A roof exhaust fan motor must operate continuously at the highest point of a building, directly exposed to the open atmosphere. Whether used for kitchen exhaust, general ventilation or moisture extraction, motor selection here depends not only on power but also on outdoor protection, vertical discharge, the airflow-pressure curve and the duty type. A poorly chosen motor either fails quickly under moisture and UV exposure or cannot deliver enough airflow to meet ventilation demand. This guide explains step by step how to specify the right motor for a roof exhaust fan.

What Is a Roof Exhaust Fan and Which Motor Does It Need?

A roof exhaust fan is mounted on the roof plane and draws air out of the building, discharging it upward into the open atmosphere. It usually uses a centrifugal impeller; air is drawn from below, accelerated by the impeller and discharged through a vertical or radial damper. The motor position is critical: the motor is most often placed inside the fan housing, out of the airstream, so that moisture, grease or heat from the exhaust air does not contact it directly.

This arrangement differs completely from an in-line axial duct fan. In a duct fan the motor usually sits within the airstream and connects to an internal duct; in a roof fan the motor sits outdoors, in a vertical discharge position. If you are considering an in-duct application, our duct-type axial fan motor selection guide is more suitable. In a roof fan, the core challenge is operating the motor safely in the open air.

Effect of Vertical Discharge and Mounting Position on the Motor

In roof fans, air is usually discharged upward, vertically. The motor shaft is most often vertical, which directly involves the motor mounting position (IM V1 or a similar shaft-up/down arrangement). In vertical mounting, rainwater must not reach the bearing, drain holes must be correctly positioned, and the seal type must be appropriate. You can find these details in our separate vertical-mounting motor selection article. An incorrect mounting position leads to early bearing failure at a hard-to-access point like a roof.

Outdoor Protection: IP55 or IP65?

The most critical decision for a roof exhaust motor is the protection class. Standard industrial motors come with IP55 protection, meaning full dust protection and resistance to water jets from any direction. In roof positions exposed directly to rain, snow and UV, IP55 is sufficient for most general ventilation; however, in heavy rainfall, snow accumulation or coastal/aggressive environments, IP65 should be preferred. IP65 adds resistance to low-pressure water jets on top of full dust protection.

We cover how to choose the protection class and the differences between IP55/IP65/IP66 in depth in our IP protection class selection article. If you need to upgrade from standard IP55 in dusty and wet environments, our IP65/IP66 protection upgrade content offers a practical roadmap. Our catalog motors are produced as standard with IP55 protection and Class F insulation; higher protection is available on request.

Extra Measures Against UV, Moisture and Condensation

An outdoor motor warms during the day and cools at night, forming condensation inside the frame. Over time this moisture weakens the winding insulation. Roof fans that do not run continuously and stop intermittently during the day face a higher condensation risk. In such cases an internal space heater or a tropicalized winding is recommended. For details, see our tropicalization and humidity protection article. Class F insulation provides high-temperature endurance and leaves a margin against the thermal swings of the outdoor environment.

Airflow, Pressure and Correct Power Matching

The basis of sizing a fan motor is the fan's operating point: the required airflow (m³/h) and the total pressure (Pa) developed while delivering it. The fan's power demand is found approximately by multiplying airflow and pressure and dividing by the fan efficiency. Roof exhaust fans in general ventilation operate at low pressure and high airflow, so 4-pole (1500 rpm) or lower-speed motors are usually preferred. In applications such as kitchen exhaust that carry greasy and hot air, both the power margin and the temperature endurance are increased.

To see the required-kW calculation for pumps, fans and conveyors step by step, our motor power calculation article is a good start. Because fans present a variable-torque load, the power demand changes with the cube of speed; that is, reducing the speed slightly cuts the power significantly. We address this relationship in our load-type-based motor sizing article.

Pole and Speed Selection: Balancing Quiet Operation and Efficiency

In roof fans the speed choice determines both noise and airflow. High speed (2-pole, 3000 rpm) provides high pressure in small-diameter fans but is noisier and increases aerodynamic sound. For general ventilation, 4-pole (1500 rpm) or 6-pole (1000 rpm) motors run quieter and last longer. Low speed is preferred in large-diameter, low-pressure fans. We compare the effect of pole count on efficiency and torque in our pole count and efficiency article. For the right pole choice, our 2/4/6 pole selection guide is also useful.

Continuous or Intermittent Operation? Duty Type (S1/S3)

A roof exhaust fan runs continuously (S1) in some buildings and intermittently (S3) in others, cycling on during the day. The duty type determines the motor's thermal behavior and therefore its power margin. A continuously running fan needs a motor suited to S1 duty that can reach thermal equilibrium at full load. In fans that start and stop frequently, the high current drawn at each start heats the motor; in that case, the duty type and the number of starts per hour must be considered.

We explain what the duty types mean in our S1-S6 duty type selection article. To avoid exceeding the thermal limit in intermittent duty, our S3/S4 intermittent duty content provides guidance. In a continuously running exhaust fan, efficiency directly affects energy cost, so an IE3 or higher efficiency-class motor should be preferred.

Differences Between Kitchen Exhaust and General Ventilation

In kitchen exhaust, air carries grease vapor and high temperature. Even if the motor stays out of the airstream, the ambient temperature is high, so a higher insulation class and temperature endurance matter. In general ventilation (offices, parking entrances, warehouses) the air is relatively clean and the temperature is moderate. In both cases, outdoor protection must not be neglected. If fire and smoke extraction is involved, special high-temperature fan motors are required; our fire exhaust and pressurization fan motor article covers this in detail.

Noise and Vibration in Roof Fans: Avoiding Neighbor Complaints

Roof exhaust fans usually run above or near living spaces, so the noise level is a criterion that should not be overlooked during selection. Fan noise comes from two main sources: aerodynamic (impeller and airflow) and mechanical (motor bearing, imbalance). High-speed motors produce more aerodynamic sound, so in buildings that also operate at night, the lowest practical speed is preferred. The motor's own vibration level also matters; a well-balanced rotor does not transmit vibration to the roof structure and, over the long term, prevents loosening and fatigue cracks.

We gathered all aspects of selecting a low-noise motor in our noise and vibration article. We covered how the motor's vibration class is determined and its acceptance values in our vibration and balance (ISO 10816/20816) content. On lightweight, resonance-prone structures like a roof, a low motor vibration value keeps the fan running quietly too.

Commissioning and Maintenance: Account for Difficult Roof Access

A roof is one of the hardest points to reach for maintenance. Therefore, long maintenance intervals and durability should be prioritized in the initial motor selection. Under the effects of rain, snow and thermal cycling, the bearing grease life can shorten, so motors with re-greasing capability (grease nipples) are preferred in larger sizes. Before commissioning, the rotation direction, insulation resistance (megger) and vibration should be checked. You can perform these checks at stock entry and before installation using the steps in our incoming and acceptance inspection article.

Correctly setting the bearing re-greasing interval and grease type significantly extends the life of a roof motor; for details, see our bearing greasing content. In motors operating outdoors, the fan cowl and protective guard must also be robust against dust and foreign objects; our fan cowl and guard selection article is useful on this point.

Mechanical Compatibility and Order Details

If you are replacing the motor of an existing roof fan, the new motor's frame size, shaft diameter, mounting type and flange dimensions must match the old one exactly. Otherwise the fan impeller will not seat correctly. To read all nameplate data correctly, our nameplate and label reading guide prevents ordering errors. To determine the mounting code (IM B3, IM V1, etc.) correctly, our IM mounting code reading article is handy.

Roof exhaust fans mostly run with centrifugal and axial impeller types. For power and speed matching by fan type, our centrifugal and axial fan motor selection content is complementary. For other outdoor fan applications such as cooling-tower and chiller fans, our cooling tower and chiller fan motors article also details outdoor IP protection.

Roof exhaust fan motor outdoor mounting and IP protection

Summary of Roof Exhaust Fan Motor Selection Criteria

For the right choice, follow this order: first determine the required airflow and pressure, then select the kW and speed suited to that operating point. Next, decide the protection class (IP55 or IP65) based on the outdoor conditions. Clarify the duty type (continuous/intermittent) and add extra protection if there is a condensation risk. Finally, verify mechanical compatibility with the existing fan. This approach safeguards both ventilation performance and the motor's long life on the roof.

Regarding efficiency class, an IE3 or IE4 motor significantly reduces energy cost in continuously running fans. You can find when each efficiency class becomes mandatory in our IE3 and IE4 efficiency mandate article. For aspiration and dust collection applications, our aspirator and dust collection fan motor selection content is also a related resource.

Selecting motor power by roof fan airflow and pressure curve

Frequently Asked Questions

Is IP55 enough for a roof exhaust motor, or is IP65 required?

For most general ventilation applications, IP55 protection is sufficient; this class provides full dust protection and resistance to water jets from any direction. However, where heavy rainfall, snow accumulation, coastal exposure or an aggressive chemical environment is involved, IP65 should be preferred. The decision is made according to the climate and exposure conditions of the roof where the fan sits.

Which speed is more suitable for a roof fan motor?

General ventilation needs low pressure and high airflow, so 4-pole (1500 rpm) or 6-pole (1000 rpm) motors run both quietly and efficiently. Small-diameter fans that need high pressure use 2-pole (3000 rpm) motors, but noise increases. The fan's operating point (airflow-pressure) determines the speed choice.

What should I watch when replacing the motor of my existing roof fan?

The new motor's frame size, shaft diameter, mounting type and flange dimensions must match the old motor exactly; otherwise the fan impeller will not seat correctly. Read the kW, speed, mounting code and frame information on the old motor's nameplate correctly and order the same. Also preserve the vertical mounting position and the protection class.

Get a Quote

Contact us to supply from stock the right motor in the correct power, speed and protection class for your roof exhaust fan. Send us your airflow, pressure and mounting details, and we will determine the suitable motor for your application together. Phone: +90 (532) 345 49 86. For a detailed quote, you can use our contact page.

Pre-Purchase Checklist

  • Have the required airflow (m³/h) and total pressure (Pa) been determined?
  • Have the kW and speed (pole count) suited to the operating point been selected?
  • Has the protection class (IP55/IP65) been clarified for the outdoor conditions?
  • Has the duty type (continuous S1 / intermittent S3) been defined?
  • Is a space heater or tropicalization needed for condensation risk?
  • Have the vertical mounting position and shaft direction been set correctly?
  • Has frame, shaft and flange compatibility with the existing fan been verified?
  • Has the efficiency class (IE3/IE4) been assessed for continuous operation?