A spark arrestor and rain cap on a chimney top is designed to prevent wood embers from floating up the chimney and landing nearby on combustible materials namely a neighbors wood roof, dry grass, dry tree branches or other things that will easily catch fire. These hot embers from the fireplace can start a fire very easily. The rain cap portion is designed to reduce rainwater from entering into the chimney area. A chimney cap will keep rainwater from coming down your home's chimney. Rainwater can damage chimneys with stainless steel liners, chimney dampers, and chimney mortar joints. Beyond the damage from the water itself, moisture from rainwater can produce mold and mildew.
If the wind is blowing in a certain direction, it can introduce a downdraft. If you don’t have a chimney cap and you’re using a wood-burning fireplace at the time, a wind-induced downdraft has the potential of blowing smoke back into your home. The flat-shaped top of a chimney cap is designed to help prevent wind-induced downdrafts.
Another advantage of having a rain cap and spark arrestor on top of the chimney is that it helps keep leaves, squirrels and other rodents or animals, including birds out of the chimney itself. Occasionally, birds will build a nest, or debris or animals will block the chimney, thus preventing it from drafting properly. Should it not draft properly, smoke may fill the room, or gather on the face of the fireplace or ceiling.
Local home improvement stores, hardware stores, and fireplace stores usually carry a variety of these rain caps and spark arrestor. Generally, rain caps and spark arrestor's are easily placed on top of a chimney.
A basic chimney cap isn't very elaborate. Made of sheet metal, it provides a barrier that prevents rain and snow from coming in through the chimney and collecting behind the fireplace walls. Some caps have an aerodynamic design that prevents the wind from creating a downdraft. Also made of sheet metal, these caps feature baffles as well as a solid top. As the fire sends creosote-laden particles out the chimney, these can accumulate on the cap, especially if it has baffles, and eventually block air flow. Cleaning off this creosote build-up is the main repair issue with sheet metal caps.
Sheet metal chimney caps aren't very expensive, so if yours has a hole or is heavily damaged, it's a good idea to simply replace it with a new one. Set the cap over the flue liner and secure it with screws. Tighten until snug, but no more. Excessive pressure can crack the clay liner.
Over the time, it may happen the cap and spark arrestor is broken and you need to be prepared for this occasion. Externally it is due to the animal, weather, winds, whereas internally from creosote. As the fire sends creosote-laden particles out the chimney, these can accumulate on the cap, especially if it has baffles, and eventually block air flow. Cleaning off this creosote build-up is the main repair issue with sheet metal caps. The occasion of the accumulation of creosote may affect the chimney. So what if the chimney cap and spark arrestor are not working anymore? It needs to be repaired as soon as possible. The cost of the repair is around US$ 850.