Window shelves – unimportant or the most important part of window?

Purchasing a window is a process that requires quite a lot of customer's engagement. Many details need to be defined and many decisions need to be made. If we still collect energy to define everything at the window, all too often, we run out of energy when we come to the outside shelves. In today's construction, the window shelf is no longer just a decoration, but an important part of the wall - window – façade connection, where most often water leakage is present. Shelves are necessarily evil for many, and not much attention is given to them. But it should be. The outer shelves are the part on the building envelope, which presents a direct contact between the climatic conditions and the basic construction, and therefore it is important to know which to choose and why.

In Germany and Austria a number of damages were found on the base load-bearing structure (entry of water into the interior of the building, decay of wooden load-bearing elements) or facades, due to improper installation of shelves. In 2011 a norm was written that obligates how the shelves must be properly installed. We need to stick to the 5 basic rules:

1. The side ends must be designed in such a way that they can withstand the thermal expansion of the window shelf and prevent the water to enter under the shelf.

Figures of two appropriate side ends

2. When contacting the window shelf and the window itself, the CT seal (left figure below) or pre-compressed BG1 sealing band (right figure below) must be installed to prevent condensation and water entrance.

3. Window shelves must be installed under the angle of 5°. With a smaller inclination, the water does not flow across the entire surface of the shelf, but drains off only at the sides, which can be quickly seen as traces of dirt on the facade. Also, all other contacts between the facade and the shelf must be made with pre-compressed strips.

4. The shelf must have an overhang over the façade of at least 40 mm – if the overhang is less, the water drains to close to the façades and leaves the mark on it.

5. The façade under the shelf must be prepared in such a way that it ensures the correct inclination of the shelf and the waterproofing under the shelf.

All five rules are statutory in Austria and must be respected regardless of the wishes of the final customer. In Slovenia, legislation is not so far prepared to make this mandatory. Experience here also shows that incorrect installation of shelves is present on many facilities, and the problems caused by this kind of installation can become a nightmare for every individual.

On the pictures below there are few results of incorrect shelf installation:

The side ends are without dilatation – no space for aluminum (left) and decay of wooden beam (after 8 years) due to incorrect installation: no gasket in the side ends and behind the shelf (right)

Water does not leak of the shelf due to insufficient inclination (left) and too small overhang over the façade (right)

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4226 Žiri

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M SORA d.d. je podjetje z dolgoletno tradicijo. Njeni začetki segajo v leto 1948, ko je bila oblikovana kot zadruga. Mizarsko podjetje se ji je pridružilo leta 1985 in s tem zasadilo osnovo današnjega uspešnega podjetja poznanega predvsem po izdelavi lesenih in les-alu oken in vrat. Podjetje (brez hčerinskih podjetij) ima 180 zaposlenih in je razdeljeno v tri glavne enote: Mizarstvo, Norica (dobavitelj in zastopnik številnih materialov potrebnih za izdelavo oken) ter Trgovine (trgovine z živili, trgovine z gradbenim materialom). 


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