I install a microwave and vents back into my kitchen.i would like to vent it through the roof. I noticed several existing actic vents already in place. Instead of cutting a new hole in my roof can I vent out through existing attic vents?
Yes you can do that. If there is duct work existing you can reroute it to an existing attic vent. You might need to add an additional duct fan if the existing fan doesn't have enough force for the duct extension.
If this is a stand alone microwave, they require 3 to 4 inches of venting space on the sides, top and back and vent outward usually through the bottom. You could enclose the entire microwave and install a duct through the ceiling and into an attic vent but you'd also need a duct fan because the microwave fan is a circulating fan and not a directional one and you'd have to get the hot air out of the enclosed area.