The least expensive prevention device against water penetration of your chimney, a chimney cap keeps water from rain and melted snow out of the interior of your chimney. The mesh siding of chimney caps also serve as spark arrestors, which prevents a hot spark from jumping out of your chimney to possibly ignite a fire on your roof, your neighbor’s roof, and your yard. Chimney caps can also help you deal with draft issues if you live in a windy area.