“Slender Roofs” Not Just An Issue For Commercial Solar

Rooftop solar panels

Australian commercial rooftops are reportedly increasingly being built without the strength to bear solar panels – is this happening with new houses too?

According to a recent article on the Financial Review (paywall unfortunately), as much as 70 per cent of industrial roofs are being built “slender” (to the minimum specification). To strengthen a commercial roof in order to enable solar panels to be installed can be anything from very costly to totally non-viable.

It’s a situation of developers missing an opportunity, as industrial rooftops are prime real estate for electricity generation given the decreasing cost of commercial solar power and the desire of businesses to rein in their energy costs. The ability of a commercial roof to support solar panels is an issue that SQ blogger Ronald touched on in his post, Hints, Tips And Tricks For Commercial Solar.

“Slender Roof” Houses?

This cost-cutting may not just be confined to commercial rooftops.

When I recently built with a largish South Australian builder, I was surprised to discover that the roof of the 3-bedroom house design I chose from their range would not support a 5kW solar system – which isn’t a huge system these days. This had never been a problem in several of the older homes I had previously lived in, which didn’t require any sort of roof structure modifications.

Modifications for the new home had to be included as a build contract variation, which was noted as:

“Upgrade roof trusses (including calculation and design) to accommodate future roof solar panels supplied and installed by Owner after handover”.

The cost for this variation was $300.

Caveat Emptor

As to how common this little upsell is (or whether I was just duped), I’m not sure, but Australians preparing to build and intending to add a solar power system should flag their desire to install solar panels when the plans are being drawn up in case it’s not raised by whoever they are working with.

If in the market for an existing home, it’s also worth checking if the roof can handle a 5kW (or whatever size) system as it’s not safe to just assume this even if there’s plenty of roof space.

Being proactive in both scenarios could save some nasty and perhaps far more expensive surprises down the track – or prevent home owners from installing the system they want altogether.

It seems a little odd in this day and age when energy efficiency is such an important issue that developers/builders can still get away with roofs on houses with good access to solar resources unable to support what is rapidly becoming an average sized solar power system; especially given these homes are going to be around for a very long time.

Maybe that will change after NCC 2019 is finalised, but it’s not looking likely at this stage.

About Michael Bloch

Michael caught the solar power bug after purchasing components to cobble together a small off-grid PV system in 2008. He's been reporting on Australian and international solar energy news ever since.


  1. Bret Busby in Western Australia says

    It is just part of the Australian shonky building standards, designed to make buildings more dangerous than they should be.

    It is like the similarly shonky feature of Australian houses, where gutter edges are lower on the building side, than on the side away from the building, so as to deliberately cause gutters to overfolw back into the rooves, when a decent rainfall occurs, so that, where a house has eaves, and, the eaves contain electrical wiring, electricity shorts occur, and, where dwellings do not have eaves, the design is so that the excess rainwater is made to run back into the walls, or into the building, instead of away from the building.

    Unfortunately, the objective of standards australia, appears to be to make things as dangerous and life-threatening as possible.

    I would be fearsome, if a house, as described above, can not support a rooftop PV system, and, therefore, moreso, can not support a solar water heater, if someone kicking a football around, outside the house, whether it be a soccer ball, or one of those pointy ellitpical balls, or, throwing a basketball around, causes it to land on the roof, that it would cause the roof to collapse, under the force of the impact of the ball.

    Kind of like having a roof made out of those shopping bags that break before you get the shopping into the car, to cause the shopping to fall on the ground an break, so you have to go back to the shop, and replace the shopping that fell through the shopping bag.

    And, if the roof is not strong enough to support a 5kW PV system or a solar water heater, what happens when you get a breeze of about 30 km an hour? Do you have to go running around, chasing your roof, as it gets blown away?

    It is unfortunate that Australia does not have proper building standards.

    But, then, public safety is regarded as being of no significance, by the legislators.

    That is why so many buildings in Australia, have cladding like the Grenfell cladding (is the incidence of that cladding, in Australia, the origin of the line from the song, “Burn, baby, burn” ?).

    • I think the fact is that ‘legislators’ tout their concerns about ‘public safety’ as the basis for creating legislation at all: particularly in the area of building ~ including such considerations as the esoteric ‘streetscape’ etc.
      That entirely ignores entirely the reality that building one’s home has been going on much longer than there have been ‘legislators’. (Consider the superbly designed-for -purpose construction of beehives, wombat-holes and platypus-dens.) It’s seriously doubtful the pyramids would ever have been allowed ~ or possible ~ if they’d been subjected to today’s legislated/bureaucratic processes.. ….and certain that NONE of the dwellings built in Victoria pre-1960 would’ve been allowed. (one of which near here ~ an ordinary 3-bed weatherboard with attractive well-established gardens, on a quarter-acre ~ recently sold for $3.7 mil.)

      The only thing more perplexing and disgraceful is that people cop it sweet, and evangelise the “qualified” label as if it were a religious truism.
      The same people seem to have forgotten that ‘duly qualified’ is the SECOND-least-guarantee of competence ~ marginally ahead of the competence/qualification required to be a ‘legislator’, which lays on the ‘zero’ level..
      The validity ~ or otherwise ~ of any methodology can be measured by the ‘compulsion-factor’…..and NOTE that voting and dunny-building are both activities subject to legally-enforceable compulsion.
      But look on the bright side: One day, in the evolutionary distance we might become as smart as the average wombat.

    • Mike Gaskin says

      Here in Mexico our roofs are made of reinforced concrete. Mine is 4 inches thick with rebar and heavy chicken wire reinforced. You could park a truck on it so my solar hot water heater and 4k of panels and a.c. compressor dont make a dent.

  2. Bret Busby in Western Australia: not related to Casper Jonquil are you?

  3. Bret Busby in Western Australia says

    dRdoS7 – The name Casper Jonquil is unknown to me.

    Your pen name – is that to do with DR-DOS? From what I remember, DR-DOS 5 was the last version, then DR was taken over by Novell, and the next version was Novell DOS 6.

  4. “As to how common this little upsell is (or whether I was just duped)”

    Yep, sad to say! No builder or tradie will do anything for $300.

    But when it comes to construction we are indeed a silly country in many ways, basic housing design is cheap European style with no Australian character and little to no accommodation for Australian climate. The conventional stick frame construction system hasn’t changed in 30 years since I was a kid – other than to get worse. Now it’s a bit lighter weight, dark roofs are common and less or no eaves overhang… but wait, now we have environmental codes unlike those hick builders eighty years ago with their verandas and high ceilings.

    On the commercial side here is a nice clip of a roof peel happening: http://www.kansas.com/news/state/article207939694.html

  5. Truss roofs are designed to specific loads and the truss suppliers should be making allowance for photovoltaic panels in the minimum and perhaps other rooftop installations such as solar hot water panels.
    In my Architectural/Engineering experience the truss design doesn’t change for solar panel allowance, but it must be there on the calculations to meet compliance regulations.
    The pressure or builder’s desire to build to minimum standards only is crazy and if the average home owner fully understood the implications they would be pushing for much better construction.
    $300 seems a typical builder charge for an engineering check, it could have been much worse (in my experience). Perhaps the project home companies could make it a “feature” that their roofs are designed including allowance for future photovoltaics.

  6. “In any bureaucracy, the people devoted to the benefit of the bureaucracy itself always get in control, and those dedicated to the goals the bureaucracy is supposed to accomplish have less and less influence, and sometimes are eliminated entirely.[Pournelle’s law of Bureaucracy]”
    ― Jerry Pournelle

    We are, unfortunately, well down this continuum…..

  7. Ganesh Wilborn says

    The NCC national construction code) requires roof to stand up to the 50 year design wind, and their own weight. In addition to this the roof must support an extra 0.5kPa (50kg/m2) for maintenance. A solar panel should be on the order of 0.25kPa and more importantly are non traversable so there should not be an issue for compliant residential roofs. Furthermore lightweight steel clad roofs are usually designed for a significant uplift wind event rather than a downward one. So the extra load from the panel will only overload non-compliant or non standard roof designs.

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