The most popular single-family houses nowadays are structures with usable attics. The rooms with slants are very cosy and full of charm. However, they have some arrangement requirements. One of these requirements is the selection of appropriate roof windows that enable optimum use of daylight and adequate lighting of the room. Choosing the wrong number and size of roof windows and their incorrect placement can result in years of torment in a non-functional interior instead of enjoying the beauty of your home.
roof window dimensions
Roof windows illuminate a much larger part of the room than vertical windows with the same glass area, and they can let in up to 1.8 times more light than façade windows. They can be installed in different places on the roof, individually or in groups, which gives you a lot of freedom in arranging rooms.
Which roof window should I buy?
When building a house or renovating an attic, most of us are faced with the question of what type of roof window to buy. Considering the wide range of products on the market, choosing the right roof window can be challenging. There are many factors to consider, such as the manufacturer, size, glass package, material and accessories you would like to equip your roof window with to enjoy its use.
There are many sizes of roof windows to choose from. Standard widths of roof windows are 55cm, 66cm, 78cm, 94cm, 114cm, 134cm and typical heights of roof windows are 78cm, 98cm, 118cm, 140cm and 160cm. Typical roof window sizes will therefore be combinations of these widths and heights. The most common roof window sizes are 78×140 cm and 78×118 cm, based on the rafter spacing. With this width of the window, you do not need to make an exchange, i.e. a transverse beam that supports cut rafters, but you insert it between the remaining rafters to transfer loads on the edges of the opening to the structural elements. It should be remembered that this is the total external dimension of the window frame and not its sash; hence the light of the opening after opening the window is smaller than the window dimension, e.g. 78×140 cm. The glazing area is even smaller as part of the pane is fixed in the sash.
Unfortunately, roof window manufacturers do not make life easy for us and mark the same roof window size with different symbols, and, e.g. 78×140 window in VELUX is marked “MK08”, in FAKRO and OptiLight “07”, in RoofLITE “M8A” and in SkyLight “7/14”.
So what size roof window and number of windows should we choose for our rooms?
This is no easy task. The correct amount of light in a room through a roof window is determined by the ratio of the window area in the clear opening to the floor area. The rule of thumb is that 1 m2 of glazing illuminates 8 m2 of floor.
Example of calculation of the number of roof windows needed for daylight in a room of 20 m²
? = 20 (20 x 1) : 8 = 2,5
This means that in order to illuminate a room of 20 m², you will need 2.5 m² of window area in terms of the clear opening.
This surface area will be achieved by using.
4 windows 78 x 118,
3 windows 78 x 140,
2 windows 114 x 118
To increase the amount of light entering the room, it is worth installing several windows next to or above each other or installing several small windows in different parts of the roof slope. This will give better light than a single large window with the same glass surface.
However, remember that the number and size of windows should not be exaggerated. More heat escapes through windows than through walls. Therefore, when choosing roof windows, it is worth considering economic considerations and investing in warm roof windows. Currently, on the market, many 3-pane windows are meeting WT2021 technical conditions effective from 2021. According to this standard, the Uw value cannot exceed 1.1 W/m2K.
How do I install a roof window, and what should be the size of the roof window opening?
Most roof windows are installed on battens – they have mounting angles at the top and bottom of the frame. There are also roof windows that have structures fixed to the rafter with brackets on both sides.
The length of the roof window is chosen according to the roof pitch. It is assumed that the bottom edge of the window should be installed at a height of approximately 100-120 cm above floor level, and the top edge of the window at 180-200 cm. A roof window installed in this way ensures good visibility, both when looking through it while standing and when sitting. To meet the condition of comfortable operation and a good view from the window, its length is chosen the smaller its inclination angle.
The width of a roof window should always be chosen to not interfere with the roof framing. Ideally, this width should be slightly (up to 5 cm) smaller than the distance between rafters. For example, a rafter spacing for a 78cm roof window would be approximately 83cm.
However, if the skylight is larger, position it so that only one rafter needs to be cut. Two adjacent rafters should not be cut as this would unduly weaken the roof structure. After missing a rafter below and above the window, perpendicular beams, called exchanges, are installed. These transfer the load from the cut rafters to adjacent whole rafters, thus relieving the burden on the weaker part of the roof.
And how high should windows be installed?
It would help if you considered at what height you would like to install the window so that it performs its function and at the same time provides the comfort of use. Imagine a 78×160 roof window with a top-hung opening mounted at 1.30m. The handle will probably be beyond the reach of even the tallest member of the household. Windows with the handle at the bottom are normally installed at the height of 130-150 cm, and those with the handle at the top at the height of 80-130 cm. The top edge of a top-hung window should be within easy reach, i.e. at a level of about 200-220 cm. The window should be at such a height that it enables a standing person to look ahead and not only upwards.
In buildings with storeys below 25 m above the ground, the distance between the bottom edge of the opening window and the floor should be at least 85 cm. However, on storeys higher than 25 m above the ground, this distance should be no less than 110 cm. It should be noted that the higher the window is installed from the floor, the better the room lighting.
When deciding on roof windows, you should also pay attention to the height of the knee wall and the roof pitch. The higher the knee wall, the higher the bottom edge of the window will be. In this case, to ensure a good view, it is advisable to install a vertical window under the roof window to create a knee-high combination. It is best to position the window so that the bottom edge is about 120 cm from the floor. The handle, located at the bottom of the sash, is easily accessible to the user, and the window performs its primary functions perfectly. Such a height will also make it convenient to place furniture underneath it and pass by it without bending (even when the sash is open). It will also be easy to open.
To sum up:
- Roof windows illuminate the attic more effectively than vertical windows.
- When arranging your loft, take the time to plan and choose your windows carefully.
- The best results will be obtained by using several windows located at different roof points rather than windows grouped in one place.
- The higher a window is installed from the floor, the better and more evenly it illuminates the room.
- High roof windows provide an even distribution of light in the room and guarantee visual contact with the surroundings.
- By choosing and positioning roof windows in the right way, you can gain spectacular sunlight, good ventilation, thermal and acoustic comfort without any significant increase in costs.
- To increase the comfort of using the loft, it is worth equipping our windows with roof window accessories such as awnings for roof windows or roller blinds for roof windows.