Build a windowsill


The outer sill (Sohlbank) should be installed by a specialist. But the inner sill can also be built by an ambitious DIY enthusiast. In the following instructions, we will explain step by step how to build a window sill made of wood or natural stone.

Differences windowsill inside and out

In the case of the outer sill, in addition to the proper installation, it is above all the connection to the thermal insulation composite system (ETICS) that is important. For more information, see Sealing the windowsill. By contrast, you can simply build the inner window sill yourself. For this, the following building materials or materials are available:

  • Wood
  • Natural stone slabs

The sill should have a height of 3 to 5 cm. Laterally, the new windowsill should reach 1 to 2 cm into the soffit. Accordingly, you must strike a groove on the left and right. For a wooden sill, attention must be paid to the high quality of the wood and to a preferable type of wood. Under no circumstances should you use cheap wood composite materials such as laminated wood.

Pay attention to suitable wood

You should also use the hardest possible wood. Very suitable are extremely hard tropical woods. These are so dense that they can absorb virtually no water. However, you still need to treat your new wooden windowsills before installation. Some do-it-yourselfers make the mistake of installing a new wooden sill first and then impregnating or letting it in.

Treat the wood in advance

But then the glued bottom can no longer be protected. As a result, it is likely that she will forgive the wooden sill over time. Therefore, it is mandatory to treat the new windowsills made of wood around before installation.

Step-by-step instructions for building from a windowsill

  • Wood or natural stone tiles of suitable size
  • Installation glue
  • some plaster for lateral plastering
  • Silicone for sealing butt joints
  • Impregnation for the wood (eg linseed oil varnish)
  • possibly wooden wedges for aligning the windowsill
  • Hammer and chisel for the groove
  • silicone Press
  • spirit level
  • Hammer and rubber mallet
  • if necessary tool for cutting the wood or natural stone

1. Preparatory work

Take the measurements for the new windowsills. You can cut wood yourself with suitable tools. Natural stone slabs can be ordered to measure or cut by a stonemason. If necessary, you can also borrow a stone saw. Especially if you let the sill in the side reveal, the cutting edge is no longer visible, which can not be absolutely perfect with such small saws.

2. Prepare the window

Now you can beat the lateral grooves. Depending on the condition, you may need to smooth the surface with mortar. Bear in mind that the sill must be in the water.

3. Install the windowsill

Now apply the adhesive and insert the windowsill. In particular, a light wooden sill will have to complain even more. Now give the assembly adhesive enough time to harden. Please follow the instructions of the manufacturer.

4. Theses

Now you can still grout the rear as well as the side and the lower edges to the soffit.

Tips & Tricks

Even for professionals, wooden boards that were produced for stairs have become particularly well established as a self-made windowsill. Especially here it is usually about very hard wood, which is already pretreated accordingly and will not forgive until hardly ever.


Video Board: How to Make Window Sills & Aprons