0.42BMT or 0.48BMT colorbond thickness?

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…