Chimney and Roof Connection
Where the chimney passes through the roof, provide a 2-inch clearance between the wood framing and masonry for fire protection and for expansion due to temperature changes, settlement, or slight movement of the chimney during heavy winds.
Figure 15.—Method of flashing. Sheet metal, h, over the cricket, extends under the shingles k, at least 4 inches and is counter-flashed at l in joint. Base flashings b, c, d, and e and cap flashings a, f, and g lap over the base flashings and provide watertight construction. A full bed of mortar should be provided where cap flashing is inserted in joints.
A chimney must be flashed and counter-flashed ([fig. 15]),[3] to make its junction with the roof watertight. When the chimney is not located on the ridge but on a sloping roof, a cricket, j, is built, as detailed in [figure 16], high enough to shed water around the chimney. Corrosion-resistant metal, such as copper, galvanized metal, zinc, or lead, is best for the flashing and counter-flashing. When tin is used, paint it well on both sides.
[3] See p. 26, Farmers' Bulletin 1751, Roof Coverings for Farm Buildings and Their Repair, for method of installing flashing.
A feature, said to have originated in colonial Williamsburg as a precaution against fire hazard, is to build the upper section of outside chimneys 18 inches to 2 feet away from the gable ends of the house ([fig. 17]). This is not only a safety factor but a practical one because the chimney can be more easily flashed, small windows can be used in the walls of upper story rooms behind the chimney, and framing the roof is simplified.
Capping the Chimney