What thickness colorbond roofing to use?

Bluescope makes blank coil for roofing manufacturers in several thicknesses. People use two ways to describe the thickness of their roofing material:

  1. TCT (Total Coated Thickness). If you were to put a measuring calliper on the roofing sheet, this will be the reading that you will see. It is the actual thickness of the material including the paint coating.
  2. BMT (Base Metal Thickness): The strength of the roof sheet is dependant on the thickness of the actual steel. The protective coatings virtually contribute nothing to the strength of the roof sheet. So when engineering calculations are performed, this method of describing the thickness is what counts.

So, the best way to talk about thickness is to use BMT.
Most manufacturer’s span tables use BMT

Roofing manufacturers use these common thicknesses:

  • 0.35 BMT – for wall cladding
  • 0.42 BMT – the thinner choice for common roof profiles
  • 0.48 BMT – the thicker choice for common roof profiles
  • 0.55 BMT –  for standard and custom flashings
  • 0.60 BMT – for corrugated curved roofing
  • 0.70 BMT – special profiles like Lysaght Longline

What is the difference between 0.42 BMT and 0.48 BMT roof sheeting?

A typical layperson will think that the thicker roof will last the longest. The truth is that once rust starts to eat into the steel, the 0.06mm extra thickness does very little to slow down rust holes from developing.

So what is the advantage of using 0.48 BMT?

If you had a huge warehouse roof, it is economical to use as little steel as possible in the structural framing. The designer will choose to increase the roof spans to decrease the number of purlins. The thicker roof sheeting will allow this. This also translates to less roof fasteners and a quicker roof installation. For big roofs, all these add up and 0.48 BMT is the way to go.

However, the purlins/battens on a residential roof are usually spaced to give the roof more stiffness and ease of foot access. Therefore, they are more closely spaced and the thinner 0.42 BMT roof sheet will be strong enough.

That’s why the thicker roof material is usually associated with commercial roofs and the thinner gauge for residential roofs.

When should 0.48 BMT roofing be used on residential roofs?

I used to value cost savings more than functionality.

And this meant that I would use the thinner 0.42 BMT roofing on all my residential roofs.

Gradually, I realised the fallacy of this dogma.

Most of our skillion roof replacements fall in the 30 t0 40 square metre range. The savings in material costs in using the thinner gauge roof sheeting is in the $50 to $100 range.

Whilst the 0.42 BMT material has the strength to span between 900mm and 1200mm, it’s resistance to localised buckling due to heavy foot traffic is far lower than the 0.48 BMT.

Inexperienced foot traffic on residential roofs (TV, Air conditioning, plumbing, gutter cleaning…) will result in localised buckling of the roof.

In the long run, I would rather spend the extra $50 to $100 up front to reduce the likelihood of buckling damage during the service life of the roof.

I now quote all my roof replacement work based on using 0.48 BMT roof sheeting. It is cheap insurance against local buckling damage.

… But there is an exception. The ‘kliplok’ profiled roof sheets have a much higher resistance to buckling of the ribs – and sometimes, I would pick the 0.42 BMT over the 0.48 BMT.

… All except the Lysaght ‘Kliplok 700 HS transverse fluted’ roof sheet.

This is explained why on another specific post…

Why replace a tile roof with a colorbond roof?

Why fix it if it ain’t broke?

This is what Tom did!

Tom had an old  tiled roof.

It happened to be old concrete roof tiles and he often wondered about its age and whether it was time for a new roof. Initially, it was a low priority item for him because he had just spent a few bucks on a new kitchen.

… Until a storm showed up.

The kitchen ceiling started to drip. It was an emergency but there were no roofers to be found.

“… it is against Occupational Health and Safety regulations to send a roofer up on a wet roof in windy and wet conditions…” – was a typical response.

Only the SES could help (apparently they worked under a different set of laws?).

“ An old dried up tree branch fell off the back gum tree. It cracked a couple of roof tiles. We have put a tarp over the area and you will need to get a roofer out when the weather gets better…” The SES guys were great.

Storms keep roofing crews busy – and it was a good two weeks before a roofer was able to peel the tarp off and take a look.

“…Yep. A couple of cracked roof tiles. They are ‘Boral Windsors’. Bloody hard to find replacement for these now since Boral stopped producing Windsors a few years back…

Lucky for you I have about 5 in my yard”

This was the good news.

The roofer had more to say….

“ You will need more work on your ridge capping because the broken tiles are directly under the ridges… Also, there are quite a few loose ridges. Maybe the SES or just a maintenance thing…

Probably about $3000 to do a proper fix.”


Time to make an executive decision.

For Tom, it was decision time.

He had a healthy balance on his line of credit.

The roof showed multiple dips (maybe because the roof tiles were too heavy?)

The roofer had said that if there was sarking under this roof tiles, then maybe it would have kept the kitchen dry.

The roof was hot – even though he had put a couple of whirly birds in a couple of summers before…

So Tom started researching.

He was after a roof that could withstand a falling branch. Looked good and straight. Had less maintenance issues …and a bit cooler.

Initially, he was looking for a new replacement tile roof. Then he read all the talk on the internet about colorbond roofs.

He Googled ‘replace tile roof with colorbond roof’…. And up popped a whole page published by a roofing materials supplier called ‘No 1 Roofing’.

All the pros and cons of tile vs colorbond roofs.

Tom made a decision. It was too easy…

It was going to be a new colorbond roof!

… Now, all he had to do was find a good roofer.



replace terracotta roof with colorbond roof

It was 2014 and Terry saw the Colorbond ad on TV.

One day…. He was going to be able to run out in his pyjamas and admire his new colorbond roof. His old terracotta tiled roof looked crap, the tiles were very fragile. And the look of the tiles did not match with the other metal roofs that were hanging off it…

Terry also thought: Maybe new guttering…and colorbond fascia covers…

That was Terry’s dream.

In 2017, It came true…



When roof tiles just don’t work on a roof…

replace leaking low pitched roof tiles with colorbond


“… it is just criminal how builders and roof tiles get away with their ignorance and laziness about where tiles just don’t work!” – my roofers regular rant.

It is a fact that most roof tiles struggle to keep the heavy rain out when they are laid at anything less than 20 degrees.

Roof tile manufacturers say that putting sarking under their tiles will stretch the minimum tile roof slope to as flat as 15 degrees. If you read between the lines, this reads: “We know that the tiles will leak – but the sarking will catch the leaking water and carry it to the eaves. Hopefully, no water will reach the internal ceilings…”

… And they are mostly right.

Until 10 or so years go by.

The battens that sit on the top of the sarking, goes through hundreds of wet/dry cycles and start rotting. The roof tiles lose their support and collapse to a flatter position and cause more leakage. This is when water starts dripping onto the ceiling.

When it comes to roof pitch versatility, Colorbond metal roofs come out the clear winner over roof tiles. You can build a colorbond metal roof from 1 degree to a 90 degree pitch. You just cannot do it with tiles.

Some tiles are better than others…

Terracotta tiles are generally better at handling the lower pitches. The interlocking upstream edge will prevent water from overflowing the top edge of the tile.

Concrete roof tiles do not come with any ridging on the upstream edge. Water will run over the top edge as the roof pitch dips below 20 degrees.

The flat profile concrete tiles tend to be worst. They leak everywhere…

…. And then you have leaves.

Trees are great.

But they drop leaves on the roof.

Leave clog up the water courses of roof tiles and block the natural flow of water down the tiles (especially flat concrete tiles).


Colorbond roof replacement leaves on flat concrete tiles

In our many years of replacing tile roofs with colorbond roofs, we consistently come up against the two main enemies of roof tiles.

Trees and low pitches.

So when you are considering a tile roof, you need to take these into account first. You may end up going with colorbond.

I often tell people that I am extremely biased towards colorbond – BUT they need to take it with a grain of salt because we only do colorbond roof replacements.

Of course we are biased.

The No 1 Roofing post I mentioned earlier is an independent view about replacing tile roofs with colorbond. It does have a lot more information about the topic than I have covered above.

Pop over there for a read: http://www.no1roofing.com.au/replace-tile-roof-with-colorbond/

Then come back and make contact if you are thinking of a colorbond roof replacement.




Newtown, in Sydney’s inner west has unique problems with their roofs and here is an innovative roofing repair solution for a faulty roof and gutter problem

I get roofing repair enquiries from Newtown fairly often. Situated in Sydney’s inner western suburbs, Newtown is a densely packed inner city suburb with lots of old terrace houses and narrow one way streets. It can be a difficult Sydney suburb to attempt roof repairs because of the complex access […] Read more »