How to tell frass from regular sawdust
- Texture: frass is finer and more uniform than carpentry sawdust.
- Composition: frass contains dark insect debris (legs, antennae, dead worker fragments) — sawdust is pure wood.
- Pattern: frass piles up directly below a gallery in a small concentrated mound. Sawdust is usually larger volume and irregularly distributed.
- Freshness: frass replenishes — sweep it up, check 7 days later, fresh frass means active colony. Sawdust doesn't replenish.
What's happening when you find frass
Carpenter ants tunnel galleries in moist wood. As they excavate, they push the debris out through small 'kick-out holes' — typically pinhole-sized openings in the surface of the gallery wall. The frass falls and accumulates below the hole. The size of the pile correlates roughly with colony age and activity: small pile (a teaspoon) means a young or low-activity colony; larger pile (a tablespoon or more) accumulating over a few weeks means a mature, active colony.
The most common frass locations in Metro Vancouver homes
- Below window frames on west-facing or north-facing walls — these catch the most rain and often have failed flashing.
- At the base of exterior walls where siding meets the foundation — the sill plate area is the first wood component to absorb rising damp.
- In basement stairwells adjacent to exterior walls — wall moisture migrates to interior faces.
- Below deck ledgers — where the deck attaches to the house is a chronic water-intrusion point in BC construction.
- In attic insulation near eaves — plugged gutters direct water into the soffit-fascia junction and the wood above it.
- Under bathroom floors or kitchen soffits — plumbing leak histories leave residual moisture that persists for years.
Using sound and frass together to locate colonies in walls
Carpenter ant colonies in wall voids can sometimes be detected by sound. Press an ear against the drywall at night in a quiet room — active colonies in walls sometimes produce a faint rustling or crinkling sound as workers move through galleries. Tap the wall — hollow or unusually soft sounds from specific areas of drywall (compared to adjacent panels) can indicate galleries. This acoustic method works best in larger, active colonies and is not definitive, but when combined with frass location, it narrows the gallery area before drilling inspection holes.
Taking moisture readings
A pin-type moisture meter is the most reliable way to confirm wood moisture above threshold in suspected frass locations. Readings above 19% in framing lumber indicate elevated moisture; readings above 25% indicate active wetting. Home-grade meters (available for under $50 at hardware stores) are sufficient for screening. If you find frass and the adjacent wood reads above 19% moisture, you have confirmation of the conditions for carpenter ant establishment, regardless of whether you've seen the ants themselves.