Why does the wood in my older home sound hollow when I tap it?

What hollow-sounding wood in an old house is telling you

Wood that sounds hollow when you tap it is wood that's been eaten out from the inside. The surface looks fine, but there's not much behind it.

In an older home, termites are the leading cause. They tunnel through the interior of a board and leave a thin outer skin intact, which is exactly why the sound changes from a solid thud to a papery drum.

This is one of the few pest symptoms I'd call an emergency, not because your house falls down tomorrow, but because the clock has already been running for a couple of years by the time you can hear it.

The good news: confirming it is cheap and fast, and a targeted treatment on a focused area often runs $150–$600. The trap is waiting, because damage keeps compounding while you decide.

What's actually eating the wood

Four culprits produce hollow wood, and telling them apart is mostly about the evidence they leave behind.

Subterranean termites are the top suspect in almost every region. They need soil contact and build pencil-thin mud tubes up foundation walls and piers. Break a tube open, and if it's resealed in a few days, the colony's alive.

Drywood termites skip the soil entirely and live inside the wood. No mud tubes. Instead they kick out tiny six-sided fecal pellets that pile up like coarse pepper below the infested wood. They're common along the Gulf Coast, in the Southwest, and in California.

Carpenter ants don't eat wood, they hollow it for nesting, usually in wood that's already damp. Their galleries are smooth and clean, and the sawdust they push out (frass) has bits of dead ant mixed in.

Then there's plain old rot. Fungal decay softens and hollows wood near leaks and windows, and it feels spongy and wet rather than clean-hollow. Rot and termites travel together because both need moisture, so finding one doesn't rule out the other.

• Mud tubes on foundation or piers: subterranean termites
• Pepper-like pellets and pinholes: drywood termites
• Frass with insect parts, large ants: carpenter ants
• Soft, wet, spongy wood, no tunnels: fungal rot
• Fine powder and tiny round holes: wood-boring beetles

Why older homes get hit harder

It's not bad luck, it's how these houses were built and how they've aged.

Homes built before the 1990s usually never got a soil termite pretreatment, which is standard on new construction now. That barrier just isn't there.

Decades of landscaping and mulch tend to raise the grade against the foundation, sometimes right up to or above the sill plate. That puts wood in easy reach of soil-dwelling termites.

Old galvanized and cast-iron plumbing leaks. Crawlspace vents get blocked or were undersized to begin with. Every one of those creates the damp wood termites and rot both want.

You also get more direct wood-to-soil contact in older builds: porch posts set in dirt, wood steps, buried form boards left in place after the pour. Each is a highway into the structure.

What to do right now

Start by mapping the problem so you're not guessing when the inspector arrives.

Tap the entire run of trim or framing, not just the spot that caught your attention. You want to know if it's one board or ten feet of wall.

Do the screwdriver test in a few places. Clean tunnels packed with soil point to subterranean termites. Hollow smooth galleries suggest carpenter ants. Wet mush is rot.

Get into the crawlspace or basement and scan the foundation, piers, and sill plate for mud tubes. Photograph everything.

Sweep the floor near suspect wood and windowsills. Bag any pellets, wings, or frass. Those samples let an inspector ID the species fast.

Hunt down the moisture. A dripping trap, a downspout dumping at the foundation, or grade sloping toward the house all feed the problem.

Then call two or three licensed pest companies. Many do free or cheap termite inspections, and comparing scopes protects you from overselling.

What this costs in 2026

Your DIY costs here are diagnostic, not extermination. That's it for confirming what you're dealing with.

A targeted professional treatment on a focused area, say a single infested wall or a specific foundation section, typically runs $150–$600.

A full perimeter subterranean treatment on an average single-family home usually lands between $1,200 and $2,500, depending on linear footage and whether they trench-and-treat or install a bait system.

Bait systems often add an annual monitoring fee, commonly $250 to $400 a year, which is fair for the ongoing protection.

Drywood termites in one area may be spot-treated for a few hundred dollars. Widespread drywood infestations can require whole-house fumigation, which runs roughly $2,500 to $6,000 based on square footage.

Wood repair is separate and depends entirely on what's damaged. Swapping a length of baseboard is cheap. Sistering joists or replacing a section of sill plate can run $1,000 to $5,000 or more once you're into structural carpentry.

• Professional repair typically runs $150–$600. This is not a DIY job, so plan on hiring a pro.
• Targeted spot treatment: $150–$600
• Full perimeter subterranean treatment: $1,200 to $2,500
• Bait system annual monitoring: $250 to $400/year
• Whole-house fumigation (drywood): $2,500 to $6,000
• Structural wood repair: highly variable, often $1,000 to $5,000+

What you can do yourself and what you can't

You can do all the detective work: tap-testing, probing, spotting mud tubes and pellets, and tracking down the moisture source. That's genuinely valuable and it saves the inspector time.

You can also fix the conditions that invited them. Regrade so soil slopes away from the foundation, pull mulch back off the siding, repair leaks, and clear crawlspace vents. Do this regardless of who treats.

What you shouldn't do is try to kill the colony with hardware-store products. Foaming a mud tube or spraying a baseboard feels productive, but it usually just relocates the colony to a spot you can't see.

Termite treatment is a licensed trade for a reason. It involves trenching and injecting soil termiticides around the foundation, drilling and treating slabs, or fumigating an entire structure. The chemicals and methods aren't homeowner-grade.

So the honest line is this: diagnosis and prevention are yours, extermination and structural repair belong to licensed pros.

Can this wait?

Short answer: no, not really. If the wood already sounds hollow, the colony has had a couple of years to work, and it's still working.

The cost curve is brutal. A problem caught at the trim stage might mean a $300 treatment and a $60 board. Let it reach the joists and sill, and you're looking at structural carpentry that runs into the thousands.

Every wet season pushes subterranean activity harder against your foundation, and every spring swarm can start a fresh colony nearby.

If you've found active mud tubes, live insects, or fresh pellets, put this at the top of the list. Book an inspection this week.

If the damage looks old and inactive (no fresh evidence after a week of monitoring), you have a little breathing room, but still confirm it with a pro before you assume it's dormant.

Safety while you investigate

Hollow structural wood can fail. If a joist, beam, or sill plate sounds hollow and the floor above feels bouncy, keep weight off it until it's assessed.

The same damp spots that draw termites grow mold and fungal rot. Wear an N95 and gloves when you're probing into rotted or frass-packed wood.

Crawlspaces are their own hazard. Watch for standing water near live wiring, low clearance, and old insulation. If water and electrical are mixed, shut the circuit or wait for a pro.

Leave the chemistry to the licensed applicator. Termiticides and fumigants are effective because they're strong, and they're not meant for casual use.

How the pros actually confirm and treat it

A good inspector doesn't just look at the hollow board. They read the whole house for conducive conditions and access points.

They'll sound the wood, probe suspect areas, and use a moisture meter to separate wet rot from dry termite galleries. Many carry infrared cameras to spot moisture and activity inside walls without opening them.

For subterranean termites, the two main treatments are a liquid soil barrier (trenching and injecting termiticide around the foundation, drilling through slabs where needed) and bait stations placed in the soil that the colony carries back and feeds to the queen.

Liquid barriers work fast and protect a large perimeter. Baits are slower but eliminate the whole colony and provide ongoing monitoring. Plenty of pros combine them.

For drywood termites, spot treatments (injected foams and dusts) handle isolated pockets, while widespread infestations get whole-house fumigation under a tarp.

After treatment, a reputable company documents the work, often provides a warranty or bond, and schedules follow-ups. Keep that paperwork. It matters at resale and for any future claim.