Chimney Cap Screen Mesh

You may be wondering why single flue chimney caps and multi flue chimney caps have two options for mesh thickness. Standard mesh thickness for chimney caps is 3/4". Most chimney caps come with an option for a smaller mesh of 5/8". The mesh screen serves two functions. First, it keeps rodents and birds from entering into the chimney while still allowing for the chimney to exhaust properly. Secondly, it serves as a spark arrestor, not allowing sparks and embers to escape from the chimney and into the environment where it could cause a fire.
Many localities - primarily California, but also in other areas around the nation - have spark arrestor codes that call for 1/2" spark arrestors.

The California state building code calls for such spark arrestors on all new construction. Other California codes call for it in "wooded areas" or "within 200 feet of wooded areas" and when it comes time to sell your home, you can also bet the inspector will require you to update your chimney cap to comply with the spark arrestor codes.
To make such a reduced spark arrestor that does not clog, chimney cap manufacturers follow the interpretation that a sphere larger than 1/2" in diameter should not penetrate the screen. This is wording the National Fire Protection Association uses, as well as the one being referred to by several California localities.

Thus, the spark arrestor screens on these chimney caps measure 5/8" from center to center, but still won't let a 1/2" sphere through. This makes them somewhat clog resistant while still meeting major code interpretations.
You can check your local building codes to see what mesh thickness is required in your area.