Window Sills

By Mitch Bougard – Senior Advisor
Canadian Masonry Services | Ottawa, Canada

For over five decades, Canadian Masonry Services has provided expert restoration and masonry craftsmanship across Canada and the Caribbean. Our work is guided by one principle: “Restoring yesterday’s works to today’s standards.”

Most people never notice their windowsills — until the masonry below starts to fail. The sill’s main job is to protect the wall beneath by catching and shedding water before it penetrates. In Ottawa’s harsh climate, that protection is vital.

What Does a Windowsill Actually Do?

A proper sill projects beyond the wall, slopes downward (10–15°), and includes a continuous drip edge underneath. This simple detail prevents water from tracking back onto the wall face, keeping the brick or stone below dry.

A windowsill must be waterproof and non-porous — its job is to shed water, not absorb it. Materials such as dense natural stone, cast concrete with low water absorption, or engineered CSA-certified cementitious units are ideal. Clay brick or CMU absorb moisture, leading to staining, spalling, and freeze–thaw damage.

The drip edge must project ≥ 25 mm (1 in.) beyond the face of the wall. The drip groove should be continuous, 6–10 mm deep and cleanly formed to break surface tension effectively.

Two Common Mistakes in Sill Installation

Most windowsill failures trace back to one of two installation errors:

• Interrupted drip edge — breaks between sill joints let water cling to the wall rather than falling cleanly away.
• Protruding masonry below — when the brick or stone beneath the sill is not recessed, runoff lands directly on the wall face, causing staining and freeze–thaw damage.

The fix is straightforward: keep the drip edge continuous across the full sill width, and ensure the masonry below is recessed so water falls cleanly away from the wall.

Material and Mortar

Use dense, non-absorbent materials like natural stone, concrete, or Adair limestone. Mortar type S for strength, type N for flexibility. Always use full-bed mortar and tight joints.

For custom cast sills, CSA-approved mixes guarantee low absorption, high compressive strength, and superior freeze–thaw resistance — critical in Ottawa’s climate.

Installation Methods

Custom Cementitious (CSA-Certified) Sills

Clean the cavity, remove all debris, and install a CSA-certified cementitious sill with a formed drip edge. A waterproof membrane is placed between the sill and the masonry cavity to prevent freeze–thaw damage from trapped moisture.

Arriscraft (Calcium Silicate) Sills

Arriscraft sills are calcium-silicate, autoclaved under steam and vacuum — dense and dimensionally uniform, but not waterproof. They still require proper slope, a waterproof membrane, and continuous drip edge detailing.

Both OBC 9.20.13.3 and NBC Part 9 require flashing or membrane separation at horizontal masonry interfaces to control water entry and isolate the sill from trapped moisture.

Freeze–Thaw in Ottawa

Ottawa’s climate is particularly hard on masonry. Damage occurs in the narrow band around +1°C to –1°C, when moisture trapped in the wall repeatedly freezes and thaws. Each cycle expands the trapped water, breaking the bond between brick and mortar and causing spalling.

Proper sill design limits water saturation in the wall below, which is the most effective way to prevent this type of damage.

Code and Standards

All Canadian Masonry Services windowsill work meets or exceeds:

• OBC 9.20.13.3 — slope and projection requirements
• CSA A371:14 (2024) — masonry construction for buildings
• NBC 2020 Part 9 — moisture control and flashing integration

Craftsmanship, Experience, and Integrity

True craftsmanship lies in the details — slope, jointing, drip edges, alignment. Each sill by Canadian Masonry Services is hand-tooled, bonded, and leveled for long-term strength and durability.

Decades of work across Canada and the Caribbean have taught us how sun, salt, and freeze–thaw cycles affect masonry differently. Different climates demand different materials and detailing — knowledge earned only through field experience.

Final Thoughts

Next time you look at a building, check below the windows. If the masonry is clean and sound, it’s thanks to the unsung hero above — the windowsill. Built right, it protects the wall for decades.

A windowsill that looks good today is not necessarily one that will perform well five years from now. Proper slope, drip edge detailing, membrane separation, and material selection make all the difference.

At Canadian Masonry Services, we focus on long-term performance, not just immediate appearance. That is why our process is more detailed — and why it lasts.