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How to measure for a new or replacement Rooflight

How do I know what size skylight I need?

It can be confusing when purchasing a new rooflight or skylight, so take a few minutes to read this very easy guide on how to buy the correct rooflight today.

So, lets say for example you're after a new dome skylight, where do you start?

Firstly, are you looking to install a new Rooflight or replace an existing?

If you are looking to replace an existing Skylight then you may already have an upstand (Or Kerb) on the roof. If this is the case then you want to take measurments from the External Edge-to-edge of this upstand. You will also need to take note of the Internal of the upstand also.

The External Dimension is the vital measurement in this case, once you have this you can start looking at options like Dome Only, Flat Glass Rooflights, Roof Lanterns. All of which will secure directly to your existing upstand on the roof.

On each product page we list a few measurements, inclduing the required External Upstand size.


DeliveryNext Day
BrandPermaroof UK
FinishClear / Diffused
External Upstand Size500 x 500mm External

External Upstand Size - A builders curb is another term for a timber upstand. Typically a builders curb is created using 2" timber. So, if you're going to create you own curb or you already have an existing curb then the external size of this is needed.

The External Upstand size is also the dimension you want to use when installing a new Rooflight but you plan to create your own upstand.

Its really that simple!
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But I am looking to install a new Roof Window and I want one of your Insulated Upstands!

In this case, if you are looking to install a new Rooflight with one of our pre-formed insulated upstand then you want to make note of the Roof Opening or Aperture in the roof. For example, if you want a 2000 x 1000mm Aperture/Opening then this is the measurement you will need.

You can then start looking at options like Fixed Domes or Opening Dome Windows, which come with uPVC Insulated Upstand. You will then find the Internal Aperture size relating to the specific product, like in the below example...

DeliveryNext Day
BrandPermaroof UK
FinishClear / Diffused
Internal Aperture2000 x 1000mm Internal


If you plan to create your own timber upstand, we advise contacting us to confirm correct dimensions.

Timber upstands must always be at least 150mm High. Widths will vary depending on the product being installed.

As always, if you get stuck - Call us today!
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