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.]