Cal. Code Regs. tit. 17 § 13605
Mechanical exhaust ventilation equipment shall be provided over all cooking equipment as required to effectively remove cooking odors, smoke, steam, grease and vapors. The ventilation shall be adequate to provide a reasonable condition of comfort for employees. Grease filters or other means of grease extraction are required and shall be of steel construction, or other approved material, and shall be readily accessible for cleaning. Every joint and seam shall be substantially tight. No solder shall be used, except for sealing a joint or seam. Every hood shall be so designed and installed to provide for thorough cleaning of the entire hood. When grease gutters are provided they shall drain to a collecting receptacle fabricated, designed, and installed to be readily accessible for cleaning. All ducts in the exhaust system shall have a slope of at least two inches per lineal foot. All seams in this duct work shall be substantially tight to prevent the accumulation of grease. The ducts shall have sufficient clean-outs to make the ducts readily accessible for cleaning. Make-up air shall be provided at the rate of that exhausted. It may be accomplished from screened service openings, vents in the ceiling, or mechanically through an air-conditioning system, but not through open doors or openable windows.
Cal. Code Regs. Tit. 17, § 13605