How do I keep water and moisture from rotting the underside of my deck?

Where the water comes from here

Carlsbad gets little rain, so people assume decks stay dry. They do not. From late spring through summer the marine layer condenses on every surface overnight, and a deck can be wet at 7 a.m. for weeks on end. Irrigation adds more, often daily, and sprinklers aimed at a lawn routinely soak the posts and the underside of a deck beside it. Then there are the planters and hose bibs that live on decks. The total is a lot of water, delivered in small, frequent doses, which is exactly how rot likes it.

The damage lands in predictable places: the tops of joists under the gaps between boards, where drips land and cannot evaporate; the ends of boards and joists, where end grain drinks; the ledger area at the house; the bottoms of posts and stair stringers; and anywhere a skirt or a planter keeps air away.

Drainage: getting water out

Start with the ground. Soil under a deck should slope away from the house, and a layer of gravel over landscape fabric keeps weeds down and lets water pass without splashing mud onto the frame. Downspouts should not discharge under a deck. Irrigation heads should be adjusted or capped so they do not wet the structure; it is the single cheapest fix on the list.

On the deck itself, boards need a consistent gap so water falls through instead of pooling, and the frame should have a slight pitch away from the house on a low deck or over a slab. Where a deck sits over a living space or a patio you want to keep dry, an under-deck drainage system of sloped panels or membrane over the joists collects the water and carries it to a gutter. Those systems also keep the joists dry, which is a structural benefit as much as a comfort one.

Ventilation: getting air in

A deck frame that gets wet every morning is fine if it is dry by evening. That takes moving air on both sides of the boards and all around the joists. Composite and PVC manufacturers make this a requirement: most call for open ventilation under the deck and a minimum clearance under the joists, and some void the warranty if the deck is sealed. Wood wants the same thing for the same reason.

The enemy of ventilation is solid skirting. A closed lattice packed tight, a solid board skirt with no vents, or stacked firewood and storage bins under the deck all stop the air. Our answer on fascia and skirting covers how to close the underside visually while keeping it open to air.

Protecting the wood where water will sit anyway

The balcony we rebuilt in North County had none of this. The joist ends had been wet for years behind an unflashed wall connection, and the railing falling apart was just the visible symptom. Every item on this list would have prevented it.

• Joist tape or a metal cap on every joist, beam and ledger top before the decking goes on.
• End-grain sealer on every cut end of framing and decking.
• Post bases that hold posts an inch above the concrete, never buried.
• Flashing over the ledger, lapped into the wall's weather barrier.
• A gap and flashing where decking meets stucco or siding, so boards do not wick water into the wall.
• Stair stringers landing on a concrete pad or footing, not on soil.

An annual half hour

Once a year, ideally before the May marine layer sets in, get under the deck with a flashlight. Clear leaves and debris from between boards and off the joist tops. Look for staining under the ledger and dark streaks down the posts. Probe any soft-looking wood with a screwdriver. Check that sprinklers still point where they should. If the underside is too low to see, a camera on a phone slid between the boards does the job.

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