Flat and Low Pitched Roofs

My Canberra Building Inspections ACT


A flat roof is not actually flat, but rather a few degrees. And, low pitched roofs can be nine to twelve degrees.

The advantage of flat and low pitches roofs is that they can span great distances, and are cheap to put up. And because they generally have metal as their roof cover they are light. But being light means that the number of trusses installed to hold them is less. Roof battens are less too. So the whole roof structure is light.

Flat roofs work great in sheds, and factories. Commercially, most offices use flat roofs. The quality of roof sheeting is generally higher in commercial roofing.

In a domestic situation the flat roof has meant that greater indoor ceiling spans can be attained, walls can easily go higher, and cost savings are available. And to a greater degree they can be very stylish and modern looking.


The disadvantages of low pitched roofs is that they leak more easily than a higher pitched roof. A high pitched roof has gravity pulling the water away. A flat roof is using overflow as a means of water runoff.  A pitched roof will get water off the roof faster, and a flat roof will get the water off slower. So, a flatter roof has more water on the roof. And while there is more water on a roof, it has more time to be blown into unsealed flashing corners, and then run down inside the building.


Inspectors cannot see behind a flashing. They cannot always tell if the flashing will hold away water or not. Often during a staged inspection an inspector can pick up problems before they happen. But after construction, it can be hard to tell.

Water can also get blown back up a flat roof more easily than a higher pitched roof. Such water then can back up to a flashing, and get blown into and under the flashing more easily.

Lost Technology

For ten years Canberra had it very dry. During that time the technology to keep water out from a roof was partially lost as roofing tradesmen did not expect their flashings to be subjected to heavy rains. And when in 2020 Canberra was subjected to heavy rains some of these roofs leaked.

Over the next year those tradesmen regained much of their ability to flash a flat roof and to correctly weather heavy rain.

The flashing shown in the photo here is open and not well made. Obviously, water can get in. While the roof is dry inside now, the roof sheet should be turned down more so water does not blow back up. That would be so the top flashing does run the water off, rather than have the tendency to hold the water.

In 2020 the drought ended and the wet seasons have continued since. Consequentially, the tradesmen, having now repaired their own earlier work, have been able to teach new tradesmen how to flash buildings again.

Loss of Architect Oversight

Decades ago architects were often seen on these sites, checking the quality of construction. Today, that expertise is not wanted due to the cost. Back then the tradesperson had to build to the architect’s specification. The architect would inspect often. Now, for economic reasons, that expertise has gone.

Box Guttering

Often flat roofs are accompanied by big box gutters. These are traditional when the design of the buildings do not have the guttering exposed to simply overflow outside. Box gutters are excellent for factories and commercial buildings that abut boundaries. In domestic designs they are flawed however, often by simply being there.

Think of a box gutter as being a very long bath tub. It fills up with roof water and needs to then be let out.

Here, you can see from the picture here that the box gutter fills with water which must exit. But if it blocks at the drain, or at the gutter ends, there can be problems. The water can then go up under the flashing on the parapet wall.

So, buyer, be aware

So, buyers, you need to be aware. That is what you can do. But, you can also get a specialist roof plumber to look at your roof all over. It is recommended, as well as having a building inspector.

Should you buy a new building, and the builder will not repair it’s defects, you can discuss it with the Department of Fair Trading here.

And regards what we have to say about flat roofs in our own Terms and Conditions, please read them. If you are buying a flat roof or low pitched roof, please understand them.

All Building and Pest Inspectors are registered here at Access Canberra. Our number is 111.