Chimneys extending above the roof are exposed to the wind and may sway enough during a gale to open up the mortar joints at the roof line. Openings in a flue at this point are especially dangerous because sparks from the flue may come in contact with the woodwork of the roof. It is therefore good practice to make the upper walls 8 inches thick (fig. 11) by starting to offset the bricks just below the intersection with the roof. The brickwork around all fireplaces and flues should be laid with cement mortar, as it is more resistant than lime mortar to the action of heat and flue gases. A good mortar to use in setting flue linings and all chimney masonry, except firebrick, consists of 1 part portland cement, 1 part hydrated lime, and 6 parts clean sand, measured by volume. Slacked-lime putty may be used in place of hydrated lime; firebrick is best laid in fire-clay. [Illustration: Figure 10.--This Williamsburg chimney shows the pains taken to make the chimney attractive. The three flues are arranged as a T with well-bonded withes between them. Often four flues were used in the form of a cross.] _Openings Into the Chimney_ No range, stove, fireplace, or ventilating register should be connected with the flue being used for the heating apparatus because this is a frequent cause of unsatisfactory operation. Fires may occur from sparks passing into one flue opening and out through another where there are two connections to the same flue. If an abandoned fireplace chimney is to be used for a range or stove, close the fireplace flue tight about a foot below the smoke pipe hole. [Illustration: Figure 11.--Greater resistance to the weather is provided by building the exposed upper section of a chimney with 8-inch walls. Also the mortar joint, in which the counter-flashing is embedded, is not so likely to fail as it is when the wall is only 4 inches thick.] Gas-fired house heaters and built-in unit heaters, if not connected to a masonry chimney, may be connected to flues of corrosion-resistant sheet metal not lighter than 20-gage, properly insulated with asbestos or other fireproofing material that will comply with the recommendations of the Underwriter's Laboratories, Inc. Such flues should extend through the roof.