Why your chimney needs a cap
Not only will it keep out critters and leaves, but it will protect a masonry chimney from the devastating effects of the weather

By Jay Hensley

An open chimney is an invitation to trouble, and sometimes to disaster… Any experienced chimney sweep can tell you why:

Rain Damage

Rain coming down an open flue mixes with soot or creosote, causing unpleasant odors and deterioration of flue walls.  It can run down into the stovepipe, stove, fireplace insert or furnace, where it can cause rust damage.  It can also rust out a fireplace damper.  Rain plus coal soot from sulfuric acid, which is particularly destructive.

Even in a clean chimney, rain can damage flue walls and the smoke shelf in the area down behind.  Moisture getting into cracks and mortar joints freezes and expands, making the cracks larger.

Unwanted guests

All sorts of pesky critters will crawl, slither or fly down an open flue.  Chimney sweeps have been called out to evict (among other things) squirrels, cats, raccoons, ducks, snakes, barn owls and even a goose.

Baby raccoons are cute, but that big wild mama trying to protect them can be ferocious and rabid.  Raccoon feces often contain the eggs of a roundworm parasite, bayliscaris procyonis, which can be lethal to humans.

Birds and their highly flammable nests are another common problem.  The nests can block a flue or even fuel a chimney fire.  An Ohio sweep removed a dozen dead birds from a blocked gas flue one cold winter’s night.  The family had shut down the furnace because their homes carbon monoxide alarm had sounded.

Types of flue caps

Stainless Steel Copper Powder Coated

A flue cap and screen will keep out leaves, critters and rain.  It can prevent flaming balls of creosote from spewing out onto your roof if there is a chimney fire.  A cap can also prevent the occasional downdraft.  Some caps are engineered to solve more serious problems with the draft.

Prefabricated metal chimneys come with their own cap.  For masonry chimneys, a variety of cap styles, sizes and prices are available to cover everything from a single flue to an entire multi-flue chimney.  They can be made of stainless steel, aluminum, copper, or electrostatically painted galvanized iron.  They should be mounted securely, but not permanently, as they need to be removed when the flue is inspected and swept.

What about chimney pots?

Vitrified clay chimney pots are also available for topping off a flue.  Originally designed for use on flues to coal fires, these are durable and handsome, but they can restrict the draft and are not necessarily suitable for a chimney serving a woodburning fireplace, furnace or stove.

Some local fire codes require spark arrestors (a type of screen) on all chimneys.  They are often required or especially recommended in wooded areas or when a house has a shake roof.  Chimney pots are difficult to screen properly.

Call a chimney sweep

Installing a cap is a task best left to a chimney sweep, who is adept at using ladders and crawling about on roofs.  Ask your chimney sweep to show you the types of chimney caps that are appropriate for your chimney.

The National Fire Protection Association (NFPA) recommends annual inspection of flues serving gas, oil, and wood fueled heating systems.

When you call your chimney sweep about installing a cap, it makes perfect sense to go ahead and schedule an inspection of all your flues, fireplaces and heating stoves.

A little professional and preventive maintenance, preferably in the spring of the year, can go along way toward keeping your family safe and heading off the possibility of extensive (and expensive!) chimney repairs later on.

Reprinted, with permission, from the May-June 2008 issue of The Chimney Sweep News, an independent trade magazine for chimney service professionals.  Jim Gillam, editor/publisher.  541-882-5196. www.ChimneySweepNews.com