Custom Air Filters for Rooftop Packaged Units with Non-Standard Dimensions

Rooftop packaged units are often installed where space is tight, access is awkward, and the original housing was built around a specific filter size. When that unit needs a replacement, a standard off-the-shelf filter may be close but not quite right, and even a small mismatch can affect fit, sealing and day-to-day maintenance.

For mechanical contractors and facility managers, the key is knowing what makes a rooftop filter unique, what to measure, and what details to confirm before ordering. The goal is not just to get a filter that fits the slot, but one that suits the unit, the airflow path and the site conditions.

Why rooftop packaged units often need bespoke filters

Rooftop packaged units are commonly selected for plant flexibility and compact installation, but the filter section is not always standardised across brands or older equipment. Some units have shallow filter rails, unusual access doors, limited media depth or a frame arrangement that was designed for a particular maker’s cartridge or panel.

That means the “same” nominal filter size can still be wrong if the actual housing differs. In practice, custom air filters are often needed because of:

• •non-standard internal dimensions
• •limited clearance for frame depth or handle depth
• •older units with discontinued filter sizes
• •modified housings after service or retrofit work
• •multi-stage filter banks with irregular cell sizes
• •access restrictions that make rigid filters difficult to remove

A bespoke filter can help you match the equipment rather than forcing the equipment to accept a near-enough size. That matters for sealing, installation ease and reliable servicing.

What to check before you order a replacement

Before placing an order, confirm the physical details of the housing rather than relying only on the old filter label. Labels are useful, but they are not always accurate after years of servicing, replacement parts or small alterations to the unit.

Measure the opening, not just the old filter

Measure the clear internal opening of the filter slot or rail system, then check the dimensions of the old filter as a reference. If you need guidance on measurement points, see how to measure an air filter.

Check:

• •width, height and depth of the available space
• •whether the filter slides into rails, rests on ledges or sits behind a retaining frame
• •how much clearance there is for removal and insertion
• •whether the filter must flex slightly during installation or remain rigid

Be careful with nominal sizes. A filter described as one size may have a different actual size, especially if it was made for a specific rooftop unit housing.

Confirm the frame style and sealing method

The frame matters as much as the media. Some rooftop units need cardboard, metal, plastic or rigid box-style frames. Others use bag, pleated or panel formats that rely on a particular edge profile to seat correctly.

Ask whether the unit needs:

• •a rigid framed filter or a flexible panel
• •gasketed edges
• •a header or handle for removal
• •a reinforced border for repeated handling
• •a washable product, if specified by the equipment instructions

Some filters are designed to be washable - check the product instructions before assuming a replacement can be cleaned and reused.

Check airflow direction and installation orientation

Many rooftop filters can be installed only one way. The airflow direction arrow should align with the unit design, and the filter should sit squarely in the frame without twisting or bowing.

If the filter bank is part of a larger ventilation or air conditioning arrangement, confirm the intended filtration stage with the mechanical services engineer or building owner. If the project is being reviewed against AS 1668.2:2024, site requirements still need to be confirmed case by case rather than assumed from the standard alone.

Why fit matters as much as filter media

A filter that is too loose can allow bypass around the edges, while one that is too tight may buckle, tear, or sit unevenly in the housing. In a rooftop unit, that can create maintenance headaches and make routine servicing harder than it should be.

The correct media grade is important, but it only works properly when the filter is installed as intended. For example, if a housing was designed for a deep pleated filter and a shallow panel is fitted instead, the seal and effective surface area may both be affected.

When reviewing replacement options, consider:

• •whether the filter area is exposed to heavy dust loading from roads, worksites or plant rooms
• •whether the unit sees strong airflow fluctuations or high face velocity
• •whether the frame needs extra stiffness to hold shape under service handling
• •whether the access door can close without compressing the filter unevenly

The right product is the one that matches the unit’s dimensions, frame arrangement and operating conditions, not simply the nearest catalogue size.

What to tell your filter supplier

Good information saves time and reduces the chance of ordering the wrong item. If you are sourcing a bespoke replacement, send clear details rather than just a photo of the old filter.

Useful information includes:

• •unit make and model, if known
• •existing filter dimensions, including depth
• •frame material and style
• •whether the filter is panel, pleated, bag or another format
• •airflow direction and any labels on the old filter
• •any clearance issues around the access panel or retaining rails
• •the site environment, such as dust load, coastal exposure or industrial activity

If the old filter is damaged, distorted or missing, a site visit or careful measurement of the housing may be the best starting point. For ongoing support, our filter servicing and commercial filter servicing pages may also be useful when you are planning maintenance around a busy site.

Media choice and performance considerations

Once the size is confirmed, the next step is choosing a media type suited to the rooftop unit and the conditions around it. Different filters are designed for different duties, and rating systems should be interpreted carefully.

MERV, EN 779 and ISO 16890 do not convert directly, so avoid assuming one label equals another. EN 779 classes such as G3, G4 and F5-F9 are still commonly used in Australia, even though ISO 16890 is now the current European standard. The right selection depends on the system design, filtration objective and site conditions.

When a project involves higher-efficiency filtration, check the exact requirement carefully. For example, H13 and H14 are HEPA classifications under EN 1822, with H13 at greater than or equal to 99.95% and H14 at greater than or equal to 99.995% efficiency at the most penetrating particle size. Those filters are not automatically suitable for every rooftop packaged unit, so confirm compatibility with the system design before specifying them.

If you are unsure which product is appropriate, start with the equipment documentation and then work with a supplier who can translate the dimensions and operating requirements into a practical replacement.

Custom filters for difficult housings

Some rooftop housings are straightforward to measure, while others have worn rails, distorted frames or retrofitted access panels that make standard replacements impractical. In those cases, a custom-made filter is often the simplest path to a proper fit.

At Total Air Filtration, a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, we make and supply custom filters for unusual housings, including rooftop packaged units with non-standard dimensions. We can help you compare an existing sample, interpret measurements and identify a suitable replacement format.

If you are working on a rooftop unit and need the right fit, our products and custom air filters are a good place to start, or contact us to discuss the housing details.

For a replacement that suits the unit rather than fighting it, call 07 3800 3448 or request a quote.