Accurately measuring the size of your skylight is essential to ensure you order the correct dome for your flat roof. But don’t worry — it’s easier than you think! This guide has been specially created to walk you through the steps to determine the correct size of your skylight. We’ll clarify terms like daylight size, outer size, and roof opening, and show you how to measure them yourself. Follow the instructions and you’ll be confident you’ve chosen the perfect skylight for your flat roof. Let’s start measuring!
Determine the daylight size
The daylight size is always measured from the inside, at the top of the inner edges of the existing upstand (see photo). To simplify the ordering process in terms of dimensions, we’ve decided to work exclusively with daylight size and roof opening. All dimensions shown on this webshop refer to the daylight size.
If you are measuring from the outside...
When renovating a skylight, you will likely start from the dimensions of the existing dome. Keep in mind that the outer measured size is 160 mm larger than the daylight size (B + 160 mm = measured size). This is because the dome has an 80 mm flange all around. Example: An outer 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!
