Symptoms
- Board flexes or feels spongy when stepped on
- Discoloration, dark staining, or fungus growth on the wood
- Wood crumbles or splinters easily when probed
Likely causes
- CommonProlonged moisture exposure without adequate drying timeBoards in shaded or poorly ventilated areas stay damp longer and rot faster.
- CommonFailed or missing sealant/stain allowing water absorptionUnsealed wood absorbs water directly into the fibers.
- PossibleDebris trapped between boards holding moistureLeaves and dirt between boards trap water against the wood.
- PossibleGround contact or poor drainage beneath the deckBoards near ground level or low spots stay wet longer after rain.
What to do
- Probe suspect boards with a screwdriver; if it sinks in easily or the wood crumbles, the board is compromised.
- Mark all soft or rotted boards for replacement rather than attempting to patch them.
- Check the joists underneath rotted boards as well, since rot can spread downward.
- Remove the damaged boards and replace with new, pressure-treated lumber matched to the existing deck.
- Seal all new boards, including cut ends, with an exterior wood sealant before or immediately after installation.
- Improve drainage and airflow under the deck if standing water or poor ventilation contributed to the rot.
Tools and parts
- Screwdriver (for probing)
- Circular saw
- Drill/driver
Pressure-treated deck boardsSearch: “pressure treated deck boards”
Exterior wood sealantSearch: “exterior deck wood sealant”
When to call a pro
If rot has spread to the structural joists or support posts, a licensed contractor should assess the deck's overall safety.
Safety
Use caution
Avoid stepping directly on soft or rotted boards, since they may not support your weight and can cause a fall.