How to Specify Air Filters for Rooftop Units on Industrial Buildings

Rooftop units on factories and plants work hard in tough conditions. They sit in full sun, wind, rain and dust, and they are often awkward to reach for inspection and change-out. That means filter selection needs to account for more than just airflow and particle capture. It also needs to suit the cabinet, the maintenance access, and the way the building is actually operated.

Start with the rooftop environment

Industrial roofs are not benign locations. Filters installed in rooftop units can be affected by heat from direct sunlight, moisture from driven rain, airborne dust, and debris that blows across the roof. If access is limited, a filter choice that looks fine on paper can become difficult to inspect and maintain in practice.

When specifying filters for this kind of equipment, consider:

• •Whether the unit is fully exposed or partially sheltered
• •How much dust the site generates internally and around the building
• •Whether wind-blown debris is likely to load the filters early
• •How the maintenance crew reaches the unit and opens the filter section
• •Whether rain ingress, roof leaks or condensation could affect the filter bank

A filter should suit the conditions around the unit, not just the air handling design. If the rooftop arrangement changes, such as after a process upgrade or a roof replacement, the filter specification may need a fresh look.

Match the filter to the system, not just the label

Industrial buyers often see a rating on a product sheet and stop there. In reality, the right filter depends on the fan capability, the filter frame arrangement, and the pressure drop the system can tolerate as the filter loads with dust.

For most rooftop units, the key questions are:

• •What size and depth of filter does the housing accept?
• •Is the frame designed for panel filters, pleated filters or another format?
• •What final resistance can the fan and motor handle before airflow becomes an issue?
• •Does the unit need a coarse pre-filter to protect a finer secondary filter?
• •Is there enough room to fit deeper media if a longer service life is needed between inspections?

If you are replacing filters in an existing unit, measure carefully. A slight mismatch can cause bypass, poor sealing or installation difficulty. Our guide on how to measure an air filter is a useful starting point when dimensions are unclear.

For new work or replacements with non-standard dimensions, custom air filters can be the practical option when you need a specific size or construction to suit the rooftop cabinet.

Choose a filtration level that suits the space

The right filtration level depends on what the building is serving. A factory with light dust exposure and general ventilation needs is different from a plant with process dust, welding fumes or sensitive production areas. If the unit supports general building ventilation, the design should align with the project brief and the mechanical services engineer’s requirements.

Where standards are being considered, AS 1668.2:2024 may be relevant to ventilation design, but site requirements should always be confirmed with the mechanical services engineer or building owner. It is also important to remember that MERV, EN 779 and ISO 16890 ratings do not convert directly, so do not rely on simple crosswalks between them.

Some industrial rooftop units use a staged approach:

• •A coarse pre-filter to take the heavier dust load
• •A finer filter downstream to handle smaller particles
• •A final filter stage where the application calls for it

EN 779 classes such as G3, G4 and F5-F9 are still widely used in Australia, even though the standard has been superseded by ISO 16890. If you are comparing products, make sure you are comparing like with like, and check the manufacturer’s performance data rather than assuming one label is equivalent to another.

Think about weather and construction details

Filters on a rooftop need to hold together in a harsh setting. Heat can dry out some media and seals, while wind and vibration can loosen poorly fitted panels or create leakage around the frame. Rain and moisture can damage some filter types if the unit is not well protected.

That makes construction details important:

• •A rigid frame can help the filter keep its shape in demanding rooftop locations
• •Proper sealing reduces bypass at the filter edge
• •Corrosion-resistant components are worth considering where the housing is exposed
• •The media should suit the expected dust type and moisture exposure

Some filters are designed to be washable - check the product instructions. That option may suit some applications, but it is not universal and should never be assumed. If a filter is not intended to be washed, doing so can damage the media and shorten service life.

Plan for access and maintenance before you order

A rooftop filter specification should make maintenance realistic. If technicians need to carry filters up ladders, through tight roof hatches or across long roof runs, large or awkward formats can slow the job and increase the chance of damage on the way in.

Before ordering, confirm:

• •How the filter will be delivered to the roof
• •Whether one technician can handle the filter safely or whether two people are needed
• •Whether the access door allows removal without bending or forcing the frame
• •Whether there is space to inspect the gasket, track or holding frame
• •Whether the site has a safe method for carrying used filters back down

Inspection frequency depends on the system, site conditions and dust load. Follow the filter and equipment manufacturer instructions, and monitor pressure drop where gauges are fitted. On a dusty industrial site, a rooftop unit may need much more frequent attention than a cleaner administrative area in the same facility.

If maintenance is outsourced, share the equipment details with the contractor and keep the filter specification consistent across all units. Our filter servicing and commercial filter servicing pages may also help if you are looking at ongoing support.

Work with a supplier who understands industrial rooftops

For factories and plants, the best filter is usually the one that fits the unit properly, handles the dust conditions, and can be maintained without unnecessary hassle. That requires clear dimensions, a sensible filter stage, and an honest look at the rooftop environment before purchase.

If you are comparing options across multiple buildings or trying to replace an older, non-standard filter arrangement, it helps to have a supplier who can check the size, format and application with you. Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, and we work with industrial buyers, facility managers and mechanical contractors on practical filter solutions.

To discuss a rooftop unit filter specification or replace an existing filter, call 07 3800 3448 or request a quote. You can also view our products or contact us if you need help identifying the right fit.