This page shows some close up photos of some of the most common hazards that can be found in a standard chimney. Regular chimney inspections are the most common way in which these hazards are identified, before they cause extreme damage.
Cracked flue tiles are one of the most common chimney hazards. They are most often caused by chimney fires, but can also occur is a foundation shifts, placing stress on the ceramic tiles. They are a hazard because they can allow dangerous gases to escape into the chimney structure, and into the home.
Similarly to the cracked flue tiles, missing mortar between flues tiles is dangerous because it allows dangerous gases and flammable particles to escape the flue.
This image shows creosote leaking to the exterior portion of the chimney structure in an attic. If you see something like this you should have your chimney check immediately by a licensed professional. Creosote build-up can be flammable, and if a hot or flaming particle were to make it outside to the chimney flue into the chimney structure a chimney fire and possibly even an explosion could occur.