Portal VS Purlin: 7 Considerations for Purlin Mounts

Purlins are a common enough sight in timber - or modern steel-framed roofs, stretching parallel to the eaves and providing a combination of support to the structure and to the roof above. Sometimes called “roof battens”, when you load directly onto the roof, it’s the purlins that do the heavy lifting.

Below are 7 considerations when considering a purlin mounting system.

Purlin mounting relies on structural support

This will be no surprise. When architects and engineers come together to discuss the loading on a roof and the placement of a platform or mount, there is often the need to boost the number of purlins in specific places. Purlins mounts are ideal for new buildings, since extra support purlins can be indeed during the design and engineering phase.

Fast installation and flexible design

As long as you’ve got the purlins, installing a mount is typically a matter of lifting a completed platform to place and fixing it down. And we’ve made things even better by the exceptional flexibility in design with our modular purlin mounts, ready for any situation. That flexibility also means that, almost regardless of the size, you can relocate a purlin mount in five years or a decade with minimal roof work required. It’s easy future proofing.

Fixing options – the good and the bad

You’ve got to remember that your roof is a weather shield, not a work surface or a structural interface – so you’ve got to treat it with care if you want to utilise it over its 50- year design life.

Mounting off your purlins means tampering with your roof, so it stands to reason that every connection point with your roof is a risk. These connection points can cause long term corrosion and damage, or immediate water ingress issues. That’s why it’s important to minimise the number of connection points a purlin mount platform has with the roof/

The option we recommend for purlin mounts is our RC01 fixing detail mount, a secure system that penetrates the roof (with ingress protection in place). It’s fast, reliable, and provides excellent long-term performance. Its load bearing capacity is high, meaning fewer roof connections to support any given platform.

The second option is an industrial adhesive tape applied to the trusses. While this method is faster, and perfectly safe long- term, the removal or relocation of a purlin mount secured this way can be more challenging.

Where you mount matters, however, and here’s where it’s easy to get caught out. We always mount our RC01 through the rib (i.e. the peaks of a textured roofing) rather than the pans (i.e. the troughs). Some companies out there like to mount through the flutes – after all, this will transfer the load directly to the purlin. But here’s the catch: they create a spectacular debris trap in the flute, increase the potential for water ingress – and if your purlin mount is only a few inches off your roof, that’s a long-term hassle that’s better to avoid.

Let’s consider another aspect. You can purchase platforms that space the trusses at 600mm centres or closer – but additional, closer trusses might be hiding the fact that their trusses are of weaker, lower-quality materials and designs. The risk of roof damage is high, and the ability to maintain the roof is severely reduced.

Need to know: adhesive solutions and roof raisers

The roof raiser is a non-structural part of the roofing. In essence, it creates a space between the purlins and the roof sheet. They’re becoming more and more popular in New Zealand and Australia– and they’re also something that you need to be aware of if you’re looking to load up your roof.

If roof raisers are installed and you want to fit a purlin mount using the adhesive tape method, well, you’re out of luck. Yes, the roof raisers can be strengthened to take the additional load, but certainly not after they’re already fitted. You’ve got to know beforehand – which is why you should involve your mounting specialists at the design stage.

We recommend avoiding the use of adhesive tape on principle, as the risk of becoming involved in a non-compliant building assembly is high.

Under pressure

When a platform is designed by engineers, it’ll just be a platform situated on the roof to support equipment. The HVAC specialists and engineers will know the kinds of loading – and vibration, as we discussed in our ebook Anti-Vibration Understanding and Isolation Vibration – to expect and therefore design for. This tells them how to design the purlins in the areas where the additional forces are anticipated.

Something that may be overlooked, however, is the need for acoustic screening. While acoustic panels – like our HushMonkey solutions – aren’t a significant load, a smart engineer will need to anticipate wind loading from the screens.

Let’s paint a picture: You’re in a typical three storey town or city building, and experience a gusting wind speed of 45m/s (common enough across 90% of Australia and New Zealand). Your 1.8m high screen receives up to a 2.8kPa wind force acting on the screen and a 4.3kN force acting either up or down on your mounting point and to the 1.2m centre purlins below. And our HushMonkeys are often 3m tall, almost doubling that force! Screens generate high overturning moments and put specific purlins under extreme loading like gusts, so engineers and designers need to take into account the bigger picture when purlin or portal mounting, and not simply apply a blanked 2.5kPa vertical loading to the purlins below the platforms.

Purlin mounts are best suited for new buildings

Mounting to the purlin is a common enough enterprise when significant weight isn’t a major consideration, which is why you’ll often see pergolas and solar panels mounted this way in residential applications. But for commercial or industrial buildings, where the weight of equipment can climb into the order of tonnes, existing purlins simply may not be strong enough.

So why not add extra purlins? In new buildings, that’s easy. But for an existing roof, sometimes it’s not feasible or practical to get a 9m member into a roof space, nor is the cost justifiable – so consider a portal/rafter mount, where the platform can be strategically designed to transfer the loads back to the stronger building elements and bracing system, bypassing the weaker elements. there’s a better chance of strategic loading, especially about knee joints.

For any roof mounting system, great or small, a Monkeytoe Purlin-mount Plant Platform ensures the simple and effective installation and mounting of condenser units, large package units or any heavy-duty equipment.

The Monkeytoe Purlin-mount Plant Platform

The Purlin-mount Plant Platform is the ultimate prefabricated solution that can be seen across Australasia in applications from rooftop mounting for air conditioning units to, viewing decks, and landings for roof area entry and exit. Anywhere that reliable strength is needed, you can find a Monkeytoe.

Because the Purlin-mount Plant Platform is constructed from high-tensile T6 marine grade aluminium, it’s significantly lighter than steel structures giving you the same performance with less weight on the roof.

Best of all, we’ve developed the modular system to require no onsite welding or hot works. We prefabricate everything to be assembled and crane-lifted on site to get you and your clients into their spaces in no time.

Find out more

Click here to download our latest ebook “Portal vs Purlin: The Mounting Showdown” to find out more about mounting solutions. Understand the science behind why portal / rafter mount? Why purlin? And what’s going to come out on top for your needs – now and into the future?

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