Symptoms
- Board feels spongy or gives when stepped on
- Visible dark staining, cracking, or fungal growth
- Screwdriver sinks easily into the wood
Likely causes
- CommonStanding water or poor drainage under and around boardsDebris trapped between boards or against the house holds moisture against the wood.
- CommonFailed or worn-out sealant/stainUnprotected wood absorbs rain and stays wet longer, accelerating decay.
- PossibleGround contact or poor airflow underneath the deckLow clearance under the deck traps humidity against joists and boards.
What to do
- Probe suspect boards with a screwdriver across the deck to map out how widespread the rot is.
- Remove and replace individual rotted boards rather than patching them, since rot doesn't stop spreading on its own.
- Check the joists underneath rotted boards too, since rot often continues into the structure below.
- Improve drainage by clearing debris from between boards and ensuring water can run off the deck surface.
- Sweep out the gap between the deck and the house (the ledger area) where debris commonly collects and holds water.
- Apply a quality exterior wood sealant or stain to all deck boards, including cut ends of any replacements, once repairs are done.
- Reapply sealant every 1–3 years depending on the product and sun/weather exposure.
Tools and parts
- Screwdriver (for testing)
- Circular saw
- Drill/driver
Pressure-treated deck boardsSearch: “pressure treated deck boards”
Exterior deck sealant/stainSearch: “deck sealant stain”
When to call a pro
Call a deck contractor if rot has spread to the joists, beams, or posts, since that affects structural safety.
Safety
Use caution
Avoid walking on boards you suspect are rotted until they're replaced; a soft board can give way underfoot.