precast concrete sills with a caulked mortar joint

What is the best way of fixing a deteriorated brick window sill, and what is the importance of fixing it soon?

In masonry, window sills are quite often where you see deterioration when there is no sign of deterioration anywhere else on a building. The biggest reason for this is the “freeze-thaw cylce”. In cold climates window sills absorb water from melting snow and ice and when the temperature goes below freezing, the water that penetrated the masonry expands and causes masonry window sills to spall and crack.

It is important to fix a deteriorated window sill as soon as you see a sign of cracked brick, spalled brick or missing mortar joints. These symptoms tell us that the window sill is no longer doing its job to flow water away from the inner wall of your home and away from the masonry that is below the sill. Water can cause the bricks below the sill to deteriorate, the same way water deteriorated the sill.

When choosing a product to replace a deteriorated sill, I strongly recommend a precast concrete product. I do not recommend brick window sills because they are more susceptible to water damage. The other advantage of a precast concrete sill is there are less mortar joints, if any. Mortar joints in sills are susceptible to erosion and sometimes cracking.

“Through wall flashing” should be installed underneath window sills, before the actual sill is installed. In masonry flashing is used to wick water that was absorbed by the masonry away from your home and also in the case of window sills, away from the masonry below the sill. Holes in the mortar joint called “weeping holes” should be left under the sill to aid in the exit of any water that the flashing has gathered.

It is also a good idea to caulk any mortar joint with a polyurethane caulk. This will prevent the mortar joint from eroding.​

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If you have a deteriorated brick window sill, and are in the Ottawa area, Tundra Masonry can help! Call or email us for a free estimate and consultation.