Rain caps help prevent wildlife, downdrafts, and rain from entering the flue. The screen also acts as a spark arrestor.
Wildlife
Squirrels and raccoons often gain access to chimneys. Birds are drawn to the warmth and will build nests in a chimney that isn’t capped. The screen on a rain cap helps keep them out.
Animals that get stuck inside a chimney often can’t climb back out and die there, which creates a horrible stench.
Downdrafts
Wind on a rooftop can create a downdraft, which causes smoke to back up into a home. A rain cap helps prevent this, along with the cold air that comes with it.
Rain
Without a cap, rain pours into the flue and can damage the attic and the interior ceilings and walls. A rain cap helps keep it out.
Spark Arrestor
The screen helps prevent a hot ember that escapes through the chimney from setting the roof or other nearby combustibles on fire.
Annual chimney inspections include attention to chimney caps and removing any obstructions.