Can You Vent a Dryer Through the Roof?

While there are no rules preventing a dryer vent through a roof, it’s a good idea to weigh the benefits against the risks and challenges of such a project.

Benefits

Aesthetics are one of the more popular reasons to run a vent through the roof. By limiting the number of utilities in view, your home’s curb appeal is less cluttered. Other good reasons to vent your dryer through the roof include:

less accessibility by small critters and rodents
situational proximity to property lines
reduced chance of damage from curious children, bored pets, or falling tree limbs

Risks & Challenges

Before you cut a hole in your roof and use it to push hot air up and out, there are a few issues to consider. There are also some structural and environmental factors that will impact your decision. Here are a few safety issues and challenges we’ll cover later in more depth:

building code limitations
condensation
lint build-up
maintenance & safety

What Is Code for Venting a Dryer?

Most municipalities in the United States use the International Residential Code (IRC), however, some are known to use different regulations. It’s important to always consult your local residential building codes or building department.

IRC Section M1502, Clothes Dryer Exhaust covers dryer vents. It makes no mention of whether you can or cannot vent a dryer through the roof. It simply states, “dryer exhaust systems shall be independent of all other systems and shall convey the moisture to the outdoors.” If all other code requirements are met, a dryer vent terminating above a roof is an acceptable solution.

Aside from having to vent your dryer outdoors, there are a few important code requirements you’ll want to focus on:

Material and Size

You must use a 28-gauge (minimum) metal duct with a smooth interior that is 4-inches in diameter.

Duct installation

The entire duct system must be secured and supported at no less than 12-foot intervals. Duct sections cannot be joined with screws that enter the opening more than ⅛” so as to prevent lint and debris from collecting and blocking the exhaust flow. Every section must fit inside the adjoining section in the direction of airflow. Each seam must be mechanically joined and sealed with an approved mastic or foil tape.

Duct length

The Code limits the length of dryer vents to 35 feet. If you’re unable to vent in a straight line and use elbow fittings, your calculated length will have to be adjusted. Fittings create friction and reduce airflow. For each fitting used the following distance will have to be subtracted from the maximum allowable distance:

45-degree elbow – subtract 2.5 feet
90-degree elbow – subtract 5 feet

Example: If you use a 90-degree and 45-degree fitting, your maximum allowable measured dryer vent distance gets reduced to 27.5 feet (35 – [5+2.5]).

Duct termination

Where the duct exits the structure, the opening must remain undiminished in size and fitted with a backdraft damper not less than 12.5 square inches. Screens placed over the opening of the exit point are prohibited. Excess lint can get trapped in the screen and interfere with damper performance.

Placement of the duct outlet is commonly overlooked. The Code requires installing it no closer than 3 feet from openings to any buildings including windows, doors, crawl space openings, or ventilated soffits.

How Far Can You Vent a Dryer Vertically?

Now that you know you can vent a dryer through the roof, the next issue may be, is 35 feet enough distance to vent vertically through the roof. If you’re in a single-story home with an attic, chances are good that you’ll have no issues. A typical roofline measures 12-15 feet. However, multi-story structures can be more challenging. If you have a dryer in the basement and want to go through the second story into the attic and out through the roof, you’ll likely exceed the 35-foot maximum.

Let’s assume you decide to put the dryer on the first floor of your 2-story house, and you have an attic, so the termination through the roof will occur 7 feet higher. With 9-foot ceilings, that’s just 25 feet. Unfortunately, you realize you can’t run the duct in a straight line. The duct needs to be redirected 4 feet over to the back of the house so the vent can’t be seen from the road. You decide to use 2 90-degree fittings.

Recalculating the total allowable distance, you realize you will exceed the 35 foot maximum. Those two 90-degree fittings shortened your maximum allowable distance by 10 feet to 25 feet! With that 4 feet of extra horizontal run to get to the back of the house, you have to cover a distance of 29 feet. It won’t pass inspection.

Your options are to:

Use the shorter, 25-foot straight-line path.
Reconfigure the ductwork in the attic using 2 45-degree fittings. At a loss of 2.5 feet each, subtracting 5 feet from 35 allows 30 feet of total allowable distance.

Regardless of direction, the maximum distance for dryer duct systems is 35 feet.

What Happens If You Can’t Vent Your Dryer Outside?

There are cases where a dryer just can’t be vented. Some older apartments were built without dryer vents and specifically ban their use. Some historic building preservation rules restrict these types of alterations. In many cases, the cost of installing a new dryer vent system is just too expensive.

Unfortunately, the only option available is to use what is known as a ventless dryer. The IRC Code explicitly provides exceptions for condensing (ductless) clothes dryers because they do not push warm, moist air back into the living area.

They are extremely efficient and have been used extensively for years in other countries. With recent attention to environmental impacts, these machines are only now starting to gain in popularity within the US. However, they do have a high up-front price tag compared to vented dryers.

Two common types of ventless dryers are:

Condenser Dryer

This dryer condenses the warm moist air as water from the clothes is collected and drained from the machine. As the air cools, it gets reheated in a highly efficient drying cycle.

Heat Pump Dryer

This dryer simply keeps pushing warm, “room temperature” air across the clothes rather than cooling and reheating. Because it operates at a much lower temperature, it can take a long time to complete a load.

Is It Safe to Vent an Electric Dryer Inside?

Electric dryers work by evaporating moisture from clothes and pushing that warm, moist air out through a vent. Exhaust air from any dryer must be directed outdoors. With dryer temperatures reaching 130 degrees Fahrenheit, it’s just not safe to be released inside.

Bacteria, mold, and fungal growth can result in severe health issues. Moisture from routine condensation can damage window sills, cause your wallpaper to bubble, warp and rot wood, and even damage your paint. Without exception, electric dryers must be vented outdoors.

Can You Vent a Dryer Through the Attic?

When considering running a dryer vent through the roof, you almost always have to go through an attic first. An ideal installation of a dryer duct through an attic is a straight vertical line that runs from the dryer directly through the attic to the rooftop. Bends, turns, or horizontal sections in an unheated attic can create unwanted challenges related to condensation and lint build-up.

Condensation is a major concern for the section of duct running through an unheated attic. During the summer when attics can exceed 100°F, there is no real issue of condensation. Moist, hot air from the dryer being pushed through a 4-inch metal duct will always rise and carry moisture with it.

However, when the attic becomes cooler, warm, moist air from the dryer quickly condenses in that colder section of the metal duct before it has a chance to escape. Water will build up inside the duct walls and run back down towards the dryer and collect.

However, any horizontal or lateral attic ductwork can present a much bigger concern. If horizontal sections of the duct are perfectly level or sag in the direction of the outlet, condensation collects and becomes trapped. Worse, as your dryer runs, tiny lint particles get trapped by the condensation and accumulate like a rolling snowball that gets bigger and bigger.

The growing mass gradually restricts airflow, making your dryer work harder. Eventually, the duct fills completely where continued use results in a continually growing debris clog downward toward the heat source. If the blockage is not cleared, chances increase exponentially for a significant and costly fire.

You can vent through the attic, but it’s important to take proper precautions to make sure condensation and lint build-up never become an issue.

Install all ducts with a slight slope backward toward your dryer. You want any moisture to drain downward toward your dryer.
Wrap the metal dryer vent ducts with insulation to reduce the chance of condensation forming on the cooler metal surfaces when attic temperatures start to cool.
Keep vent duct angles to an absolute minimum. Clearing blockages in a duct system with 90-degree elbows presents a lot of challenges and could require a complete dismantling and reassembling of the attic section.

Can You Vent a Dryer Through a Soffit?

Just as dryer vents can run through an attic to a roof, a dryer duct system can run through a soffit – with certain exceptions. The construction of the duct must fit securely inside the soffit and follow all dryer vent code requirements. The same precautions suggested for an attic install should also be incorporated.

Existing soffit vents cannot be used as a point of termination. Soffit vents are considered intake vents. Code requires all dryer vents to terminate not less than 3 feet from any opening to a building and explicitly mentions staying away from ventilated soffits.

Where is the Best Place to Vent a Dryer?

The shortest and straightest route at ground level is always the goal when deciding on the best place to install your dryer vent. When possible direct your vent on a horizontal path through the closest exterior wall.

A slight downward slope towards the outlet will keep condensation from draining back towards the dryer. Ideally, the vent outside is no less than 18-inches above ground level but no more than 3 feet. Easy outdoor access to the dryer vent makes for quick maintenance.

Should You Vent a Dryer Through the Roof?

In most cases, a dryer vent directed through a roof should only be considered as a last resort. However, when the roof offers the shortest and straightest path to the outdoors, there is no reason to exclude it from consideration as a viable option. The good news is it’s easy to mitigate concerns about the effects of condensation and potential damages with proper attention to just a few construction details.

Hopefully, you found this information helpful and you have a better understanding about venting a dryer exhaust through a roof. If you liked this article and think it could help others, please take a moment to give it a share. More importantly, if you’d like to share any of your own experiences or have any questions, please drop me a note below.

Should a Dryer Be Vented Outside?