Preventing flying embers from escaping your chimney and igniting a roof surface or neighboring debris should be part of any responsible chimney owner's safety plan. Spark arrestors offer a proven and safe method of fire prevention where chimneys are used. Basic information about spark arrestor screen specifications can be gleaned from local building code requirements.
Chimney spark arrestor screens are designed to control and stamp out embers that escape from the chimney flue. The spark arrestor is a mesh screen cage that is attached to a simple base — the mesh cage extends upward and over the flue opening. As the embers rise and attempt to float away from the flue, the mesh acts as a barrier and prevents the embers from leaving the cage area. Spark arrestors come in two styles: capped or uncapped. A solid cap adds an extra barrier and helps prevent debris from entering the flue.
Spark arrestors fit over the chimney flue and attach to the chimney surface — the flue is a duct that slips inside the chimney. Because chimneys and flues come in many different combinations of shapes and sizes, each spark arrestor is designed to fit a specific set of flue sizes and chimney shapes. This indicates the arrestor cage may be adjustable in length, width or diameter. For instance, a rectangular-shaped unit may fit a flue sized 6 by 9 inches up to 14 by 22 inches. Mesh screen sizes vary also and can be found in dimensions of a 1/4 to 1 inch; however, 1/2- or 5/8-inch mesh is a standard size for spark arrestor screens.
Screens are typically made of galvanized metal to provide heat resistance and prevent corrosion and damage from the elements. Some manufacturers offer spark arrestor units in a powder-coated or sealed finish as an added preventative to corrosion and heat resistance and to provide an easy-to-clean surface. Capped spark arrestor units usually feature a pitched or sloping cap to stop debris and rain buildup.
Building Code Requirements
Some states and local governments have requirements for chimneys and spark arrestor screens in their building and fire codes -- California is one such state. The codes outline minimum design requirements that may include directions on how and where the unit is attached to the chimney, what the minimum and maximum mesh size must be and how high the screen must sit above the flue. Contact local building and fire departments to verify the dimensions and mesh sizes for your jurisdiction.
For instance, Daly City, California, provides an elevation drawing and a table of dimensions to illustrate its code requirements for spark arrestors. Daly City requires the screen mesh to be no larger than 1/2-inch and no smaller than 3/8-inch. The city also requires the unit to be made of corrosion- and heat-resistant materials equal to 19-gauge galvanized, 24-gauge stainless steel or 12-gauge wire. Required screen height runs between 4 and 12 inches depending upon the dimensions and shape of the chimney.