Flat Roof Ventilation: Ultimate Guide

As the homeowner of a flat roof house, you discovered that your roof is not fitted with any vents.  Questions you might have probably center on whether a flat roof needs ventilation and if so, what is the effect if there have no ventilation vents been installed, and what type of vent should you install to rectify any recent and possible future problems. 

The main benefit of ventilation is that it gives your flat roof a longer lifespan. Proper roof ventilation helps prevent too much condensation from building up. Condensation can lead to serious moisture damage and the forming of mold. Fortunately, there is a variety of vents available for flat roofs to choose from.

In this article, we’ll discuss why roof ventilation is needed for flat roofs and list the most commonly used vents. Some “related terms, like “cold” and “warm roofs,” “wind tunneling,” and “vapor control layers” will also be explained.

A flat roof with ventilations

Cold roofs and warm roofs

When you start looking at ways to ventilate your flat roof, you have to know what the difference between a cold and a warm flat roof is.  Although both these terms describe insulation techniques, they are both relevant as well when talking about ventilation. Let’s start by explaining what each type of roof entails. This will help you to follow the ventilation discussions.

Cold roofs

A roof is called a “cold roof” because of the insulation technique used.  With this technique, insulation is laid between the rafters. Because the only thing between rafters is empty space, this is a relatively low impact way of insulating a roof.

But why call it a cold roof?  The reason is that although the major part of the ceiling below the roof is well insulated, the rafters are not insulated. As a result, the cold will conduct through the rafters on cold winters days into the room below. 

This type of insulation is relatively easy to install when it is fitted while building the house, but it can be very time consuming and tricky to install in an existing flat roof home. When you want to insulate an existing flat roof with this technique, you have to rip up any existing roof surface to enable you to lay the insulation in between. It also requires that the roof surface has to be replaced or redone.

The benefit of a cold roof is that it has ventilation, which allows heat to escape from beneath the shingles.

A warm roof

As opposed to the cold roof technique of insulating between rafters, you simply install insulation on top of existing surfaces with the warm roof technique. If the existing roof surface is still in good condition you only have to brush off all the debris and then secure insulation boards to it with adhesive. You can then lay any type of flat roof covering on top of it.

As the entire roof structure is insulated with this technique, it is more energy-efficient than a cold roof.

Why is roof ventilation needed – even with flat roofs?

Your day-to-day activities in your house produce a lot of warm, moist air. Activities such as cooking, showering, and laundry washing all contribute to the warm and moist air.  Come to think of it – it is quite a lot of warm and moist air that is produced in a home with say 2 adults and 3 children. Five people are taking a shower every day, cooking is done most of the days and there is a lot of laundry being done every week. 

All this warm and moist air must go somewhere and without good flat roof ventilation, this air can be the source and the cause of much damage to your building and your health.

To avoid condensation

The warm air rises and naturally collects in your home beneath the roof. This air can become trapped in your roof space if there’s nowhere to go. If your house has a hot roof, the warm air will change the temperature of the roof decking because there is no ventilation to relieve the heat. Condensation is then caused by the difference in temperature between the roof deck and the outside air.

During the winter, you might think that your roof doesn’t need vents as the vents allow the comfortable warm, conditioned air to escape to the outside. But ventilation is always necessary to prevent some problems that might develop because of condensation.

To understand the concept, think of your car on a cold morning. When you turn on the heat in your car, the window develops some frost. In other words, warm air at one side of a surface can cause frost on the side of the same surface. 

To avoid rot of roofing materials

The thin layer of condensation in a roof can become a big problem. It can slowly rot out the decking and all your roofing materials. The moisture can even drip down into the rooms beneath it causing a lot of damage. So moisture and heat need a way to vent outside. Vents will ensure that the roof and the air outside are at the same temperatures. 

Ventilation is also necessary during the summer because differences between the roof temperature and the outside air can still cause condensation.

Therefore your home’s flat roof needs ventilation. 

To keep mold out

The temperature differences that cause condensation can apart from starting the rotting of the roof material, lead to the forming of mold. The mold can form in the roof and then spread to the rooms beneath the moist places in the roof. This can have an influence on your and your family’s health. It is an expensive task to get rid of mold in your house.  

The roof has cooled off due to external temperatures and the hot air condensates on contact and create moisture in the air. Ventilation is necessary to let the hot air that contains the most moisture escape. Then cool, fresh air replaces it. This will stop the water from condensing and prevent the forming of mold.

How much ventilation does my flat roof need?

The type of roof you have will determine how much ventilation is needed. Generally, cold roofs require less ventilation than warm roofs. Cold roofs are built with natural ventilation, but additional ventilation is still necessary. 

To ensure sufficient ventilation, there must be enough space to allow for the movement of air. Some local authorities’ building regulations require that a ventilation gap of at least 50mm must be in place between the insulation and the underside of the deck of the flat roof.

Flat roof venting options (How to ventilate a flat roof?)

Let’s look at possible venting options for flat roofs. 

Cant Vent

With this venting option, you install the vent between the roof joists. You can use it in a perfectly flat and horizontal environment with a built-in cant that allows for water runoff. 

Pop Vent

The louvered pop vent is very commonly used in homes that have a flat roof. The Pop Vent either brings air into the roof space or allow heated air to be eliminated from the roof space with electrical fans. The Pop Vent is a very effective venting system. 

Powered Bathroom Vents

To get rid of steam and heat many people install an exhaust fan in their bathroom. A flat roof is ideal for this type of vent as the vent can then be placed directly above the bathroom ceiling. The Bathroom Vent can be driven by wind power but usually, it is electrically powered.

Mushroom Roof Vent

The Mushroom Roof Vent is also a commonly used vent for a flat roof. It is a long, slender aluminum vent in the shape of a mushroom when fully installed. Mushroom Roof Vents help to not only stabilize the temperature within the roofing system but also allow moisture to escape.

Source

Box Roof Vent

This is a square or rectangular shaped basic aluminum roof vent. Although Box Roof Vents are used primarily in commercial buildings, they are also used in larger flat-roofed homes. The box of the vent covers all the piping with a large cover and shields the vent from rain and snow. It is popular because of this shielding feature.

Boot Vent

Many residential homes with flat roofs make use of the Boot Vent. This is a very basic venting system and acts as a base and holder for the piping that is coming through the roof. The rubber boot fits the exact size and shape of the pipe. This keeps water from leaking in. Built-in flashing holds the vent in place.

Breather Vent

The Breather Vent allows moisture to escape while eliminating any moisture from entering. Breather Vents use billows that open and close according to the moisture content in the roof.

Aura Vent

The aura vent is the ultimate choice for a flat roof vent. It has a rounded head that goes around to take the hot air out of the roof and at the same time bring in fresh, cooler air. 

What to look for when you want to install vents for a flat roof

When you have to decide on the vents you have to install it is good to check whether they have the following features. You will most probably not get a vent that has all the features, but it can make the comparison between different vents easier. 

  • Look for a low-maintenance vent. Preferably it should not have moving parts, and if there are moving parts like a fan, the fan should preferably use wind and not electricity. 
  • Check the level of noise created by the vent.
  • The vent should be rodent-proof: Preferably the vent should have a built-in wire mesh screen.
  • Find out whether the vent is stormproof and can withstand extreme wind, rain, cold, and heat conditions.
  • Read the warranty to see what is included in the warranty. 

What is Wind Tunneling?

When reading and researching about roof ventilation, you may come across the term “wind tunneling”. Simply put, this is when air flows through one vent and directly out of another. 

Usually, this occurs when two vents have been installed at the same level but on opposite sides of a roof. It is more a technique used in pitched roofs but the principle can be applied to flat roofs as well. 

Wind tunneling is ideal to provide a strong supply of clean air. However, you should install the vents at different levels if you want to successfully deal with a condensation issue. 

What are vapor control layers?

“Vapor control layers” is another concept you will hear about when looking at ventilation problems and solutions. In simple terms, vapor control layers are designed to prevent moist air to pass into your roof space. Nowadays, these layers are usually an automatic feature in roof ventilation systems. 

Previously, before control layers have been developed, vapor control barriers have been used. But the problem with barriers is that they don’t let moisture in, but also don’t let moist air from inside the house out. The moist air inside the house and roof has nowhere to go and simply condensates and can cause havoc in the roof space. By using modern vapor control layers instead of barriers, you don’t create an environment where the moist air can be trapped and cause condensation.  The layers allow moist and hot air to escape to the outside. 

Conclusion

As indicated in this article, flat roof ventilation has many shapes and forms. But the bottom line is that there have to be space and the necessary “tools” in the flat roof to always ensure that there’s a proper amount of air circulation throughout the year. 

It’s not so essential for houses with warm flat roofs, although it is recommended that even warm roofs must have some ventilation. 

Cold flat roofs need ventilation because they collect higher amounts of condensation. 

We hope that this article will help you to find the right type of vent for your flat roof home. 

Charles John

Experienced HVAC technician with 8 years of experience in the industry. Capable of handling all sorts of heating and cooling equipment as well as proficient in operational management, construction-related techniques such as preventative maintenance, electrical troubleshooting and AutoCAD

Latest Posts