The earliest signs of termites on a deck are mud tubes running up posts or piers, small piles of sand-grain-sized pellets beneath the wood, and a hollow sound when you tap boards that look intact on the surface. Catching these signs early matters: average termite treatment runs $1,000 to $3,000, while replacing structural wood can cost $2,000 to $10,000 or more if damage has spread.
Mud Tubes on Posts and Framing
Subterranean termites, the most common type to attack decks, build pencil-width tubes out of mud along any surface that connects soil to wood. These shelter tubes protect the colony from open air and sunlight as workers travel between their underground nest and your deck lumber. You’ll most often spot them running vertically up deck posts, support piers, concrete footings, or along the ledger board where the deck attaches to your house.
Sometimes the tubes are obvious. Other times, the only visible clue is small spots of dried mud on the edge of a board. These are “pinholes,” created when termites accidentally cut through to the outside and quickly patch the opening with mud. If you scrape away one of these mud spots and find a tiny hole underneath, mark it and check back in a few days. If the hole gets sealed again, termites are actively working inside that wood.
Frass Piles and Kick-Out Holes
Drywood termites leave a different calling card. Instead of building mud tubes, they push their waste out of tiny openings in the wood called kick-out holes. The waste, known as frass, collects in small piles on surfaces below the infested boards. Look for these piles on the ground beneath your deck railing, along joists, or on the deck surface itself near post connections.
Drywood termite frass consists of tiny hard pellets roughly the size of a grain of sand. Under magnification, each pellet has six concave sides, giving it a distinctive shape. The pellets are dry and uniform in color, usually matching the shade of the wood the termites are eating. This is the key way to distinguish termite frass from carpenter ant debris, which looks more like coarse, rough sawdust mixed with insect body parts and random fragments. Termite droppings are smooth and pellet-like; carpenter ant waste looks shredded.
One reliable test: clean up the pile completely and check the same spot a few days later. If a new pile appears, termites are still actively feeding and pushing waste out of their galleries.
Hollow or Soft Wood
Termites eat wood from the inside out, leaving a thin outer shell that can look perfectly normal. Tapping suspect boards with the handle of a screwdriver reveals what’s happening underneath. Solid wood produces a firm, sharp sound. Infested wood sounds hollow or papery.
You can also press a flathead screwdriver or probe into the wood surface. If the tool sinks in easily or the wood crumbles, the internal structure has been consumed. When you break open damaged wood, look for tunnels (called galleries) running along the grain. If those galleries contain dried mud or soil, that’s termite damage. If you see live, pale-bodied insects inside, the infestation is actively progressing.
Structural symptoms show up in later stages. Deck boards that feel spongy underfoot, railings that wobble when they didn’t before, or visible sagging in the framing all point to significant wood loss that has been building for months or years.
Swarmers and Discarded Wings
Once a year, usually in spring or early summer, mature termite colonies release winged reproductive adults called swarmers. These insects look similar to flying ants but have straight antennae, a thick waist, and two pairs of wings that are equal in length. After landing, they shed their wings almost immediately.
Finding piles of small, translucent, equal-sized wings on your deck surface, around light fixtures near the deck, or on nearby windowsills is a strong indicator that a colony is established close by. The swarmers themselves are often mistaken for flying ants, but the wing shape and body proportions are distinct if you look closely.
Where to Look First
Termites need moisture, and they enter through soil-to-wood contact. That makes certain parts of your deck far more vulnerable than others.
- Posts where they meet the ground. Any deck post sitting directly in or on soil is the single easiest entry point for subterranean termites. Exterior woodwork should sit at least 6 inches above ground level to allow for inspection and reduce risk.
- The ledger board. This is the horizontal board that bolts your deck frame to your house. Moisture often gets trapped behind it, and termites that reach this point can move from the deck into your home’s framing.
- Stair stringers. The angled boards supporting your deck stairs frequently rest on or near soil, creating direct access for termites.
- Joists near planters or stored items. Anything that traps moisture against the wood, including planters, tarps, firewood stacks, or garden equipment, raises the humidity around those boards and attracts termites.
Cellulose debris under the deck also increases your risk. Cardboard boxes, scrap wood, old mulch piled against posts, or even thick leaf accumulation can serve as a food source that draws a colony closer to your structure.
How to Do a Basic Inspection
You don’t need special equipment for a first pass. A flashlight, a flathead screwdriver, and 20 minutes will cover the basics. Start at ground level and work your way up. Check every post where it contacts the soil or footing, looking for mud tubes, soft spots, or mud-patched pinholes. Run the screwdriver along the bottom edges of joists and beams, pressing into the wood every few inches to test for softness. Tap boards and listen for hollow sounds.
Pay extra attention to any wood that stays damp. If a section of your deck doesn’t dry out well after rain, or if you notice existing water damage like discoloration or soft rot, termites are more likely to target that area. Some species feed exclusively on wood that has already been weakened by moisture, so water damage and termite damage often show up in the same spots. A cheap moisture meter (available at most hardware stores for under $30) can help you identify problem areas that look dry on the surface but hold moisture inside.
Professional inspectors follow a similar process but with more experience reading subtle signs. They typically start at the exterior foundation perimeter, then move to the crawlspace, checking every connection point with a probe and flashlight. An annual professional inspection runs roughly $300 to $800 as part of a termite protection plan, depending on the size of your home and deck.
Old Damage vs. Active Infestation
Finding galleries packed with dried mud doesn’t necessarily mean termites are there right now. If no live termites are present and no fresh mud tubes or frass appear, the damage could be from an infestation that was treated or naturally died off years ago. The wood is still compromised structurally, but active treatment may not be needed.
Signs that an infestation is currently active include live termites visible in broken wood, mud tubes that are moist or rebuilt after you remove them, fresh frass piles that reappear after cleaning, and swarmers emerging from the deck structure. If you’re unsure whether damage is old or active, a professional inspection can confirm it quickly with tools like moisture meters and more thorough probing of hidden framing.