In a rainstorm, water falls into the fireplace even when the damper is closed. Can I put a metal cap on top of the chimney or will it interfere with the draft and smoke emission? How can I attach a metal cap to a brick chimney that has a flush concrete cap? Herman Leder
When fireplaces with metal dampers are improperly built -- that is, with no provision for differential movement due to the temperature between the brick masonry and the damper -- thermal expansion of the metal damper may crack the surrounding brick masonry.
If this happens, it can allow water to penetrate the chimney assembly at the smoke shelf. Fill any exterior masonry cracks before investigating a rain cap.
If the chimney flue liner and the concrete cap are flush, as you say, the resulting water problem may be occurring right there. The flue should project a minimum of two inches above the cap so as to provide resistance to water from wind-driven rain.
If wind-driven rains are indeed the cause of the water problem, a rain cap may be insufficient unless the flue liner is built up two or more inches.
There is an engineering relationship between chimney height and draft requirements. This data is applicable in determining whether there is sufficient chimney height to add a rain cap and still allow an adequate draft.
Why don't you send for two booklets put out by the Brick Institute of America , 1750 Old Meadow Road, McLean, Va. 22102. Phone (703) 893-4010. They are Technical Notes on Brick Construction, No. 19 and Energy Efficient Fireplaces, No. 11.
Additional information may be obtained from the Fireplace Association of America, 111 East Wacker Drive, Chicago, Ill. 60601.