What is the minimum pitch for corrugated iron roofing?

The rule of minimum pitch for corrugated iron roofing is the most common disregarded piece of engineering logic.

If you have a metal roof on your house – there is a good chance that it is made of corrugated iron roofing. The corrugated profile dates back to 1820 and it is the most used metal roof profile in the world…

It is no wonder that most house builders use corrugated roof sheeting when there is a metal roof to be installed. Here is a horror story about attempts to repair leaks on a flat corrugated roof.

Corrugated roofing is a great material on traditional pitched roofs – but it has severe limitations when the pitch gets below 5 degrees. That is why all roofing manufacturers recommend the 5 degree pitch as the minimum slope and they will not provide any guarantees if it is installed flatter than this.

There are two main reasons why corrugated roof sheeting and low pitches do not mix:

  1. During heavy rain, the valleys of the corrugations fill up to the height of the crest or even above that. This means that at the side laps of every sheet, water overflows under the lap. And fills up the gap between adjacent roof sheets. Then it trickles over the edge of the roof sheet into the roof cavity.
  2. At the upstream end of the corrugated iron roof sheeting, the valleys are turned up to prevent water flowing backwards and running off the upstream end. During heavy rain (or if there is wind combined with rain), water fills up the valley and overflows the turn up. Fairly heavy leaking can result from this.

Flat skillion roofs are the most common way to increase the size of a traditional bungalow house and many builders and roofers simply slapped on some ‘corrugated iron’ for the roof. This is the cause of most of the roof leakage problems with skillion metal roofs.

Modern architecture has birthed the demand for low profile roofs and builders and roofers still have a problem in this modern era of simply not understanding the limitations of corrugated roofing. I see countless examples of corrugated roofing on brand new low pitched roofs. These are water leakage time bombs.
… and many roofers think they can fix these roofs with a bit of roofers silicone – until they find that roofers silicone is not always the way to do a roof repair properly.

Roof sheeting manufacturers have long since overcome the limitations of low pitched roofs by inventing special profiles like “Lysaght Trimdek”, ‘Lysaght Kliplok’, and other profiles. But people still cannot grasp that when the pitch of the roof starts getting low, the look of the roof profile is less important than the function…

The problems with corrugated roof sheeting and low pitched roofs must be the number one metal roof problem because of all my videos on my Youtube, the one below is the most viewed….

Corrugated roofs are not the only type of roof that suffer from leakage due to being installed at low pitches.

Even though some roof repairers have some understanding that the ‘trimdek’ and ‘Kliplok’ profiles are suited for low roof pitches, they still lack the expertise to fully detail these types of roofs so that they do not leak. For more videos about problems that these types of roofs (like the one below), head over to our new ROOFING PORTAL channel:


Answer to a smart question: “As a roof gets longer, does the slope need to increase?”

The answer is: “YES. It has to do with how much water the roof is carrying. A longer roof had more water at the downstream edge. Also, what is not usually taken into account is the average rainfall for the area the roof has been installed.

Therefore, in dry areas, you can have a much longer corrugated roof that can be laid at 5 degrees.
If your roof is in Bathurst, NSW, you can have a 25m long roof at 5 degrees.
But if you live in Coffs Harbour, a corrugated roof at 5 degrees cannot be more than 10 metres long.”

If you are interested in other roof profiles and maximum lengths, look at the picture below (courtesy of Stramit book of Answers).
minimum roof pitch and rainfall intensity
Length of roof against roof profile and rainfall intensity