5 Signs Your Deck Joists Are Rotting Underneath (Check Now)

TL;DR: Rotting deck joists usually show up as a bouncy or sagging deck surface, soft or spongy wood when probed with a screwdriver, dark staining or mushrooms near the ledger board, and rust streaks around fasteners. If a screwdriver sinks more than 1/4 inch into a joist with light pressure, that wood has lost most of its strength and the deck needs inspection before anyone stands on it.

_Last reviewed: July 2026 · 7 min read_

A deck that looked fine last summer can be quietly failing underneath, and most owners don't find out until someone falls through. The joists hold the whole structure up, but they're hidden under the decking boards, so rot has years to spread before it's visible from above. Here are the five signs that tell you what's happening below without tearing up a single board.

Okoniq Property Hub logs your deck's build date, lumber type, and inspection dates so you're not guessing how old the joists are or when they were last checked.

What does a bouncy or sagging deck floor actually mean?

A soft bounce when you walk across the deck means the joists have lost stiffness, and that only happens when wood fiber has started to break down. Healthy joists, whether pressure-treated pine or cedar, should feel solid underfoot with no give. If you notice a trampoline-like flex, especially in the middle of a span between beams, the joist there has likely lost 20% or more of its structural integrity to fungal decay.

Sagging is a step further than bounce. If you kneel down and sight along the bottom edge of the deck from one end, joists should run in a straight line. A dip of even half an inch in a 12-foot span usually means the wood is compressing under load because it's rotted through part of its cross-section. This is the same mechanism that shows up with slab leaks under a floor — moisture works invisibly until the structure finally gives.

How do you check if the wood is actually soft?

Push a flathead screwdriver or an awl into the joist at a shallow angle and see how far it goes. Sound, dry wood resists penetration and you'll feel firm contact within an eighth of an inch. Rotted wood lets the tool sink in with almost no resistance, sometimes an inch or more, and it may come out damp or crumbly rather than splintered.

Test every joist near the ledger board (where the deck attaches to the house), at the ends where joists meet the rim board, and anywhere the decking above shows discoloration. These are the three spots where water collects and rot starts first, roughly 70% of deck failures investigated by structural engineers trace back to the ledger connection. Homeowners often assume metal flashing problems are cosmetic, but they behave the same way downspouts dumping water in the wrong place do to a foundation: slow, silent, then sudden.

What do stains, mold, and mushrooms near the joists tell you?

Dark streaking, white fuzzy growth, or actual mushrooms sprouting from a joist mean active fungal decay is underway, not just old water damage. Wood-decay fungi need four things to thrive: oxygen, a food source (cellulose in the wood), a temperature between 32°F and 100°F, and moisture content above 20%. A deck built over a shaded, poorly ventilated crawlspace hits all four conditions for most of the year.

| Sign | What It Usually Means | Urgency | |---|---|---| | Gray/silver surface graying | Normal UV weathering | Low | | Dark brown-black staining | Active moisture retention, early rot | Medium | | White or orange fungal growth | Decay fungus actively feeding | High | | Mushrooms or soft crumbling wood | Advanced structural decay | Immediate |

If you see mushrooms, treat it the way you'd treat 8 warning signs of a roof leak before water comes in: the visible sign is late-stage, and the damage underneath has usually been building for a year or two already.

Why do rust stains around fasteners matter?

Rust bleeding from screws or joist hangers means metal connectors have been sitting in moisture long enough to corrode, and that moisture is touching the wood too. Galvanized joist hangers are rated to resist corrosion for the life of a treated-lumber deck, roughly 15 to 25 years, but if they're rusting in under 10 years, something is trapping water against the connection, usually a clogged gap, missing flashing, or ponding water from poor drainage.

Corroded hangers also lose holding strength well before they look obviously broken. A hanger that's lost 30% of its steel to rust may still hold static weight but fail under a dynamic load, like a group of people or heavy furniture. Check every hanger you can see from below, and if more than a couple show heavy rust, budget for full hardware replacement, not just spot fixes.

Should you worry about the wood species and age of your deck?

Yes, because untreated or improperly treated lumber rots faster regardless of how well you maintain it. Pressure-treated pine typically lasts 15 to 20 years structurally if properly sealed and drained, while untreated joists or lumber with expired treatment can fail in half that time. If your deck was built before 2004, it may use older chromated copper arsenate (CCA) treatment, which behaves differently than today's ACQ or MCA treated lumber, and it's worth knowing which one you have when planning repairs.

FAQ

How much does it cost to replace rotted deck joists?

Replacing joists typically runs $50 to $100 per linear foot including labor, and a full joist replacement on a 300-square-foot deck often lands between $3,000 and $6,000 depending on decking removal and disposal.

Can you repair a rotted joist instead of replacing it?

Sistering a new joist alongside the damaged one works if less than a third of the original joist is compromised, but if rot has reached the ledger connection or spans more than 4 feet of a joist, full replacement is safer.

How often should deck joists be inspected?

Inspect visible joists and hardware once a year, ideally in early spring before heavy summer use, and do a closer probe test every 3 to 5 years, or sooner if the deck is more than 10 years old.

Is a bouncy deck always dangerous?

Not immediately, but any noticeable bounce that wasn't there when the deck was new signals reduced structural capacity and should be checked with a screwdriver test within the same season it's noticed.

What's the earliest sign of joist rot before it's visible?

A musty smell coming from underneath the deck, especially after rain, often shows up before any visible staining or softness and is worth investigating with a flashlight and probe tool.

This is educational information, not structural engineering advice. Call a licensed contractor or structural engineer if you find soft wood, sagging, or active decay before using the deck again.