Many of the chimneys built today are unsightly and frequently detract from an otherwise well-designed house. Within the last 100 years the size and attractiveness of chimneys ordinarily built has declined. The large old chimneys of colonial days were proportioned to suit the house and surroundings and at the same time provide for two or more large fireplaces. With reduction in the size of fireplaces and the substitution of several stoves and eventually one central heating plant, the chimney has developed into a merely utilitarian shaft. [Illustration: Figure 17.--A house in southern Maryland in which the space between the chimney and the house wall shows clearly. The practice of building the chimney in this way is common in the tidewater section of the South.] Spark Arresters Spark arresters are desirable and, where chimneys are near combustible roofs, lumber, forests, etc., they are sometimes required, depending on the kind of fuel, waste materials, or refuse that may be burned and the amount of deposits that may accumulate in the flues. While arresters cannot be depended on to eliminate entirely the discharge of sparks under all conditions; yet, when properly built and installed, they materially reduce spark hazard.[4] [4] See Standards for Construction and Installation of Spark Arresters for Chimneys and Stacks, published by the National Fire Protection Association. In general all parts, whether of wire, expanded metal, or perforated sheets, give longer service if they are of rust-resistant material. Arresters for domestic purposes should have vertical sides extending upward not less than 9 inches so as to provide a gross area of surface at least twice the net flue area. They should be kept outside of the flue area and be securely anchored to the chimney top. Openings in the screen not larger than five-eights of an inch nor smaller than five-sixteenths of an inch are advisable. Commercially made screens can be purchased which generally last several years. Arresters must be kept adjusted in position and renewed when the openings are worn larger than the normal screen openings. [Illustration: Figure 18.--_A_, A common type of arched hood; _B_, flat stone hood; note the withe separating the two flues.] ESTIMATING BRICK The number of standard-size brick (8 by 3¾ by 2¼ inches) required to build a straight chimney having only two or three flues can be estimated by drawing the flue lining to scale and then drawing lines 4 inches to 8 inches outside of the lining depending on the thickness of the brick walls. Lay out 4- by 8-inch rectangles in the space between the lining and the outside lines to determine how many brick are needed per course.