Install a window later


In particular, when you buy an existing building, the layout of the building in the fewest cases is exactly in line with your own ideas. This applies in addition to the room layout on the existing windows. For this reason, we have below a guide for you how to install a window later. In addition, you will find plenty of additional information, which is especially important in the subsequent installation of windows.

Numerous reasons for retrofitting a window

A house does not always correspond to the ideas of its current inhabitants or owners. Especially in old buildings often rooms such as toilets, bathrooms or storage rooms have been designed without windows. If a conversion or change of use takes place, it is often thought about the subsequent installation of a window. But before you begin, certain precautions must be taken.

  • Determining whether a window can be retrofitted from a static point of view
  • Verification of required official building permits
  • in many cases, the consent of the neighbors
  • Determination of the window size
  • Structural engineering for the correct construction

Static check

In order to determine whether the entire building stereotype "tolerates" a new window at the position you have chosen, you should definitely consult a structural engineer or architect. He can give you binding information about whether there are static concerns in your project or not.

The Bauamt, which is in your possession, can quickly tell you whether a building permit is required or not. In most cases this will be the case for sure. In addition, the neighbors must agree as soon as certain distances to the neighboring property are fallen below. He can refuse that if he does not want you to be able to look over his shoulder practically all the time. Here is also not crucial, whether ground floor or upper floor.

Determine window size

Based on the statements of your structural engineer and then of the accepted rules of house building, you now derive the size of your window. There are no really standardized and fixed window sizes in Germany. Rather, these are related to the room size. How exactly you determine the optimal size of a window can be found in the linked article.

Structural design of the subsequent installation of a window

In the construction journal you will find articles and instructions for actual window installation, also for plastering windows and, of course, also for window and door installation according to RAL. We also provide instructions for walls in the house journal. Therefore, the following instructions for retrofitting a window refer to the essential steps that are not affected by these detailed instructions.

Step-by-step instructions for retrofitting a window

  • Lintels in appropriate length
  • Fixing material for the lintels
  • window
  • bricks
  • mortar
  • plaster
  • depending on the window installation different building materials, which depend on the chosen installation technique
  • cover
  • Tool for working on the existing wall (chisel, concrete cutter, etc.)
  • Concrete mixer or electric drill with stirrer
  • mortar bucket
  • different spatulas and trowels to the walls
  • various trowels and spatulas (for example trowel) for plastering
  • spirit level
  • Richtlot
  • Chalk line
  • Window installation tool
  • Tool for the specifically intended Fenstersturverankerung
  • possibly wheelbarrow
  • possibly scaffolding
  • possibly rubble containers

1. First steps: slot for the lintels

When all permits have been obtained, the corresponding blueprint is also available. According to this plan, you now draw the exact position of the lintel on the affected wall, ie in the middle above the drawn window niche.

Depending on the size of the window, the slot for the falls is 30 to 40 cm wider than the actual window niche. On each of the later sides of the window, the lintel must rest on the masonry at a height of 15 to 20 cm.

2. Fit and fasten the lintels

It is talk of lintels, because with conventional strength of a wall always two falls are used side by side. So first insert the inner lintel and fasten it with the appropriate fastening material. Exactly the same way, then you proceed at the second lintel.

3. pry window opening

Now you can continue the wall break. Draw the breakthrough (also depending on the length of the falls) on each side 10 to 15 cm wider than the window will be wider later.

4. Wall reveal

Now you will laterally rebuild to the actual dimension of the actual window niche minus the required plaster thickness and seal (compi-band) to achieve a straight, vertical finish. Also on the parapet you build a horizontal stone row over the entire width until reaching the required dimensions.

5. Plastering the soffit

If you want to mount your windows in accordance with RAL guidelines, you must now level the reveal straight and fine, so that the sealing tape (Kompi tape) actually seals in the best possible way later.

6. Place and install window

Now you can set the window, fix it and finally plaster the soffit (if necessary). Finally, you have to set the new window.

7. Alternative approach

If you get the green light from the structural engineer (especially with very large, wide windows), you can make the breakthrough to the full length of the lintels and then rebuild the masonry that later sustains the falls.

Tips & Tricks

The subsequent installation of a window is a highly demanding job. Alone the installation of windows and doors in accordance with RAL guidelines is so demanding that even some (not suitably certified) craftsmen have difficulties with them. If you want to enlarge an existing window niche in order to subsequently install larger windows, do not forget to arrange for a cost-effective disposal to meet the old window.


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