Filling With Mortar
Gaps in masonry windows, such as windows with bricks or concrete frames, can be filled with mortar. If there is a slim crack, the mortar can be mixed up to a heavy paste and spread between the masonry blocks to fill it in. You can also find premixed mortar, known as leveling compound. Either mortar type can be applied to cracks with a putty knife.
Mortar as an Adhesive
If the bricks or blocks around a window fall out of the window frame, mortar may be used as an adhesive to replace them. It should be mixed according to the directions on the packaging to create a thick, heavy adhesive and spread around the inside of each hole in the window frame. Then, a brick or block can be pressed into each space around the window. Once set up, it should dry the same color as the rest of the mortar used in the design. Ideally, you won’t be able to tell that the blocks or bricks were ever missing.
The gaps around most other types of window frames, such as wood or metal, should not be filled in with mortar. Though mortar works as an adhesive substance, the material is porous and does allow both air and moisture to pass through. Gaps in the edges of wood or metal windows should be filled in with caulk, which prevents air from getting through and also provides water-resistance. If the holes in the window frame are in the actual wood frame, you can also use wood filler, which works much in the same way that mortar does to fill in a crack.
Alternatives to Mortar
For small cracks that appear in concrete and other masonry windows, you don’t have to necessarily use mortar to fill in the gaps. Grout is one alternative to mortar. Both of these materials have the same basic installation process, which is an application with either a putty knife or grout float that can be time-consuming. If the cracks are small, you can also use silicone caulk, which has a simpler installation process, just as you would on a wood or metal window.