The 15-minute checklist
You opened the cabinet under the sink and saw a dark spot, or a musty smell hit you, or both. Here's what to do right now, in the next 15 minutes, before deciding whether to call anyone.
Step 1 — Open and inspect (2 minutes)
Empty everything out of the under-sink cabinet. Use a flashlight. Look at:
- The cabinet floor (most common spot for visible growth)
- The back wall against the drain plumbing
- The underside of the sink basin
- The side walls and the inside of the cabinet doors
Take three or four photos. You'll want them later either to send us or to monitor changes.
Step 2 — Find the moisture source (5 minutes)
There is always a moisture source. Common culprits:
| Source | Sign |
|---|---|
| Drain trap leak | Stain pattern below the trap; water beads on the trap underside |
| Supply line drip | Small puddle near the shutoff valves; cabinet floor wet only on one side |
| Garbage disposal seal | Drip from the bottom of the disposal unit |
| Dishwasher line | Wet area near where the drain hose enters the cabinet |
| Caulk failure at the sink rim | Water pooling around the sink edge above; sometimes drips down inside the cabinet |
| Refrigerator water line (if adjacent) | Wet floor under the fridge as well |
Run the sink for 30 seconds with the stopper down, then look again. Run the dishwasher rinse cycle for 60 seconds, then look again.
Step 3 — Stop the source (5 minutes)
If you found a slow leak:
- Tighten the trap nuts a quarter turn (don't overtighten plastic threads)
- Replace a worn supply-line washer ($2 part)
- Replace the garbage disposal seal ($8 part, 10 minutes)
- Re-caulk the sink rim ($6, 20 minutes including dry time)
If you can't find the source, leave a paper towel on the cabinet floor overnight and check it in the morning. Damp paper towel tells you where to look.
Step 4 — Dry it out (3 minutes setup, 24 hours running)
Pull a small fan or a hair dryer (low setting) into the cabinet and run it for several hours after the leak is fixed. Open the cabinet doors. If you have a dehumidifier, set it up nearby.
The decision tree
Now you know what you're dealing with. Pick the path:
Less than 1 square foot of visible discoloration, moisture source is fixable, cabinet wood is solid and not crumbling. DIY clean is fine. Wipe with a 1:10 bleach solution (or a 3% hydrogen peroxide solution, equally effective and less harsh on cabinets), let dry, recheck weekly for 4 weeks. If it comes back, escalate.
1 to 5 square feet of visible discoloration, OR moisture source isn't immediately obvious, OR the cabinet floor feels soft. Send photos at moldremovalandtesting.com/photo-check. 2-hour reply with whether it warrants a visit.
More than 5 square feet of visible discoloration, OR you can smell it from outside the cabinet, OR the floor underneath the cabinet feels soft. Book a free inspection at moldremovalandtesting.com/schedule. At this size the question is usually how far the moisture has migrated, not whether to remediate.
Anyone in the household has an active respiratory condition, OR a baby/elderly person/pet showed symptoms before you noticed the spot. Skip DIY and book the inspection regardless of size.
Don't do this
- Don't paint over it without removing it. Mold grows through paint.
- Don't spray heavy fragrances to cover the smell. The smell is the diagnostic signal.
- Don't seal the cabinet shut. You need airflow to dry.
- Don't ignore a slow leak for a week. A 1-week leak is a wipe-down. A 6-week leak is a remediation.
Next step
Snap the photos and decide which path you're on. If unsure, send them; we reply in 2 hours and the answer is free.