Deck Rot: Where to Check, How to Probe, and When a Deck Is Repaired or Rebuilt

• The ledger. The board fastened to the house that carries one end of every joist. Water runs down the siding and collects between ledger and house framing. The 2025 California Residential Code requires flashing above the ledger that either extends at least 4 inches beyond the ledger face or reaches the face and turns down at least ¼ inch (CRC § R507.9.1.5), with the house wrap lapped over it (§ R507.9.1.6). The code's exception is a ledger held at least ¼ inch off the wall covering, so that water drains behind it. A ledger with no visible flashing is the first thing to probe.
• Joist ends at the ledger. Cut ends take up water faster than face grain, and a hanger holds water against the end.
• Post bases. A post standing on concrete without a standoff base wicks water from the footing; a post set in soil rots at the ground line.
• Under the boards at the door. The threshold leaks and the boards are tight, so the joist below never dries.
• Railing posts. A post notched over the rim collects water in the notch and splits at the bolts.

Decay fungi need moisture, oxygen and warmth. Wood kept at or below about 20 percent moisture content does not decay; serious decay occurs only above the fiber saturation point, which averages 30 percent (USDA Forest Products Laboratory, Wood Handbook, 2021 edition, ch. 14). "Dry rot" describes what the wood looks like after the fungus has eaten it — dry, brown, cracked into cubes — not how it got that way.

• Look from below. Dark streaks running down from the ledger, white or brown growth on the joists, rust streaks from hangers.
• Probe. Push an ice pick or a screwdriver into the suspect spots: the bottom edge of the ledger, the ends of several joists, each post at its base. Sound wood stops it; if the tool goes in ¼ to ½ inch with little resistance, or the wood is soft and spongy, decay may be present (North American Deck and Railing Association, Check Your Deck checklist).
• Check the flashing. From above, at the house wall: is there metal or membrane visible between the deck and the siding? If not, the ledger is the priority.

The North American Deck and Railing Association's deck evaluation guide is written as a checklist for an annual deck inspection.

The number of soft spots is not the test. What the contractor weighs:

• Where the rot is. Boards and a railing post are replaceable parts. Joists, beam, ledger and posts carry the deck; rot in them is a structural question.
• How much sound wood is left in the member and whether the load can be carried while it is replaced. A ledger can be replaced with the deck temporarily supported; whether that is sensible depends on the rim joist and wall behind it.
• The condition of what the rotted part connects to. A rotted joist end usually means the ledger or the hanger has been wet; a rotted post base says something about the footing and drainage.
• Whether the deck was ever flashed. A deck with no flashing at the house and rot at the ledger is a rebuild of that connection whatever else is sound.

Replacing boards is a repair. Replacing a joist or a post is a repair when the members around it are sound. When the ledger, beam or several framing members are decayed, the contractor will tell you whether the deck is rebuilt or the frame is replaced under the existing boards.

What Berkeley showed: the balcony that collapsed in June 2015, killing six, had been framed in 2005–2006. The state's investigation found dry rot along the top of the cantilevered joists; the framing had been built with a water-absorbent sheet material, and the construction logs made no mention of protecting it from the weather through a wet winter (Contractors State License Board findings, reported by Berkeleyside, June 2, 2017). Structural decay can be hidden from above; moisture protection at the framing is what prevents it.

• Photograph the deck from underneath, including the ledger and at least one post base.
• Photograph the strip where deck meets house from above.
• Probe three spots and say which ones gave.
• Say how old the deck is, if known, and whether it has been re-boarded.

Often, when the members around them are sound. When several joists are soft at the ledger, the ledger and its flashing are usually the cause, and that connection is what gets rebuilt.

Not always. The ledger can be replaced with the deck temporarily supported. Whether that makes sense depends on the condition of the rim joist and wall behind it, which the contractor can only judge on site.

The boards are. The joists, ledger and posts under them are usually wood and rot the same way. A composite deck with a wood frame still needs the frame checked.

The North American Deck and Railing Association's guidance is built around an annual inspection. California's balcony-inspection laws for multifamily buildings use six- and nine-year cycles (balcony inspection law guide); a single-family deck has no state rule.

Replacing boards in kind does not change the structure; replacing framing or the ledger can. See when a fence or deck needs a permit.

Cited from the 2025 California Residential Code with January 2026 errata, § R507.9.1.5 for ledger flashing and its exceptions and § R507.9.1.6 for the water-resistive barrier; the USDA Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material, ch. 14, Biodeterioration of Wood (FPL-GTR-282, 2021); the North American Deck and Railing Association, Check Your Deck consumer checklist and deck evaluation guide; and Berkeleyside, "Water-absorbent material caused fatal balcony collapse, state says", June 2, 2017.