How to Probe and Sound Wood for Hidden Termite Damage

Why this matters

Subterranean termites eat wood from the inside and leave the surface intact, so the damage a buyer finds after closing is usually damage an inspector could have heard but did not tap. Sounding and probing are how a WDI inspector finds what the eye cannot see. They are also how an inspector gets into trouble: a screwdriver run through a seller's painted trim, or a report that writes "termite damage" over a hollow sound nobody opened. The skill has two halves, finding the damage and writing down exactly what was seen versus what was only suspected. This card owns both. What termite damage, decay and carpenter ant work look like once exposed is in 'Pest Activity Signs Reference'; how the findings go on the form is in 'The WDI Report Form, Section by Section'.

Safety in the crawlspace and the attic

Crawlspace. Before entry, look in with a light: standing water, sagging or damaged wiring, animals, and any smell of gas or sewage. Water on the ground with wiring damage in it means you do not enter until the electrical hazard is dealt with by the right trade. A gas smell means everyone leaves the house and the call goes to the gas utility from outside. Wear a bump cap, eye protection and gloves; probing overhead drops debris into your face. Crawls often carry rodent droppings, so use the respiratory protection your shop specifies under a written program per 29 CFR 1910.134 and follow 'Rodent Droppings and Hantavirus for the Trades'. Tell someone you are going in, and keep your exit clear.

Wiring and pipes. Do not probe or strike within reach of wiring, junction boxes or plumbing. A screwdriver tip through cable sheathing on a sill is a shock and a fire.

Painted wood in older homes. In a house built before 1978, assume painted trim may carry lead paint. Sounding with a handle does not disturb it; probing it does. Avoid probing painted surfaces in finished areas, which you should not be doing without permission anyway.

The tools and what each one tells you

• A screwdriver with a solid plastic or wood handle, or a dedicated sounding tool. You tap with the handle and listen. Sound wood gives a sharp, solid knock; wood hollowed by galleries gives a dull, papery or hollow note, and the change is audible as you move from sound wood into damage.
• The same screwdriver's tip, or an awl, for probing a spot that sounded wrong. Sound wood resists the tip. Damaged wood lets it in with little force, and the hole shows you what is inside.
• A bright light held at a low angle, to read paint for blistering, rippling and pinholes, and to see mud in cracks.
• A moisture meter, because moist wood is both what attracts subterranean termites and what fungal decay needs. USDA Forest Products Laboratory guidance (the Wood Handbook) treats wood kept below about 20 percent moisture content as safe from decay; a pin meter reading above that at a sill or joist is a conducive condition whatever else you find.

The sequence

1. Start outside at the foundation. Look for mud tubes on the foundation face, wood in contact with soil, and grade at or above the sill. This tells you where the colony is most likely to be reaching the wood, which tells you where to tap first inside. See related: Mud Tubes: Working, Exploratory, Drop and Swarm.

2. In the crawlspace, sound the sill plate along its full length. Tap every foot or so, moving steadily so you can hear the note change. The sill sits on the foundation and is the first wood a colony reaches. Pay extra attention below bathrooms, kitchens, and anywhere a downspout or hose bib is outside.

3. Sound the rim joist and the joist ends where they bear on the sill. Then sound the subfloor around plumbing penetrations, the tub drain and the toilet flange from below.

 CRAWLSPACE SECTION, WHERE TO SOUND
   finished floor and baseboard above:
   read paint only, no probe without OK
 ==========================================
 subfloor    <- sound at tub drain, toilet
 --------+---------------------------------
 rim     |  joist  <- sound end on sill
 joist   |
 --------+---------------------------------
 SILL PLATE  <- tap full length, probe soft
 ==========
 foundation wall  <- mud tubes on face?
 --------------
 soil  <- grade vs sill, wood in contact

4. Probe only where sounding said something. Push the tip in gently at the center of the dull section and at its edges. You are trying to find out what is there and how far it goes, not to open it up. A probe in sound wood beside the damage tells you where the damage stops.

5. Read what the probe exposed. Subterranean termite galleries run with the grain and are lined or packed with soil and fecal material, giving a dirty, layered look. Carpenter ant galleries are clean and smooth, with coarse sawdust-like frass pushed out nearby. Fungal decay with no insect involvement shows as wood that is soft, stringy or crumbling, often with cracking across the grain. Drywood termite work leaves small hard pellets. Match what you see to 'Pest Activity Signs Reference' rather than assuming.

6. Read the finished areas without probing. Inside the house, look at baseboards, door casings and window trim below and around the areas where the crawl showed activity. Blistered or rippled paint, a surface that dents under thumb pressure, tiny holes plugged with mud, or a line of soil in a trim joint are cues. Tap lightly with the handle if the owner allows it. Do not push a tip through finished wood without the owner's permission. In a real estate transaction the person who hired you is often not the person who owns the house, and a hole in the seller's trim is a claim against your shop.

7. Mark every finding on the sketch as you go, with the location close enough that someone else could put a hand on it. See related: How to Draw the WDI Inspection Graph.

Visible evidence or suspicion: the line on the report

This is where the job is won or lost, and it is the reason this card exists. A WDI report describes what was visible and accessible on the date of the inspection. It is not a guarantee of what is inside walls.

• Visible evidence is what you saw: mud tubes, live insects, damaged wood you exposed with a probe, frass, shed wings. Write it as what it is and where it is.
• Suspicion is what the evidence suggests but you did not see: a hollow sound in a section you did not open, blistered paint over finished trim, damage that obviously continues beyond where you could reach. Write it as a suspicion with the reason, and recommend further evaluation.
• Never upgrade one into the other. "Hollow sound, not probed" is not "termite damage". "Damage exposed for 3 ft" is not "damage along the north wall".
• Write the limits. If a section of sill was hidden behind insulation, stored items or a finished wall, say so. Areas you could not see are a finding.

How those categories map onto the specific fields of the form your state or the lender requires is owned by 'The WDI Report Form, Section by Section'.

Worked example: a crawlspace ranch with a bathroom on the north wall

The house. A 1960s single-story ranch on a vented crawlspace, inspected for a buyer. Outside, the north wall has a downspout discharging at the foundation and mulch banked within an inch or two of the siding. The hall bathroom is on the north side.

Sill plate, north wall. Tapping from the northwest corner east, the note is solid for the first 13 ft, then goes dull for a section of about 3 ft directly below the bathroom, then solid again. Probe at the center: the tip goes in about 1/2 in with light pressure and exposes galleries running with the grain, packed with soil. Probe 6 in either side of the dull section: firm. A dry mud tube runs up the inside face of the foundation wall to the sill at the same spot. No live termites when the tube is broken open. Moisture meter on the sill there: high, consistent with the downspout outside.

Joist ends on that section. Two joist ends bear on the damaged sill. Both sound solid and probe firm.

Subfloor at the tub drain. Solid.

A joist near the crawl access, south side. Sounds fairly solid, but the underside shows dark staining and cracking across the grain, and the probe sinks into soft, crumbly wood with no galleries and no soil. Moisture meter: high. That is fungal decay from a past leak, not termite activity. It goes on the conducive condition report, not the WDI findings, per 'Moisture and Conducive Condition Report'.

Inside, bathroom baseboard on the north wall. Paint is rippled along about 2 ft above the damaged sill section. Owner not present; the listing agent says no probing. You tap lightly with the handle: dull. You do not probe.

What the report says, in substance:

• Visible evidence of subterranean termites: damaged wood with soil-packed galleries in the north sill plate below the hall bathroom, approximately 3 ft, exposed by probing, with an inactive mud tube on the adjacent foundation wall. No live termites observed.
• Bathroom baseboard above that location: rippled paint and dull sound; not probed (finished area, permission not given). Possible damage; further evaluation recommended.
• Fungal decay in a joist near the crawl access; reported as a conducive condition.
• Conducive conditions: downspout discharging at north foundation; mulch against siding; elevated wood moisture at north sill.

What a careless report would have said: "Termite damage north wall." That claims more than was seen (3 ft is not the wall), leaves out the baseboard the buyer will find, and lumps decay in with termites, which changes the treatment and the repair bid.

What changes the method

A slab house. No crawl to sound from below. Sounding moves to door frames, baseboards and trim at slab-level penetrations, the garage sill plate, and any wood near grade, and the limits section of the report carries more of the weight.

A finished basement. Framing behind drywall is inaccessible. Read the drywall and trim, sound where permitted, and write the finished walls as inaccessible.

Drywood termite regions. Pellets and kick-out holes matter more than mud, and the attic and exterior trim get as much attention as the sill.

The finish test

Every finding is on the sketch at a location someone else could put a hand on, each one labeled as visible evidence, suspicion or conducive condition, and every area you could not sound or see is marked as inaccessible with the reason. No finished surface carries a probe hole the owner did not agree to.

References

• See related: Pest Activity Signs Reference
• See related: The WDI Report Form, Section by Section
• See related: How to Draw the WDI Inspection Graph
• See related: Rodent Droppings and Hantavirus for the Trades
• 29 CFR 1910.134, respiratory protection programs
• USDA Forest Products Laboratory, Wood Handbook (moisture content and decay)