ᐅ What should I use to secure the windowsill? What type of mortar should we choose?

At the beginning of October, the granite window sills for our home will be delivered, and we plan to install them ourselves. The wall is made of 36.5cm (14.4 inches) T12 bricks.

Can I set the window sills with regular cement mortar, or would it be better to use LM21 (lightweight masonry mortar)? What is the best option to use here?

When installing your interior window sills (I assume these are interior window sills), you need to ensure that they do not come into contact with the exterior window sills below the frame (thermal bridge).
The cavities of the Poroton bricks are sealed with masonry mortar, then mortared up to the appropriate height and smoothed off. Before installation, light-colored granite types are additionally sealed on the underside with white natural stone adhesive and then set with flexible tile adhesive.

I did this myself back then as well. For the exterior, I used regular cement mortar. Beforehand, I applied granite adhesive under the windowsill with a trowel. It is also important to seal it properly here.

Hello,

Like the member Herrmann, I did the same. This is the easiest way and it still holds up well today.

Best regards

Thank you for the tip, as I need to take care of this at my parents' house next week. Now I know what to do and can get the necessary materials.

I also used masonry mortar for the interior window sills. It is important to ensure there is enough ventilation space for the exterior window sill. Be sure to avoid thermal bridges here.