It is a common building practice to install window sills which are comprised of multiple sections of stone or pre-cast concrete. The joint between these sections are jointed using brick mortar. This is fine when the building is new but as the building ages and settles these joints often crack. Water can then penetrate the building envelope in large volumes causing damage to the wall assembly and mold growth.
As a home inspector I see this defect on just about every building that I inspect at various levels of severity. I always recommend the same solution but had actually not gotten around to fixing my own home. In an effort to be less of a hypocrite I set out to fix mine. Here is how I did it.
Step 1: Removing the Broken or Cracked Mortar From the Joint
First start out by removing any loose mortar material from the joint. I also recommend removing about 1 cm of material to make room for caulking which will be used to seal the gap.
I used the edge of a surface grinder wheel to remove the material on the top and front side of the joint to a depth of about 1 cm. Be sure to wear a face shield as this step can kick up some debris. A lot of fun when you are on a ladder at your second story windows.
To get at the portion of the mortar which the round grinder wheel could not reach I used a small punch and hammer to gently chisel out the remaining material.
This approach could probably be used to do the whole job if you do not own a grinder.
This is the what the joint looks like after the material removal phase is completed.
Clean the joint well in preparation of caulking to get good adhesion.
Step 2: Masking the Area to be Caulked
If you are like me and are a bit incapable when it comes to neat caulking, here is a good tip. Cover both sides of the sill on either side of the joint to be filled with painters tape.
Be sure to get it well adhered to prevent any caulking from squeezing underneath.
Step 3: Applying the Caulking to the Mortar Joint
Apply the caulking to fill the void that was created previously. I used a grey silicon caulking that is made to bond to masonry.
The best caulking is a commercial grade such as Tremco Dymonic in my opinion. However it can be difficult to find it at the local hardware store. I just used a run of the mill product meant for exterior use and bonding to masonry that I picked up at the local big box.
Here is what the joint looked like after the caulking was applied.
Using a small putty knife, starting at the window, smooth the caulking flush to the sill. Remove the excess material build-up on the putty knife.
Resist the urge to keep touching and fiddling with the look of the smoothed caulking, you will probably make it look worse.
Step 4: Finishing the Mortar Joint
Here is the smoothed caulking.
Carefully remove the painters tape while the caulking is still wet.
Make sure you have several wet rags on hand to clean off your hands and tools. You will need them. Amateur caulking work can be really messy!
Step 5: The Finished Sealed Mortar Joint
TADA! Here is the final product. Caulking tends to dry out in between 5 to 7 years and cracking can reappear. At that point it will need to be done again. Just pick out the old caulking and reapply using the same technique.
Maybe, we will have sold the house by then.