What kind of mortar to use to install a granite window sill?

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The granite window sills for our house will be delivered soon, we would like to install them ourselves. The wall is made of 36.5 T12 bricks.

Can we use regular cement mortar for the sills or is it better to use LM21 (lightweight masonry mortar)? Which is best to use?



When you install interior window sills (I assume they are interior sills), you also have to make sure they don't adjoin the exterior sills under the frame (thermal bridge).

The voids of the foam bricks are sealed with masonry mortar, then the mortar is raised to the proper height and troweled. Before laying, light-colored granite is further sealed on the underside with white natural stone adhesive and then installed with flexible tile adhesive.


I used to do this myself at one time. For exterior work, I used regular cement mortar. Before that, I applied granite adhesive under the sill with a trowel. It's also important to seal well here.

  • I did this the same way. This is the easiest way and still works well today.

I also used masonry mortar for the interior sills. So it is also important to get enough air to the exterior sill. Avoid thermal bridges here at all costs.

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