How to determine the correct dimensions
Measuring the size of a skylight correctly is essential to ensure that you order the right skylight for your flat roof. But don't worry, it's easier than you think!
This guide has been specially put together to guide you step by step in determining the correct size of your skylight. We will clarify terms such as 'daylight size', 'overall size', and 'roof opening', and show you how to determine these dimensions yourself. Follow the instructions and you can be sure that you are ordering the perfect skylight for your flat roof.
Let's start measuring!
Starting from an existing skylight? Then follow the steps below
If you start from a new roof opening, please scroll down.
Determine the daylight size
The day size is always measured from the inside of the top of the inner edges of the existing opening (see photo).
In order to simplify the ordering of the skylight in terms of dimensions, we have decided to only work with a daylight size and a roof opening. The dimensions provided on this online shop are always the daylight sizes in mm.
If you measure from the outside...
Understand the overall size
When refurbishing a skylight, you'll probably start with the dimensions of the existing skylight.
Keep in mind that the measured size is 160 mm larger than the daylight size (size B + 160 mm = measured size). This is because the skylight has a flange of 80 mm all around.
Example: a measured size of 1160 x 1160 mm results in a daylight size of 1000 x 1000 mm
Start from the roof opening
The roof opening (C), or the size of the hole in the roof, depends on the type of curb
Measuring the space for your skylight, or the 'roof opening' (C), depends on the type of base on which the skylight rests, which we call the 'curb' or 'upstand'. There are two types of curbs: straight and splayed.
If the curb is straight (you can recognise this by the fact that the number ends in /00), then the space in your roof, the roof opening, must be exactly the same size as the inner dimensions of the curb, which we call the 'daylight size' (B).
However, if the curb is splayed, the space in your roof must be larger than the daylight size. How much larger depends on the number with which the curb ends. For example, if it ends in /20 or /30, the space in your roof (C) must be respectively 20 or 30 cm larger than the daylight size (B).
Let's take an example: suppose you have a splayed curb that ends in /20, and the daylight size of your skylight is 1000x1000 mm (B). In this case, the space in your roof must be 1200x1200 mm (C). This means that you add 200 mm, or 20 cm, to both the length and width of the daylight size to determine the correct size of the space in your roof.
Check again
Double-check the sizes
A = Daylight size or the inner dimensions of the curb
B = Roof opening or hole in the roof
It is always a good idea to double-check your measurements to ensure that you order the correct size skylight. If you are unsure, contact a professional.
Important Tip: Make sure your tape measure is flat and straight when measuring. Small errors can have big consequences when fitting your skylight.
With these steps, you should be able to take accurate measurements and choose the right skylight for your flat roof. Remember: measuring is knowing!
Video Guide: easily measure your skylight
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