Possible causes, not a remote diagnosis
Possible sources include flashing, a crown or chase cover, masonry and nearby roofing. Condensation or another building leak may also need investigation. Several defects can coexist.
What to document safely
Take dated pictures from inside and the ground. Note rain timing, snowmelt and whether the stain grows while the appliance is being used. Keep earlier repair invoices.
Keep the appliance model, fuel, previous report and date of last service together. A clear sequence of events can be more useful than deciding in advance which component must be replaced.
Your next service conversation
Ask for leak diagnosis before authorising a coating or component replacement. Depending on the findings, a chimney contractor and roofer may need to coordinate responsibility.
Chimney leak diagnosis →Should I paint over the stain now?
Save the evidence and arrange assessment first. Cosmetic restoration can wait until the moisture source has been addressed and affected building materials assessed.
Ask for evidence before approving repairs
Request a report that distinguishes what was observed from what remains uncertain. Ask the company to identify any area it could not inspect and whether another trade is needed. If a major repair is proposed, compare a second written scope using the same appliance details and findings.
Do not treat a directory profile, an estimate or this page as clearance to use an appliance. Ask the attending professional what must be resolved before reuse.
Read the detailed guide and its sourcesTake the service question to your location
Use the local guide to prepare questions about access, building history and the authority responsible for your address. The service brief is national guidance; local permits and current burn restrictions must be checked separately.