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How Many Roof Vents Do I Need

how many roof vents do i need

I​f you’re getting ready to replace your roof or you’re building a new home, you may have to make a decision about roof vents. You might be asked how many you want, and, if you are, how are you supposed to answer that question? R​oof vents are the sort of thing that most homeowners have never had to pay much attention to. You probably didn’t even know that your roof had vents until now. Don’t worry, we’re here to help!

roof vent on house

What is a roof vent?

R​oof vents are, well, vents on the roof. But not every vent on the roof is a roof vent- the fan over your cooking range, bathroom vents, and dryer vents may all vent out through the roof, but these aren’t the vents we’re talking about. R​oofs vents exist to keep your attic well-ventilated. When properly designed and installed, they’ll pull hot air and moisture out of your attic and pull cooler, drier air into it. That means they aren’t tied into other things like the dryer vent, or the vents for your bathroom and kitchen fans.

Y​ou need two kinds of roof vent- intake and exhaust vents. A roof exhaust vent would be useless without intake vents. You can install the same kind of roof vent for both purposes, as the key difference between the two is location: intake vents have to be lower on the roof than exhaust in order to create proper airflow.

Why does my roof need a roof vent anyways?

I​f you’re wondering why your roof needs vents, you’re not alone. It’s not the kind of thing that’s self-explanatory, after all. Roof vents have a couple of primary benefits that you should know about.

F​irst, by keeping the air circulating through your attic you prevent the buildup of moisture. No matter where you live, if your attic isn’t properly ventilated, moisture will build up inside over time, and that’s going to cause damage.

S​econd, keep the air circulating prevents the attic from getting too hot. Extreme temperatures in the attic can actually damage the shingles on your roof, and that’s an expensive repair. More importantly, if the attic gets too hot it’s going to affect your home’s cooling system. Keeping the whole house cool becomes much more difficult, and your energy bills go way up.

How many roof vents do I need?

roof vents on house with skylight

N​ow that we’ve established that you do, in fact, need vents in your roof, you need to figure out how many roof vents you need. At a minimum, you need 1 square foot of ventilation space (6 square inches each of intake and exhaust) for every 300 square feet of attic space.

H​owever, that’s the minimum. You may want more than just the bare minimum here, especially if you live in warm or humid climates. In addition, the type of roof vent you install will also make a difference, since different types of roof vent perform differently.

O​n top of that, the slope of your roof can affect your ventilation needs as well. A steeper slope on the roof means that the attic has more volume than an attic with similar square footage and a flatter roof. So, if you have a steeper slope on you roof, you’ll need more ventilation.

Y​ou’ll also need more ventilation if your roof doesn’t have vapor barrier. That’s because without a vapor barrier you’re going to have more moisture buildup inside the attic. In general, you need twice as much roof ventilation if your attic doesn’t have a vapor barrier.

Y​our local building codes may also have their own specifications that need to be met. To start with, use this roof vent calculator to get a basic idea of how many roof vents you need, and then check the building codes.

What kind of roof vents are there?

T​here are six common types of roof vents that you can choose from for your home. Each has its own advantages, and a few have distinct disadvantages.

W​ind Turbines

I​f you go outside and take a close look at the roofs of the houses in your neighborhood, your bound to see a lot of these. They have a round, spinning fan on top and they look vaguely like the Queen of England’s Crown, if her crown were made of aluminum.

A​s the name suggests, these are wind-powered. The wind spins the turbine, which pulls air out of the attic. They don’t need a very strong wind to work, and they’re very effective. These are the most common roof vents that you’ll see, and you’ve probably seen them your whole life without realizing what they are.

B​ox Vents

T​hese are the simplest vents. They’re basically just small chimneys, little more than holes in the roof. Box vents rely on natural convection: hot air rises, so as the air in the attic heats up it will rise out of the box vents on the roof and be naturally replaced by cooler air through the intakes.

Since box vents don’t use any kind of fan to actively pull air out of the attic, you need more of them to get proper roof ventilation. That’s the downside. The upside is that there are no moving parts that can break or wear out, so these are effectively maintenance-free.

C​upola Vents

Y​ou’ll often see cupola vents on houses that don’t need them- they’re pretty, so lots of homeowners just have them as a decorative touch. Functional cupola are usually paired with other types of vents to improve efficiency.

S​offit Vents

soffit roof vent

S​offit vents can allow a lot of air into the attic, but they’re effectively an intake-only vent. Because the soffits are the lowest point of any roof, you would need a powered fan in the soffit vent to pull hot air out of the attic, and even then it simply wouldn’t be very efficient.

R​idge Vents

W​idely considered to be the most efficient roof vents you can install, ridge vents are installed horizontally along the ridge of the roof. In addition to efficiency, this has the added advantage of making them blend right in to the roof, so there are no visible exhaust vents along the roof.

P​ower Vents

A​s the name suggests, these vents have powered fans to draw air out of the attic. They are usually tied into thermostats and humidistats, so once they sense the temperature or moisture levels rising above a preset level, they turn the fans on. T​hese systems are probably more expensive to install, run, and maintain than most homeowners would like, and they’re also more powerful roof vents than a private residence would likely need.

Gable Vents

T​hese vents really aren’t in the roof, they’re in the gables and they run through the siding on your house. Since they fulfill the same function as roof vents, we’re including them here. Gable vents can be triangular, circular, or rectangular. Y​ou’ll often see older homes using only gable vents in their roofs, especially when their roofs have very gentle slopes. When the roof has a steeper slope to it, gable vents alone are likely insufficient to meet your needs. They still make excellent intake vents, though.

Which kind do I need?

I​f you want to maximize the efficiency of your roof ventilation, the best roof vents you can have would be a ridge vent for the exhaust, and soffit vents for the intake. This will create a lot of airflow through the attic, but with no moving parts at all. It’s an ideal system, but only if you have enough soffit vents. H​owever, that may not be an option for your home. Many older homes and a good number of newer ones use wind turbine roof vents for their exhaust, and unless you want to make major (and expensive) modifications to the structure of your roof, you’ll need to stick with these. In addition, most older homes don’t have soffit vents, and rely on gable vents for their intake. S​offit vents are a great choice for your roof intake vents no matter what type of exhaust vent you use, but depending on the design of your roof, you may not be able to install soffit vents. If that’s the case, just remember that hot air rises. So, simple box vents installed lower on the roof can function very well as your intake vents if you don’t have gable vents.

C​an I use a ridge vent without intake vents?

W​hether you install intake vents or not, your roof vents have to pull in outside air from somewhere in order to work. Roof venting is inherently a system of intake and exhaust; if you don’t install separate intake vents, then your ridge vent is going to functioning as both the intake and the exhaust.

A​ir always moves from areas of high pressure to areas of low pressure. As hot air rises, it creates an area of low pressure underneath it. When this happens outdoors, it generates wind.

W​hen that hot air is escaping through a roof vent, it will also create air movement. If you’ve got soffit vents, the air moving out of the ridge vent creates low pressure at the soffit vents, so air moves into the attic through them.

I​f you have no intake vents, then one side of the ridge vent will be the exhaust, and the other side will be the intake. This will provide some ventilation for the attic, but not anywhere near enough. For any kind of roof vent to function properly, you need separate intake and exhaust vents.

A​re roof vents really necessary for cold climates?

I​t may be hard to imagine your attic getting too hot if you live someplace best known for cold weather. And, in fact, your attic may not get hot at all. That doesn’t mean you don’t need roof ventilation though. I​n the winter, hot air from inside your home will rise to the attic. If that air has nowhere to escape, it’s going to condense in the air, leading to huge moisture problems. You’ll end up with an attic full of mold and mildew by the time spring comes. In addition, with no roof vents, the hot air trapped in the attic will heat your roof. This will melt any snow that’s accumulated on the roof, but as the water runs down to the edge of the roof it will freeze again, creating ice dams which in turn causes water to pool on the roof. This can lead to serious roof damage.

S​o, no matter where you live, your roof needs vents.

H​ow do I know if I need better roof vents?

roof ventilation in attic

I​f you’re like most homeowners, you probably don’t pay much attention to your roof vents. The signs of poor roof ventilation are clear and noticeable, but many people don’t associate them with the roof. I​f you’ve got unusually hot or cold areas in your home, that’s a strong indicator that you may have poor roof ventilation. Your heating or cooling systems in the home aren’t able to keep up with your needs, and that’s often because the roof isn’t properly ventilated. L​ikewise, if your air conditioner breaks down, you should check the roof vents. When hot air gets trapped in the attic, it can cause the air conditioner to work overtime trying to cool the home. I​n cold climates, ice dams on the roof are a sure sign of poor ventilation. Hot air trapped in the attic melts the snow on the roof, which runs down to the edge of the roof and refreezes, forming an ice dam. If you’re having a problem with ice dams, you’ve got poor roof ventilation.