A chimney cricket is a triangle shaped diversion roof built behind the chimney at roofline

If your chimney is located on the downward slope of your roof, you may experience water pooling around the base where your exterior chimney meets the roof. A chimney cricket is a small peaked roof which is installed on the back side of the chimney to deflect water and debris from the chimney. It is made from galvanized steel, stainless steel or aluminum and can be an excellent protection for your chimney.  A chimney cricket is shaped like a triangle and has a ridge on the top so that when water meets the cricket, it automatically flows down the angled sides and away from the chimney. This can be a very important part of the chimney and its roof protection system.

Protecting your chimney can be  invaluable as it would save you from unnecessary and expensive roof repairs that water leaks would inevitably require.