When you look up at a roof in a residential neighbourhood you will see what’s normally referred to as “Roof Vents” at or near the peak. The most commonly used roof vents today (in York Region, southern Ontario, Canada) are square in shape, black or brown in color, with an approximate size of 12” by 12”. As there are different manufacturers of roof vents there are also different shapes, colors and sizes. Another commonly used roof vent in this geographical area is one which resembles a large ball-like structure and twirls in the wind (commonly referred to as ‘whirlybirds’).

The basic purpose or job of a roof vent is, obviously, to vent or “let out air”. A roof vent lets out air from the house or attic space. A properly vented roof will allow air “in” at the soffits and “out” through the roof vents.

As far as how many roof vents should be on a roof, there are guidelines on this subject, the most common one being “one roof vent for every 300 sq.ft. of attic space”.

The purpose of this article is to discuss the issues with regards to the difference between a roof vent and a bathroom vent. Both are installed on a residential roof and both do vent…but as similar as their functions may be, there are also differences.

A bathroom vent is smaller than a roof vent. It is closed on three sides. It has a plate inside it that somewhat controls the flow of air, much like a damper in a fireplace or the metal plates inside the ductwork of your furnace system. A roof vent does not have such a plate.

A bathroom vent is part of the bathroom venting system which starts at the bathroom fan and vents the moisture from a bathroom to the outside of the house. The moist air is drawn from the bathroom by the bathroom fan, runs through some ductwork (piping) and vents out through the bathroom vent. This could be compared to a clothes dryer in that the moist air ends up outside the house envelope.

Bathroom vents can be bought in hardware stores or roofing supply stores. You also have the option of buying what I call an ‘adapter’. It’s a separate piece which is made to fit into the pipe which runs from the bathroom fan and ends at the roof boards or sheathing (new homes in this area usually 3/8’’ plywood). I recommend using the adapter as it can be installed to make that area more air tight and presumably help the bathroom vent exhaust more efficiently. It would be beneficial to the homeowner if the builders of new homes installed a bathroom fan which wasn’t the cheapest model available.

I believe it is important for homeowners to know the difference between a roof vent and a bathroom vent because there are literally thousands of homes in the area which have the incorrect vents installed. Instead of installing bathroom vents what we find instead is a roof vent which, of course, is better than nothing but…not the right vent. Only recently have I finally seen the proper vents being used. Despite being told by some contractors that the improper vent is “good enough” I have often ooftaken off the improper vent (in this case I’m talking about usage of a roof vent instead of a bathroom vent) and found water stained wood, sometimes blackened, and sometimes rotten. My explanation for this is that the condensation from the bathroom ventilation fan exhausts to the improper roof vent and, like a mushroom design, falls back on the roof and wood.

Luckily, the solution is simple. An inspection of your roof and the vent beside the bathroom stack pipe will determine whether or not you have the correct vent installation. Bottom line is: install the proper vent for condensation/bathroom fan ventilation…it works better!