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How Much Ventilation Does a Flat Roof Need?

how much ventilation does a flat roof need

A flat roof is a type of roof that has no slope. In order to function properly, it needs ventilation from the inside and outside in order to avoid condensation from occurring.

This is where waterproof membranes come into play – they can provide protection against moisture while still allowing for natural ventilation. So, how much ventilation does a flat roof need?

What is Ventilation?

Ventilation is air that comes into a room from the outside. It makes the air in a house better to breathe and helps remove moisture from the air.

The Importance of Ventilation for Flat Roofs

A flat roof needs as much ventilation as it can get, but often, many flat roofs are not well ventilated. If there is little, or no ventilation at all, it can have a lot of problems.

Some of these are:

  • Moisture may accumulate on the roof surface. If this happens for extended periods of time (usually months/years), problems like mold and mildew will begin to appear.
  • The water that accumulates on the top side of the roof (usually called ponding water) may leak into the structure of the building.

This means that ventilation not only makes your home/roof better to live in, but it also makes roofs last longer and saves on energy costs.    

The Materials That Can Be Used for Ventilation

There are a few ways to ventilate a roof. The following are the most popular ones among them:

Metal ventilation pipe/tubes can be attached at regular intervals to the area of the flat roof. 

There should be an inlet on one side and an outlet on another, so that when wind blows through, it sucks/pushes air through these openings.

The metal tubes can also be made with insulated walls. This way, heat will not accumulate inside the tubes and damage them. 

These are often seen to be very effective for ventilation.

Wood beams (or similar solid building materials) can run along the roof surface (flush to it), on different intervals. 

The air will circulate through these beams, because this makes it easy for wind to blow through them.

The drawback with this system is that it traps heat (which can build up over time), but they are still effective because the movement of the air will also distribute heat, instead of letting it accumulate at one place. 

However, there are some kinds of wood beams that have perforated sections in them, which will allow for more ventilation.

Roof tiles/slates can be made in such a way that they have openings on them, to make it easy for wind (and air) to blow through them and ventilate the roof surface. 

They are usually not very effective unless there is a system designed to cool down heat too.

How to Calculate How Many Ventilation Tubes You Need

It is quite easy to determine the number of ventilation tubes that are needed for a flat roof.

The first thing to do is find out how much area your roof covers (in terms of sq. footage), and multiply it by 0.09 (because you need 10% of the roof to be covered in ventilation pipes).

For example, if your roof is 400 sq. feet, you would need 36 tubes for proper ventilation (400 x 0.09 = 36). 

This number may change depending on how wet the local environment is (a very humid area will require more than 10% of the surface to be covered by ventilation tubes).

How to Install Ventilation Tubes

You should hire a professional if you want to install ventilation pipes on your flat roof. 

If you feel confident about doing it yourself, make sure that the type of material used in the installation has an R-value (heat resistance/resistance to heat) of at least 3.5 (the insulation value provided by the material) and cover the metal ventilation tubes with a layer of gravel (or other material), to make sure that there is no heat accumulating (which could melt and damage the pipes and cause problems).

Best type of ventilation for a flat roof?

The best type of ventilation system for a flat roof is the ridge and soffit system, which includes an “inverted” gable fan that vents the warm air from under the shingles.  

The ridge vent includes two or three chambers that allow air to enter at the bottom and exit through the gable end. 

This type of ventilation system can cost around $300 (including materials and labor) but does not require much maintenance.

The next best option is a “controlled” roof ventilation system, which consists of perforations in your ceiling, through which outside air enters your home, keeping the inside air cool. 

This type of ventilation system costs around $100, but does not work very well if other areas are blocked.

If you want to go even cheaper, you can opt for passive roof vents ($40), which are easy to install and maintain (you just have to clean them every once in a while) but do not work very well if you live in an area where there is hot weather most of the time.

What happens without ventilation?

When the roof is ventilated, it means that hot air can escape at its top and flow out through the ridge or soffit vents. 

Otherwise, this hot air will remain trapped under the shingles and accumulate, creating a hot roof. 

If there is no ventilation at all, heat can build up to such an extent that it melts the rubber seals on the metal vent pipes, which could damage your roof and lead to a big repair job.

How much ventilation does a flat roof need?

The bare minimum amount of ventilation for a flat roof is 1 square foot per every 150 square feet of surface area. 

In other words, if your flat roof measures 100 square feet, then you will need to install 10 square feet of ventilation. 

Keep in mind that this is the bare minimum and an inverted gable fan would be a better option.

Conclusion

How much ventilation does a flat roof need? 

The answer depends on the local weather, but in general it is advisable to have at least one square foot of ventilation for every 150 square feet of surface area. 

If you are looking for an inexpensive solution that will be easy to install and maintain, use passive vents or ridge/soffit vents. 

If you live in a hot and humid area, use the inverted gable fan.