Editor's note: This story is adapted from the U.S. Department of Housing and Urban Development's Residential Rehabilitation Inspection Guide, 2000. Click here for other stories in this series.
Chimneys should project at least two feet above the highest part of a pitched roof and anything else that is within 10 feet (3 m). A chimney should project at least three feet from its penetration from the roof (required minimum heights may vary slightly). Check the local building code. If the chimney is not readily accessible, examine what you can with binoculars from the highest vantage point you can find.
Flues should not be smaller in size than the discharge of the appliance they serve. The minimum flue area for a chimney connected to a fireplace is normally 50 square inches (320 cm 2 ) for round linings, 64 square inches (410 cm 2 ) for rectangular linings, and 100 square inches (650 cm 2 ) for an unlined chimney. Be extremely cautious about unlined chimneys; check the local building code. Flues should extend a minimum of four inches above the top of a masonry chimney. The height between adjacent flues in a multiple flue chimney without a hood should vary approximately four inches to avoid down-drafts. The same is true of a chimney with a hood unless a width of masonry completely separates every flue.
Masonry chimneys without hoods should have stone or reinforced concrete caps at the top. Cement washes with or without reinforcing mesh are also used, but they are the least durable. Some masonry chimneys have hoods over the flues. Hoods on masonry chimneys consist of stone or reinforced concrete caps supported on short masonry columns at the perimeter of chimney tops, or sheet metal caps supported on short sheet metal columns. The height of a hood above the top of the highest flue should be at least 25 percent greater than the narrowest dimension of the flue.
Check the condition of chimney tops and hoods. If a cement wash is not properly sloped or is extensively cracked, spalled, or displays rust stains, it should be replaced. Reinforced concrete caps and stone caps with minor shallow spalling and cracking should be repaired. Those with extensive spalling or cracking should be replaced. Sheet metal hood caps with minor rust or corrosion should be repaired, but if rust or corrosion is extensive, replacement is needed.
Metal spark screens are sometimes used on wood and coal-burning fireplace chimneys. Check the condition and fit of spark screens. Dirty or clogged screens adversely affect draft and should be cleaned.
Where a masonry chimney is located on the side of a pitched roof, a cricket is needed on the higher side to divert water around the chimney. Check the cricket to be sure that its seams are water-tight, that it is properly flashed into the chimney and roofing, and that it extends the full width of the chimney.
In seismic zones, check the bracing of masonry chimneys from the top of the firebox to the cap, and particularly the portion projecting above the roof. Consider consulting a structural engineer to determine the need for additional bracing or strengthening. If the chimney is prefabricated metal encased in an exterior chase of siding, check the chase top to be sure it is properly inter-locked with the metal chimney's counter-flashing so that the assembly is watertight. Also check the chase top for slope: water should drain off the enclosure. Check for the presence of a terminal metal rain cap and make certain the flue terminates not less than two inches and not more than eight inches above the enclosure top.
If the chimney is prefabricated metal and not encased, check the adjustable flashing at the roof to be sure it is tightly sealed to the chimney, preferably with counter-flashing, and check for the presence of a stack cap.
