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Hello, I wish to change a window of 1m75 high by a window of 1m45 high. How can I do to fill the void on the upper part?

The question does not specify the constitution of the structural work: exposed stone, facing brick, plaster, concrete or painted masonry, or aesthetic constraints or thermal insulation. It is therefore difficult to answer precisely this question. The only certainty is that the sleeping from the window (the fixed part), is not designed to support the weight of a masonry. Even with a light filling, for example cellular concrete, it is necessary to set up a lintelhorizontal piece which will be anchored on each side of the opening and which will support this new filling. This lintel can be cast in reinforced concrete, consisting of a similar prefabricated piece (found in the merchants materials), a wooden beam, etc.
A less "technical" solution might be to put the window and its frame and fill the void with two concrete slabs, the gap between them being filled with mineral wool.

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