Venting a Bathroom Fan Under Deck/Porch: Potential issues


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Venting bathroom fans under a deck/porch is possible, but there are potential issues. Not even treated wood is invincible against repeated exposure to hot and wet air. The deck/porch can become steamy. Snow can melt and refreeze into slippery ice. Bathroom odors can permeate entertainment areas.

Choosing the ideal location for venting bathroom exhaust can be difficult since there are many factors to consider. The International Residential Code (IRC) governs the allowable lengths of ducts, number of joins and bends, exit point requirements, etc. This means that, though it looks like you have endless possibilities of where to vent your bathroom fan, you may only actually have one or two options.

Depending on the area under your deck/porch as well as the materials used, venting under your deck or porch may be ideal for you. Below, it is discussed whether this is legal and why, as well as the disadvantages that may come with venting under your deck or porch.

What Does the IRC Say?

The IRC’s regulations regarding venting a bathroom fan under a deck or porch are indirect. There is no line or statement that says you may or may not choose this exit point for your bathroom fan’s ductwork. So, how does a homeowner handle these situations?

Well, these situations actually arise more often than not. The IRC and other building codes cannot include stipulations for all possible scenarios. Instead, homeowners need to rely on reasoning and logic in interpreting the rules that do exist.

Thus, if you wish to vent your fan under your deck or porch, it has to obey the length and dimension rules set out in the IRC. It has to be constructed of the correct materials, and follow all combined venting rules (e.g., not with the dryer).

One other valuable set of regulations that can help you determine when you cannot vent under a deck or porch can be found in Section M1504.3 of the IRC. This passage states prohibited vent locations for exhaust fans:

“1. Not less than 3 feet (914 mm) from property lines.

2. Not less than 3 feet (914 mm) from gravity air intake openings, operable windows and door.

3. Not less than 10 feet (3048 mm) from mechanical air intake openings[…]”

The first rule stated is important because it has to do with common courtesy as well as property line restrictions. Just like a tree branch extending into your neighbor’s property can create problems, venting your bathroom exhaust into your neighbor’s property can create problems as well. 

For the second rule, it is important to keep exhaust vents away from gravity air intake openings as well as openings to the home because this would allow expelled air to re-enter the home. This is a particular concern when venting through the soffit as there are usually passive soffit vents that supply air to the attic. If this supply air was hot and moist, then it could cause a whole smorgasbord of issues

The third rule has similar logic as the second one. The restrictions for a mechanical air intake vent, though, are different since gravity air intake openings are more passive with their air intake while mechanical air intake openings are actually powered and more aggressive. To terminate a bathroom fan here would be almost as bad as simply venting into another room, which is against the code.

Similar to Venting Through Soffit

Venting a bathroom fan under a deck or porch is a similar situation to venting through the soffit of your home. Both are not disallowed by the IRC, as long as they follow the exhaust regulations stated above.

The situations are also similar because they can often be more convenient. If the nearest opening to the outside from your bathroom is under the deck/porch or through the soffit, venting through here can shorten the length of ductwork needed and lessen the work, cost, and potential for issues.

Always Consult Local Rules, Regs, and Authorities

Just because your venting situation complies with the IRC does not mean it is allowed or encouraged in your specific region or area. Most states in the US do adopt the IRC, as do other areas around the world. However, it is not the only international code in effect and even those who do adopt it can make adaptations, omissions, and additions.

Printed text "know the rules" on a white keyboard key cap

Make sure to contact anyone with home or construction knowledge in your neighborhood or county to ensure that venting under your deck or porch is allowed in your locality.

This could mean contacting your HOA (if you are a part of one), a contractor, or even a knowledgeable neighbor.

Reasons It Is Possible

Although venting exhaust under a deck or porch may sound unideal or even dangerous, the content of the bathroom exhaust as well as the nature of some decks or porches makes this location acceptable for exhaust venting.

Bathroom Fumes Are Not Toxic

Sewer gas is comprised of fumes from the natural breakdown of human waste. It contains methane, carbon dioxide, ammonia, and hydrogen sulfide. Long exposure to high amounts of ammonia and hydrogen sulfide can be harmful to the human body and both methane and ammonia can be highly flammable in large amounts.

For such fumes to be exhausted below a living area, so to speak, could become dangerous indeed. However, while there is the possibility that sewer gases can be escaping into your bathroom, the amount would be insignificant. If it were more substantial, you would notice and it would indicate a problem that needs to be remedied.

The only scenario where you may have sewer gas in your bathroom is in the event of cracked pipes, clogged drains, or dry plumbing. That being said, toxic fumes would only be present in your bathroom in these situations, and it would take high concentrations and long exposures to cause any adverse health effects.

So, unlike a sewer vent or the vent of a gas fireplace, for example, bathroom vents do not carry toxic fumes that must not be permitted to enter an area where people gather. Instead, they release hot and moist air. This does come with some challenges and determining conditions, which we go through now, but it shouldn’t be dangerous.

Good Airflow in Space Under Porch/Deck

Enough area under the porch or deck means ample airflow, and it can also isolate the exhaust from populated spaces.

Little girls playing outdoors

For example, venting bathroom exhaust just to the outside of a house means that the exhaust can be happened upon by residents walking by. Hot, stale, moist, and odorous air is not pleasant by any means.

By venting under the porch or deck, residents are less likely to encounter the air.

Lumber and Fasteners Treated

A major concern with bathroom exhaust is the fact that it is warm and usually very humid. When venting under a porch or deck, which are very often constructed from wood, moisture can accumulate and may warp or damage the integrity of the wood, as well as its fasteners.

The likelihood of this decreases greatly when the wood making up your deck or porch as well as its fasteners are treated or naturally durable.

As long as the lumber and fasteners used are treated, which they usually are with outside constructions, you shouldn’t have to worry about humid bathroom exhaust damaging your deck or porch.

Deck/Porch Usually High Enough

As far as the height of your deck or porch, it is important that it is high enough to allow for ample area under the deck/porch for air circulation.

Thankfully, there are often standards for how high a deck/porch should be, varying by location. Regardless of where you live, your deck or porch is often at least 20″ (50 cm) from the ground, which should be sufficient.

Potential Issues With Venting Below Porch/Deck

As I always say, just because you can do something, doesn’t mean that you should. There are a number of negative possibilities attached to this ventilation seemingly ideal ventilation strategy. You will have to weigh the good and the bad and decide what is best for your situation.

Treated Wood and Fasteners Not Invincible

Although naturally durable or treated wood and fasteners can resist damage from moisture, fungi, and insects, it does not mean that they are invincible.

Extreme circumstances or low-quality wood can make warping or structural instability possible in your lumber or fasteners.

If the treated wood is low-quality, its seal will not last as long and the wood can begin to warp or decay. If the area beneath the deck/porch is constantly extremely humid, even high-quality wood cannot resist this amount of moisture forever.

Steamy Porch/Deck in Summer

You know from being in the bathroom after a shower, even with the bathroom fan on, the unpleasant feeling of its humid and hot air.

If hot, steamy air from the bathroom is being vented under your porch or deck, it will rise through the floor and will warm up your porch/deck since heat naturally rises.

The concern here is that your deck will be warmed throughout the year. In the summer, especially on unpleasantly hot days, your deck will be even warmer than it would be if you did not vent under your porch/deck.

Steam Will Melt Snow

Another problem that comes with your deck being warmed by hot and humid air from your bathroom exhaust is that it will melt snow in the winter.

Home with wet porch and snowy yard in Daybreak

This may seem advantageous at first since it will reduce the amount you will have to shovel, but this situation actually poses a significant safety risk.

The snow will melt, but once the fan is turned off and the deck/porch is no longer heated, this melted snow will refreeze and form ice, which is not only harder to get rid of, but also can be easily slipped on.

Ice on your deck or porch can lead to bruises, broken bones, and head injuries, which can be especially dangerous for the elderly and children.

Bathroom Odors in Entertainment Areas

Although bathroom exhaust odors will not be blown directly onto you as they would if your exhaust vent was connected to the outside of the first floor, bathroom odors can still enter your porch or deck.

As the heat and humidity rise through the floor of the deck or porch, the odorants in this exhaust travel with it.

Bathroom odors are unpleasant and can be embarrassing when having guests over, especially for a deck/porch, which is often the central place for outdoor parties.

Not Wise With Enclosed Patios/Decks

A lack of air circulation can lead to all sorts of problems involving the less-desirable parts of bathroom exhaust, namely the humidity, heat, and odor. Each of these factors can worsen when the air is not replaced by natural air circulation.

This is what would happen if you vented the bathroom fan below an enclosed deck or porch. The structural materials would suffer from build-ups of humidity, as would the deck furniture. The air would be uncomfortable to breathe in and odors can become concentrated.

To avoid these conditions, it would be wise not to vent bathroom exhaust under your porch or deck if it is enclosed.

Sources

https://codes.iccsafe.org/content/IRC2021P1/chapter-15-exhaust-systems#IRC2021P1_Pt05_Ch15_SecM1504.3

https://www.healthline.com/health/healthy-home-guide/sewer-gas#dangers

https://www.timberblogger.com/what-is-pressure-treated-wood/

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