Chimney Installation in a Cathedral Ceiling

In rooms with cathedral ceilings, the roof and ceiling are combined (see above illustration). For this type of installation, manufacturers of factory-built chimneys provide a special cathedral ceiling roof support kit with instructions that should be followed. The roof support kit usually consists of a metal band, either factory or site attached to a section of chimney, and adjustable "wings" or plates that are secured to the roof and to rafters or support pieces. The band and plates then support both the chimney below and above the roof.

The cathedral ceiling illustration below shows the use of a finishing collar, one type of chimney roof support, a roof radiation shield, roof flashing, sealed storm collar and chimney cap. The chimney that extends into the room is either painted with a high temperature paint or covered with manufacturer supplied trim sleeves.

Because of the slope of the cathedral ceiling, care should be taken to extend the chimney sufficiently far into the room so that the chimney connector will meet the minimum clearance requirement from the sloped combustible ceiling. The greater the slope of the ceiling, the farther into the room the chimney section must extend.

The roof cutaway illustration below provides an example. It shows a 6" diameter connector, joined to a 9" outside diameter factory-built chimney that penetrates a 10:12 pitch ceiling at least 3 feet from any wall. Our task is to calculate the length needed below the ceiling penetration so that the connector is 18 inches from the sloped combustible surface.

The most practical measuring points are along the lower side of the pipe, points x and y, so we first determine the distance from the lowest point (x), to the ceiling. Since the chimney is 3" larger in diameter than the connector, the distance between the outer wall of the connector and outer wall of the chimney (in illustration see t, which is 1/2 the difference in the diameters of the chimney and connector) is 1.5". Point x is therefore 1.5" closer to the ceiling than the connector. We subtract this difference from the required minimum clearance for the connector:

18 - 1.5 = 16.5".

This is the distance between the chimney and the ceiling at which the connector will be 18" from the ceiling. We can then calculate:

Multiply the horizontal run (in inches) times the pitch to get the fall:

10/12 x 16.5 = 165/12 = 13.75" (round up to 14").

The chimney must be extended 14 inches below the ceiling penetration, MEASURED FROM THE LOW POINT WHERE THE PIPING PASSES THROUGH, in order that the connector joining it will be at least 18 inches horizontally from the nearest point on the ceiling.

(Note: a roof support kit might be used in connection with a flat ceiling, for instance where there is little or no attic space.) The manufacturer's instructions for the minimum projection below this type must be followed.