Inspecting chimneys

The Old House Web

roofs and chimneysEditor'snote: This story is adapted from the U.S. Department of Housing and UrbanDevelopment's Residential Rehabilitation Inspection Guide, 2000. Clickhere for other stories in this series.

Chimneys should project at least two feet above the highest part of a pitchedroof and anything else that is within 10 feet (3 m). A chimney should project atleast three feet from its penetration from the roof (required minimum heightsmay vary slightly). Check the local building code. If the chimney is notreadily accessible, examine what you can with binoculars from the highestvantage point you can find.

Flues should not be smaller in size than the discharge of the appliance theyserve. The minimum flue area for a chimney connected to a fireplace is normally50 square inches (320 cm 2 ) for round linings, 64 square inches (410 cm 2 ) forrectangular 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 hoodshould vary approximately four inches to avoid down-drafts. The same is true ofa chimney with a hood unless a width of masonry completely separates every flue.

Masonry chimneys without hoods should have stone or reinforced concrete capsat the top. Cement washes with or without reinforcing mesh are also used, butthey are the least durable. Some masonry chimneys have hoods over the flues.Hoods on masonry chimneys consist of stone or reinforced concrete caps supportedon short masonry columns at the perimeter of chimney tops, or sheet metal capssupported on short sheet metal columns. The height of a hood above the top ofthe highest flue should be at least 25 percent greater than the narrowestdimension of the flue.

Check the condition of chimney tops and hoods. If a cement wash is notproperly sloped or is extensively cracked, spalled, or displays rust stains, itshould be replaced. Reinforced concrete caps and stone caps with minor shallowspalling and cracking should be repaired. Those with extensive spalling orcracking should be replaced. Sheet metal hood caps with minor rust or corrosionshould be repaired, but if rust or corrosion is extensive, replacement isneeded.

Metal spark screens are sometimes used on wood and coal-burning fireplacechimneys. Check the condition and fit of spark screens. Dirty or clogged screensadversely affect draft and should be cleaned.

Where a masonry chimney is located on the side of a pitched roof, a cricketis needed on the higher side to divert water around the chimney. Check thecricket to be sure that its seams are water-tight, that it is properly flashedinto 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 thefirebox to the cap, and particularly the portion projecting above the roof.Consider consulting a structural engineer to determine the need for additionalbracing or strengthening. If the chimney is prefabricated metal encased in anexterior chase of siding, check the chase top to be sure it is properlyinter-locked with the metal chimney's counter-flashing so that the assembly iswatertight. Also check the chase top for slope: water should drain off theenclosure. Check for the presence of a terminal metal rain cap and make certainthe flue terminates not less than two inches and not more than eight inchesabove the enclosure top.

If the chimney is prefabricated metal and not encased, check the adjustableflashing at the roof to be sure it is tightly sealed to the chimney, preferablywith counter-flashing, and check for the presence of a stack cap.

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