Heat builds up near the roof because warm air weighs less than the air around it. It rises until it meets the ceiling. There it forms a layer that grows hotter through the day. Solar gain through the roof adds heat from above. Machinery, ovens, welding and the workforce add heat from below. In a high-bay warehouse, that layer holds the hottest air in the building. It also moves the least on its own. Fanquip’s roof fans pull it out at the height where it sits, which is why roof-level extraction beats extraction placed lower down.
Overview.
- Warm air rises and collects at ceiling level, forming a layer that grows hotter as the day goes on.
- Two sources feed that layer: solar gain through the roof, and process heat rising off the floor.
- A field study in large warehouses measured gradients of 0.8 to 2.6°C per metre through the lower two metres.
- Roof fans extract at the height where heat sits. A wall fan draws from mid-height and misses it.
- Room volume and application set the required airflow in cubic metres per hour, which then sets fan size.
Warm air rises because it weighs less than cool air.
Warm air rises because it is lighter than the air around it. Cool air sinks because it is heavier. In an enclosed building this creates thermal stratification, a layering of air temperatures that climbs from floor to ceiling. Higher ceilings produce bigger differences.
A field study of two large industrial warehouses measured this directly. Through the lower two metres, temperatures climbed 0.8 to 2.6°C per metre. Above that, the gradient settled to around 0.5°C per metre. The same two-layer pattern turns up across buildings with different roof shapes, door types and ceiling heights. That points to physics rather than building design.
The practical result is simple. The hottest air in the building sits where nobody works. Most ventilation equipment sits too low to reach it.
Roof heat comes from two directions at once.
Solar gain pushes heat down from the roof. Process heat pushes it up from the floor. In an Australian summer both run together.
The roof is the largest surface facing direct sun for most of the day. Metal sheeting absorbs that energy and radiates it into the space below. Dark roofs absorb more than light ones.
Industrial buildings also make their own heat. Air conditioning often struggles in these spaces because heat builds continuously from machinery, ovens, welding equipment and the workforce. All of it rises. None of it leaves without help.
What feeds the ceiling layer.
- Solar gain. Roof sheeting absorbs radiant heat and releases it downward through the day.
- Process heat. Ovens, furnaces, welding bays, motors and compressors add heat during production.
- Lighting and plant. High-bay lighting and equipment add a smaller constant load.
- Occupancy. People working in the space generate heat that rises with everything else.
Trapped ceiling heat pushes down to working height.
The hot layer does not stay put. It grows through the day and pushes downward. By mid-afternoon it can reach the height where people work.
Heat exposure is a hazard the business has to manage. Safe Work Australia’s guide to managing the risks of working in heat sets out four actions to protect workers. It names four factors to assess: air temperature, air movement, humidity and radiant heat. A hot roof radiating downward feeds the last one directly.
Where the business cannot eliminate heat risk, it must minimise that risk as far as practicable. Controls run wider than hydration and rest breaks. They cover the heat itself, air movement and fresh air supply. Fanquip’s guide to preventing heat stress in the workplace covers how this applies on site.
Roof extraction removes the hot layer where it sits.
Roof fans draw air vertically from ceiling level. That is where heat and steam gather naturally. In high-ceiling buildings they outperform wall fans for heat removal. A wall fan extracting from mid-height in a 10 metre warehouse cannot reach the hottest air.
Roof fans carry contaminants out along with the heat. They also remove smoke, fumes and other airborne material from the workspace. Fanquip makes its roof fans in Australia from hot dip galvanised steel or fibreglass, depending on the variant. Weatherproof butterfly dampers close when the fan stops, which keeps rain and wind out of the penetration.
The discharge requirement decides the variant, not the fan size.
| Roof fan variant | How it discharges | Best suited to |
| Vertical discharge | Directs exhaust high into the atmosphere, away from the building | Dust, steam, smoke and noxious gases, or where discharge height forms part of an environmental approval |
| Curb base hooded | Downflow hooded discharge from a curb-mounted base | Heat and steam extraction, and cross-ventilation when positioned away from major openings |
| Profile base hooded | Low-profile hooded discharge | Dust and steam removal where roof profile limits mounting options |
| Filtered roof supply | Supplies filtered air into the space rather than extracting | High-heat areas needing clean incoming air where ceiling height limits traditional filter boxes |
The industrial roof fan selection guide covers each variant in detail, including the supply-air configurations.
Extraction and circulation solve different halves of the problem.
Extraction takes heat out of the building. Circulation stops it collecting in the first place. Each one improves how well the other works.
Giant Fans and air circulators work alongside roof and exhaust fans. Giant Fans push large volumes of air downward. Warm air rises, and roof exhaust fans expel it. The cycle keeps the ceiling layer from building to the point where it affects working height. Fanquip’s HVLS Giant Fans move high air volumes at ultra-low noise and cut perceived temperatures by up to 7 degrees. In winter the same fans push warm air back down rather than lose it through the roof.
Sometimes heat concentrates around specific work areas instead of across the whole floor. Mancoolers direct air to where people stand. That is a different problem from roof heat, and it needs different equipment.
Size extraction on the space, not the opening.
Room size and application set the airflow requirement in cubic metres per hour. That figure then sets fan size and quantity. Many sites skip this step and fit whatever suits an existing roof penetration. The building runs under-ventilated through its hottest months as a result.
Five checks before specifying roof extraction.
- Measure the volume. Floor area and ceiling height set the baseline airflow.
- Identify the heat sources. Process heat, solar gain and occupancy change the air change rate.
- Confirm what leaves the building. Heat alone, or heat with steam, dust, fumes or gases, decides the discharge type.
- Check the discharge requirement. Re-entrainment, neighbours, nearby HVAC intakes and approvals can rule out hooded discharge.
- Plan the replacement air. Extraction only works when fresh air can enter. Filtered roof supply fans or wall-mounted supply air handle that side.
One manufacturing site took this route. Filtered roof supply fans went into its high-heat areas, which improved air circulation and lowered internal temperatures while keeping the incoming air filtered. Read the full account in heat extraction fans for summer-ready manufacturing.
Fanquip’s ventilation specialists assess sites in person. They inspect the area and design a solution around the requirement and the budget. Talk to Fanquip’s team about roof heat in a specific building, or browse the industrial extraction range.
FAQs
Heat builds up near the roof because warm air is lighter than the surrounding cooler air and rises until it meets the ceiling, where it forms a stratified layer fed by solar gain through the roof and process heat rising from the floor.Â
Field measurements in large industrial warehouses recorded vertical temperature gradients of 0.8 to 2.6°C per metre through the lower two metres and around 0.5°C per metre above that, with the difference becoming more pronounced as ceiling height increases.Â
Roof fans are generally more effective for heat extraction in high-ceiling facilities because they draw air vertically from ceiling level where heat accumulates, while a wall fan extracting from mid-height is not positioned to capture the hottest air in the building.Â
Air conditioning is often impractical or inefficient in large industrial environments, because heat builds continuously from machinery, ovens, welding equipment and the workforce, which is why extraction and air movement are the more common controls.
Extraction sizing starts with the room size and the application, which determine the required capacity in cubic metres per hour and from there the fan size and quantity.Â