Chimney Caps are desirable for nearly all metal and masonry chimneys for the purpose of keeping precipitation out of the chimney. Choose a chimney cap that is equipped with a stainless steel mesh screen around the outside.The mesh screen serves two functions. First, it keeps leaves,birds,squirrels and raccoons from entering the chimney while still allowing the chimney to exhaust properly. Secondly, it serves as a spark arrestor, blocking sparks and embers above a minimum size from escaping the chimney onto the roof or into the environment where they could potentially cause a fire. If the mesh is too small it will be prone to clogging, particularly in very cold weather or under sluggish burning conditions. In contrast if the mesh is too large, birds or other animals may be able to find their way in. Chimney sweep professionals will have the equipment and different size chimney caps that will appropriately fit your chimney. Every chimney needs a cap… for the same reason every house needs a roof.