicks out over the face of the ledger. A second piece often protects the joist hangers and the top of the rim. Done right, water never reaches the wood.

What happens without it

The ledger gets wet on its back face, against the house, where there is no air and no sun. The joist ends sitting in hangers on that ledger get wet too. Behind the ledger, the house's rim joist, sheathing, and sometimes the wall plates take on water. Because it is enclosed, this wood stays damp for weeks after a storm, and the damage happens out of sight for years.

On stucco houses, which is most of Carlsbad, the problem has a particular shape. Stucco absorbs water and holds it. A ledger bolted tight to the stucco wicks moisture from it even in dry weather. If the stucco was never cut back and the ledger never flashed, the wood is in a permanent damp sandwich.

How we found it on one balcony

The owner called about a railing that kept losing pickets. The real problem was underneath: the joist ends and the ledger area had rotted where water had been sitting for years. There was no flashing at the wall at all. Levi removed the railing and decking, cut the joists back to sound wood, sistered and replaced framing, and flashed the wall connection properly before closing anything up. The railing was the symptom; the missing flashing was the cause.

What a proper repair includes

Opening the joint so the flashing can be integrated with the wall, not just surface-caulked. On a stucco wall that means cutting back a band of stucco so new flashing tucks behind the building paper. A corrosion-resistant flashing material compatible with treated lumber and the local salt air; galvanized steel does not last long near the coast, so we look at stainless or heavy-gauge coated options. Spacers or a drainage gap between the ledger and the wall where the design allows it, so water that does get in has a way out. Proper ledger fasteners, replaced if the old ones were corroded.

Caulk is not flashing. Sealant has a short life in the sun and fails exactly where it is needed most. Flashing works by geometry, which does not wear out.

• Flashing lapped behind the wall barrier, not caulked to the surface
• Material chosen for coastal exposure and treated lumber
• Ledger inspected, re-fastened, or replaced
• Joist hangers and ends checked and replaced as needed
• Rebuilt so water drains out and away

What the code expects

The California Residential Code, adopted through the Building Standards Commission from the model codes, requires flashing where decks attach to a building so water is directed away from the framing. The specifics of lapping and material are detailed in the code and in manufacturer instructions. For a homeowner, the takeaway is that a deck or balcony attached to a house without flashing is a defect, not a style choice, and fixing it is what makes a rot repair permanent.

Related questions

Sources and further reading