How do I know if my deck has wood rot or termite damage?
Short answer
Rot shows as wood that is dark, soft, spongy or crumbly when probed, often with a musty smell and a finish that peels or bubbles above it; it concentrates where water sits, at board ends, joist tops, the ledger and post bases. Termite damage shows as hollow-sounding wood with a honeycomb of tunnels inside, pellets like fine sawdust below (drywood termites) or mud tubes running up posts and walls (subterranean termites). Both are common in North County and both are repairable if caught before the structure is compromised.
Rot: what it is and where it lives
Wood rot is fungus. It needs moisture, oxygen and something to eat, and a deck frame that stays damp supplies all three. In our climate the moisture comes less from rain than from the marine layer, irrigation and water trapped where it cannot dry: on top of joists under the board gaps, inside the end grain of boards and joists, behind an unflashed ledger, and at the base of posts set in soil or concrete. What people call dry rot is the same fungus; the wood is dry by the time you find it because the fungus has already consumed it.
Rotted wood looks darker than its neighbors, sometimes with a gray or white bloom, and the finish above it often bubbles, cracks or peels. It feels soft or spongy under a thumbnail, and a screwdriver sinks in with little resistance. Advanced rot crumbles into cubes or stringy fiber. A musty smell under the deck is a hint.
Termites: two kinds in San Diego County
Drywood termites live inside the wood with no contact with the ground. They are common in coastal Southern California and leave two tell-tale signs: small kick-out holes and little piles of hard, six-sided pellets that look like coarse sand or coffee grounds, often on a board or the ground under a railing or beam. The wood sounds hollow when tapped and, cut open, shows smooth galleries running with the grain.
Subterranean termites nest in the soil and travel up into wood through mud tubes, pencil-thin tunnels of earth on the face of posts, foundation walls or the inside of skirting. They need moisture and they love a post buried in dirt or a stringer resting on the ground. Their galleries are packed with soil and look dirtier than drywood tunnels. Pressure-treated lumber resists them but is not immune at cut ends and where the treatment has weathered.
Telling rot from termites, and why it matters less than you think
Rot is soft and wet-looking and tracks water; termite damage is hollow and often in otherwise sound-looking wood with the surface intact. Pellets or mud tubes settle the question. In practice the two travel together, since termites favor wood that has been softened by moisture, and the fix for the deck is the same: find the extent of the damage, replace or sister the affected members, correct the moisture or soil contact that invited the problem, and have a pest control professional treat active termites. Structural repair and pest treatment are separate trades, and we work alongside pest companies rather than in place of them.
Where to look first
- Post bases, especially any post in soil, in a planter or in a concrete pour with no metal base.
- The ledger and the house framing behind it; probe it and look for staining above.
- Joist ends at hangers and joist tops under board gaps.
- Board ends and anywhere two boards butt.
- Stair stringer bottoms and the top stair connection.
- Railing post bottoms and the underside of the top rail at joints.
- Pergola or porch posts where they meet the deck or a slab; the porch column we rebuilt on a brick base in North County had been sitting directly on pavers.
What repair looks like
Good repair removes the damaged wood back to sound material, replaces or sisters the member with properly treated lumber and corrosion-resistant connectors, and fixes the cause: flashing where there was none, a post base where the post was buried, a gap where boards wicked water from the wall, airflow where skirting sealed the underside. On the North County balcony we cut every joist back to sound wood, sistered and replaced framing, flashed the wall connection properly, and rebuilt the railing. Patching compound has its place on a cosmetic surface, not on anything that carries load.
Catching damage early is the difference between replacing a stringer and replacing a deck. Our safety check answer gives a yearly routine; this one tells you what you are looking for when you do it.
Related questions
Can rotted deck wood be repaired with epoxy filler?
On non-structural trim or a decorative post surface, a consolidant and epoxy filler can be a legitimate repair. On joists, beams, ledgers, posts and stringers the answer is replacement or sistering with sound lumber; filler does not restore strength.
Does composite decking get termites?
The composite boards themselves do not, since there is nothing for termites to eat in a capped board, but the pressure-treated frame beneath can, especially at cut ends and where it touches soil. Keeping the frame dry and off the ground protects it.
Who treats termites, the builder or a pest company?
A pest control professional inspects for and treats active termites; a builder repairs the structural damage and removes the conditions that invited them. The two are separate trades and both are usually needed.