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I think in practice, you end up doing both jobs with the same part at the top of your chimney.
These are two separate jobs: spark arrestor to keep sparks from the fire from leaving the scene and causing a problem: chimney cap to keep the rain and snow from washing soot back down the chimney, but they are commonly built into the same part.
A spark arrestor incorporates a perforated surface of some sort. The perforations job is to hold back particulates, this would include sparks. Some are woven metal screen and others are perforated plate. The screen offers more open area, is less expensive and has less affect on the draft. Some chimney caps incorporate this feature others do not, spark arrestors also are made without a chimney cap funcion
I built a spark arrester on my chimney for no other reason that if the council or other authorities ever query my oven , I at least have done the right thing as I live in the foothills and in a fire danger area.
I have never seen any sparks run up let alone out the top of the 10 foot flue.
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