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…

What to do if you have a ‘Lemon’ roof.

Is a leaking roof a lemon?
You could have a ‘Lemon’ of a roof…

Pair a lemon with a car and you get something unpleasant. As a roofer, lemon roofs are just as foul and I just see too many of these shocking roofs being installed for unsuspecting homeowners.
Most days, I shake my head when I climb on a roof in Sydney. The faults on the roof are so blatantly obvious that it makes my job of finding where the roof leak is coming from so un-challenging.
Just as chefs should eat their own culinary creations, I think roofers should live under the roofs that they put up when Sydney gets those long bouts of stormy days.
When I stuff up and I don’t know about it – I keep thinking that I am a genius. The times that I have had to take responsibility for my errors have been the greatest learning opportunities.
I just wish that all roofers are forced get to go back to fix the roof leaks that they have been responsible for…

Some roofs are easy and some are just plan challenging.

It is hard to go wrong on a factory roof in the outer western suburbs of Sydney. Long straight sheets. Regular rectangular shapes. Easy crane access. Easy parking. Simple edge flashings.
Switch to an Inner west roof in the Erskineville and Newtown areas, and the degree of difficulty ramps up. Add in skylights, flat skinny roofs, chimneys, parapets and parking inspectors…. And mistakes plus shortcuts will eventually lead to roof leaks and premature roof lifespan.
For a better perspective on Inner west roofs, I have a special post on that subject.
We hardly get to fix any factory roofs. But we get to fix a lot of residential roof leaks closer in to the city. It is the small roofs that have the highest failure rates.

Be careful with flat metal roofs

“Keep away from corrugated roofs.”
This is my best advice for anyone contemplating the modern look of a low pitched roof. The corrugated roof profile is not designed to be laid ‘flat’. 5 degrees is usually the manufacturer’s recommended minimum slope for corrugated roofs.
Personally, I think 8 degrees should be a better limit to set. I have seen way too many leaking corrugated roofs.
Then people like cutting large holes on flat roofs and installing skylights.
There are two things to think carefully about with these skylights.

5 degrees is usually the manufacturer’s recommended minimum slope for corrugated roofs.  See what happens if you don’t follow this pitch recommendation on a separate post.
Personally, I think 8 degrees should be a better limit to set. I have seen way too many leaking corrugated roofs.
Then people like cutting large holes on flat roofs and installing skylights.
There are two things to think carefully about with these skylights.

  1. Choose the right skylight. Not all skylights can be installed on flat roofs.
  2. Design the flashings correctly. ALWAYS design and install a tray flashing .  See my leaking skylight page for tray flashing details.

Even small holes for vent pipes and exhausts can benefit from a tray flashing – especially if the hole is cut where two roof sheets lap together.
What is the best roof sheet profile? 
I still prefer the ‘Kliplok’ profile for all flat roofs. The higher and stiffer ribs combined with ‘no screws’ cuts down on a lot of potential leakage paths.

Can Lemon roofs be repaired?

I have a friend who is quite patriotic and he used to support the local Holdens. He bought a Calais (remember the premium Holden Commodore?).
After the sixth warranty repair in the first 6 months, he gave up on Holden. He sold the car and bought a Mazda. He keeps saying to this day that If Holden had just accepted the problems with the lemon car and gave him another one that worked – he would have stuck with the local manufacturer.
It is pretty much the same with a roof if there are some major design/installation problems. You can keep repairing the faults but these tend to be short term fixes.
Usually it is better to just ‘bite the bullet’ and rip the offending roof off and replace it with a proper one. It does not matter if the roof is just 6 months old.
There will always be exceptions. A good roofer will be able to assess the merits of a roof repair vs a roof replacement and make you a recommendation.

The future for roofs in Sydney?

More apartments are going up than houses. A lot of home owners will have strata managers looking at any roof problems. Small building envelopes and height restrictions means that architects are designing low pitched roofs (and making it look good).
Both situations means that it will become more difficult to repair or replace problem roofs.
Prevention is always better than cure (unless you want to live through disasters to really experience the lessons that can only be learnt this way). So, hopefully with the spread of better information