Sewer vents are a hit and miss problem among plumbers and drain cleaning services. For one, an occluded pipe cuts off the flow of air from the roof vent when water is inundating the sewer line. So, even if the pipe occlusion is in the drain arm, the ventilation will be a problem. However, the problem does not necessarily get solved through the roof vent.
Another reason sewer ventilation is difficult to finger as the culprit for a slow drain is a lot of preparation is needed to scale the roof to perform a visual inspection of the roof vent to confirm it is clear or blocked. Most people would rather try their hand at cabling the line first, than to go on a roof as part of the diagnostic protocol.
There are a few signs to follow in order to know if you are having a vent problem. If you are draining the tub, and you can hear gurgling in the sink in that bathroom or nearby fixtures, you have a ventilation problem.
If you drain a fixture higher in elevation than another, and the water slowly leaves, only to arrive in the lower elevated fixture, then quickly drains from the lower fixture, you have a vent problem.
If your roof vent is totally blocked, you won't even be able to flush the toilet. Unless you inspect the roof vent, you may be lead to believe you have a blockage in your main sewer line.