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Roof windows – a great way to light up the loft

Installing roof windows is a great way to light and ventilate the attic. Many good companies on the market offer their products, and the leading brands include VELUX, FAKRO, RoofLITE+, OptiLight and Skylight.

In the beginning, you have to choose how many roof windows to install in your room. One of our previous articles dealt with this topic: “How to choose the size and number of roof windows”. It would also help decide whether you will have wooden or plastic windows, double or triple glazed, with or without a ventilator.

Another question we ask is: What kind of flashing do I need for the installation?

The point where the window meets the roof is a potential source of problems, so the flashing has essential functions such as

Tightness – protects against external influences and acts as a barrier to water, wind, snow and dirt
Warmth – a standard flashing is an exterior flashing that does not work as a thermal barrier, so the window should be insulated from the inside, for which a ready-made insulating kit is available. There are also warm flashings available which are combined with a thermal insulation material
Aesthetics – the flashing is always visible on the roof, its components are not negligible, and the roof window flashing should be as unobtrusive as possible. Manufacturers make every effort to ensure that the flashing is neatly finished, the profiles are elegantly rounded, and their colour is similar to the most popular roofing colours.

The flashing must be resistant to the stresses caused by the roof structure, and it must also be resistant to deformation under the influence of heat and snow pressure.

Roof window REQUIRES flashing.

The flashing is necessary for the correct installation of the roof window on the roof slope. It is designed to match the manufacturer’s roof windows, ensuring a durable and tight joint between the roof window and the roof covering.

The flashing components are designed with great precision and are adapted to both the size of the window and the shape of the roof covering. Each manufacturer offers several types of flashing marked with some cryptic symbols, but they all classify them according to the same criteria :

  • roofing material – e.g. flat, profiled, high profile, curved
  • the way windows are installed – individually or in combinations (modular)
  • The depth at which the window will be installed – lowered, zero-depth, raised depth.
  • size of roof window

What is a flashing made of?

The essential flashing components for a roof window are two side sections, a top section and a bottom section, whose construction, accessories and size depend on the window model, depth of installation, roof pitch and type of roof covering.

Water flowing from the roof surface over the window…

… reaches the upper part of the flashing…

… from there it flows to the right and left to the side parts of the flashing…

… and further down to the lower part of the sealing flange…

… and finally to the roof plane below the window.

The sealing strip, or apron, is connected to the lower flashing part and maybe lead or aluminium composite. The aluminium composite aprons adhere very tightly to the roofing material, making them ideal for profiled and flat roofing materials. On the other hand, those made of the lead sheet are more rigid, thanks to which they are more resistant to tearing, and they can be shorter, which is more aesthetic. Their length depends on the type of roofing.
Drainage gutters, located above the top element and sometimes along the sides, drain condensate running down the sheathing outside the window.
Side wedges – strips of sponge or foam, sometimes factory glued to the side elements, but often constituting separate elements of triangular or trapezoidal cross-section, whose task is to cover leaks occurring along the sides of the window. The height of the gussets depends on the height of the roofing profile, and they are not needed in the case of flat roofs which adhere to the sheathing.

Flashing and type of roofing

Flashings only fulfil their purpose if they adhere tightly to the roof covering. Due to the different types of coverings, manufacturers offer several types of flashing.

Flashings for flat roof coverings

These are the simplest type of flashing, effectively protecting the under-window area. Their construction is simple, consisting of essential elements. Usually, no drainage gutters, aprons or side gussets are used. The extended lower feature is sufficient to secure the under-sill area. The profiled side elements prevent water dust from being blown underneath and are invisible after the covering has been laid.

These flashings are used on roofs covered with, e.g. tar paper, bituminous shingle, flat-panel without embossing.

Flashings for profiled coverings

In the case of tile, corrugated or trapezoidal roofing, the profile stands out from the roof sheathing. The height of the profile/wave is the height difference between the lowest and highest point (as seen from the outside of the tile), and it usually does not exceed 45mm.

Such roofing needs a wide under-window area, without which water would flow into the cavities under the roofing. In this case, an apron is required, which must be glued tightly to the folds of the covering. Side wedges are also necessary to prevent dust, leaves, snow and water from entering under the surface.

Flanges for high profile roofing

If the wave height is higher than 45mm, it is recommended to use a flange for high profile roofing, the apron of which should be as long as 22cm.
Flanges for scale coverings

Although seemingly flat in theory, slate or plain tile has a significant thickness, which makes it difficult to seal the area under the window. In this case, you can use a traditional flashing for profiled coverings, but some manufacturers have introduced special cut-side flashings for this covering. The flashing modules are laid alternately with the roofing material to provide better protection against leaks. The number of cut pieces depends on the height of the window. The number of cut pieces depends on the size of the window. Depending on the thickness of the covering, the side pieces can be flat or profiled.