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How to overlap corrugated roofing sheets?

One of the main reasons that Corrugated Roofing Sheets are so popular is because they are one of the easiest roofing options to install due to their simple overlap installation method. To install Corrapol Corrugated Roofing Sheets you simply have to overlap sheets widthways and lengthways to cover your full roof area, with minimal cuts required.

The rules around overlapping corrugated roofing sheets are similar whether you are using our Corrapol-BT Corrugated Bitumen Sheets, Awnapol Premium Bitumen Sheets or any of our Corrapol Clear Corrugated Roofing Sheets. For example, our popular Corrapol Stormproof range.

In this ‘How to Overlap Corrugated Roofing Sheets’ guide we will cover the key points for overlapping roof sheets including:

Why do you overlap corrugated roofing sheets?
Which way do you overlap corrugated roofing sheets?
How much should corrugated roofing sheets overlap? - Widthways
How much should corrugated roofing sheets overlap? - Lengthways
Why do I need to leave extra room on each side of my roof?
How much should Corrugated Roofing Sheets Overhang?
How much do you overlap plastic corrugated roofing sheets?

Why do you overlap corrugated roofing sheets?

When originally invented in the early 1800s (yes, that long ago!) corrugated roofing sheets were designed to be a more adaptable, lightweight and easy to install roofing solution to quickly install on a wide range of projects, compared to other roofing options at the time. Today it remains the case that corrugated roofing sheets, whether corrugated bitumen or clear or other styles, are one of the fastest and most straightforward sheeting options for domestic and commercial roofing projects.

Another of the main reasons for the popularity of corrugated roofing sheets is that their overlapping installation method makes it easy to calculate what is required.

However, the most important reason why you overlap corrugated roofing sheets goes back to the basic function of roofing; to create a watertight cover or shelter. In short, you overlap corrugated roofing sheets to create a watertight cover over your roof as the overlaps between sheets help seal the roof properly and avoid leaks. This is why it is crucial to properly overlap your corrugated sheets when installing to take into account specific overlap requirements based on the pitch of your roof etc.

Which way do you overlap corrugated roofing sheets?

It is advisable to overlap corrugated roofing sheets with the prevailing wind direction. Why? To increase the longevity of your roof as if you installed against the prevailing wind direction then the open side of the overlap and then fixings are exposed to greater winds and force over the roof lifetime.

If you are unsure about the prevailing wind direction at your location, just observe the weather/wind in your area over a few weeks before your roofing project to determine the most common wind direction. You can find a few tips here on how to determine the wind direction at your location.

How much should corrugated roofing sheets overlap?

Widthways

Corrugated roofing sheets should overlap a minimum of one or two corrugations depending on your roof pitch:

Pitch greater than 10° = One Corrugation overlap
Pitch less than 10° = Two Corrugation overlaps

If you have some sheet width to lose and you don’t want to cut the corrugated sheets you can overlap more than two corrugations. This also means that if you have damaged your first corrugation peak in handling it can be trimmed off or just overlap underneath and it won’t affect installation.

For help on cutting bitumen sheets, please refer to our How to Cut Corrugated Bitumen Sheets Guide.

Lengthways

When installing on a longer roof slope our corrugated roofing sheets can be overlapped lengthways as well. How much you need to overlap a corrugated sheet lengthways again depends on the pitch of your roof:

Pitch 15 degrees or more = Overlap 18 cm
Pitch between 10-15 degrees = Overlap 22cm
Pitch between 5-10 degrees = Overlap 32 cm

Why do I need to leave extra room on each side of my roof?

Always remember to allow space for your Rock N Lock Flashing. When installing a corrugated roof sheet, it is important to allow space for your Rock N Lock Flashings at both outer edges of your roof, your two gables. Whether you have a side wall abutting or open gable it is key to include space for the Corrapol Rock N Lock Wall or Rock N Lock Gable Flashing in your overlap and space calculations.

You need to allow approximately 40mm for the Rock N Lock flashing on each side of your corrugated roof.

If your roof width means that the corrugation ends further out simply overhang your Rock n Lock a little and make the deduction less than 40mm to suit.
Sometimes it is best to install your first Corrapol-BT Rock n Lock on one side before starting the run of sheets as it helps keep the sheet straight and gives you a guide.

You need to allow approximately 40mm for the Rock N Lock flashing on each side of your corrugated roof.

How much to Overhang Corrugated Roofing Sheets?

At the bottom edge or gutter edge of you roof you need to overhang your sheets slightly over the eaves to allow for drainage into your gutter. We suggest a maximum overhang on corrugated sheets of max 70mm.

The best distance for you to overhang your corrugated roofing sheets depends on a few key factors including:

How steep is your roof?
How exposed is your roof?
Do you have a gutter and how high does that sit on the eaves?

In general, the steeper your roof the less overhang you will need to ensure you don’t overshoot your gutter. Also, the more exposed your location the less overhang you will want due to the higher risk of wind uplift.

How much do you overlap plastic corrugated roofing sheets?

Corrugated plastic roofing sheets, like our Corrapol Stormproof or Corrapol PVC corrugated sheets, overlap in exactly the same way as our other corrugated sheets so either one or two corrugations depending on the pitch of your roof.

Although this How2 Video shows our Corrapol-BT Bitumen corrugated Roof sheets the same method and pitch details apply to our clear corrugated roofing sheets so simply refer to the details mentioned above.