How to Tell Marine Borer Damage From Rot Without Pulling the Pile

By Ed Brannigan, Marine contractor. Reviewed by Dana Whitcombe, technical reviewer.

Pile Driving & Piling Repair | Boat Lift Installation July 11, 1997. I was standing on a fiberglass skiff beneath a private dock in Fernandina Beach, Florida. The owner, a retired airline pilot, leaned over the rail and pointed at a creosoted pine fender pile. 'That one's dry rot,' he said. I tapped it once with a ball-peen hammer. It answered with a hollow, jangling ring, not the solid knock of sound wood. Then I scraped the green slime at the waterline and saw the pinhead holes, each with a rim of white chalk. I handed him the hammer. 'Listen,' I said. 'That's not rot. That pile is packed full of shipworms.' I'd spent the better part of two decades before that day mistaking borer damage for rot and losing money on repair quotes. By 1997 I knew better. The two look similar from a dock, but the fix is different: a rotten pile can often be cut back, treated, and spliced. A borer pile is a full replacement and the outbreak can spread to every pile in the neighborhood. You don't have to pull a pile to know which one you're buying. Here's how I've read piles for three decades. It takes ten minutes, a hammer, a long screwdriver, and the willingness to get a little filthy.

The Holes and the Dust

First, clean a patch of the pile at the waterline using a wire brush or the flat of your hand. Look for entrance holes. Shipworm damage shows as round, pinhead-sized holes, from about 1/16 to 1/4 inch across. Around the openings, especially at low tide, you'll often see brittle, white calcareous tubes that look like dried glue. Gribble, the other common marine borer, leave a dense mat of fine frass that looks like pencil shavings or sawdust. Rot, in contrast, almost never starts with round holes. It starts as cracks, stains, and a soft crumbling surface. When rot finally makes holes, they're irregular and not all the same size. Also get a whiff: rot smells like a wet cellar, musty and earthy. Borer damage has little smell. Holes plus chalky tubes plus no stink points to marine borers.

Tap and Probe

The quickest blind test is to tap the pile with a hammer. Sound wood rings. Dry rot gives a dull, thudding, papery sound. But here's the trap: a borer pile often sounds bright because the outer half-inch of wood is still intact and acting like a drumhead. So always follow a tap with a probe. I use an 8-inch insulated screwdriver, the kind electricians carry, with a fat handle. Set the tip square to the wood and lean on it. In rot, the tip sinks in steadily, with a soft, even resistance. In borer damage, you'll feel it poke in, then hit the interior of a tunnel, then break through into an empty void, then meet wood again. That alternating hard and soft feel is the signature of shipworm galleries. Rot gives you one uniform mush.

Read the Waterline

Marine borers need salt or at least brackish water, so their damage is concentrated in the tidal zone and below. Look for a band of deterioration that follows the waterline, not the structural details of the pile. In shipworm damage, the worst of it is right where the wood meets water. Rot, however, needs oxygen and moisture but not salt. It tends to attack wherever water sits, which is often the top of the pile, behind fender boards, under chocks, or at the knob where a mooring line wears. So a pile that looks sound up high but is perforated for two feet around the waterline is almost always borer work. A pile that stays wet on top, on a deck, or in a protected pocket is often rot.

The Shape That Tells

Stand back and look at the whole pile. Rot consumes the outer surface first, so a pile with severe rot gradually loses its round cross-section. The top begins to look like a blunt lead pencil, drawn down to a softer core. The wood becomes fibrous and stringy, and you'll see deep checks that follow the grain. Borer damage is the opposite. It starts from the outside but the major destruction is in the interior. The outer shell usually remains intact, so the pile keeps its full, round shape. That's why a borer pile is so sneaky: it can be a hollow pipe near the water and still look fat and healthy from twenty feet away. If the silhouette looks normal all the way down but the sound is hollow, think borers. If the silhouette is pinched and stringy, think rot.

The Neighborhood

Borers don't operate in singles. They spawn in warm water and their larvae spread through the dock, so you'll find the same pattern on neighboring piles at the same tidal height. If you feel that hollow sound on one pile, check the pile ten feet down the row. You'll likely see it there too. Rot is a more local disease. It's often triggered by a bad repair, a collection of leaves, or a crack that holds rainwater. It will show up on one side of one pile while its neighbor stays clean. When you see identical damage on several piles, all at the same elevation above or below water, that's the fingerprint of marine borers.

The Drill Is the Proof

If the evidence still isn't clear, drill a test hole with a portable brace and a 1/2-inch auger bit. Put the hole at or just above the waterline so you don't risk your hands. Pay attention to the shavings. Rot comes out as a damp, fibrous, crumbling mush that smells like soil. Borer damage comes out as coarse, ridged shavings, sometimes with fragments of the white shell-like lining. And if the drill bit suddenly spins free for an inch after fighting through a tough layer, you've just opened a shipworm tunnel. Patch the hole with a treated dowel and a plumber's plug. You've barely reduced the pile's strength. That's the last test I'll rely on before I order a replacement.

Frequently Asked Questions

I see small holes at the waterline but no sawdust. Could it still be borers? Yes. Shipworm holes are often clean because the boring dust is expelled behind them and washes away. You may see a chalky, brittle tube around the hole, but not always. Gribble leave a fine frass, but shipworm holes often look 'too clean.' What if the pile sounds solid when I tap it but the probe sinks in? Trust the probe. Marine borer damage often keeps a strong outer shell that rings like a drum. The probe will find the alternating hard and soft layers that rot doesn't have. If you feel those voids, it's not rot. Is there any definitive way to tell without cutting into the pile? A lab test on a core sample is definitive, but that's overkill. The probe and drill test above the waterline give you a decisive answer in the field. If you still doubt, dive and examine the last few feet underwater, but you don't need to pull the pile to know.