Window sill

What is a window sill?

A window sill is the horizontal element at the base of a window opening, typically sloped downward toward the exterior to shed water away from the wall. It provides structural support for the window frame and forms the bottom of the window reveal assembly. Window sills are found on both exterior and interior sides of the opening, each serving distinct functions.

The term comes from Old English and has been used in construction for centuries. In modern practice, the sill includes several components: the exterior sill (the sloped drainage surface), a drip edge or kerf (a groove preventing water from wicking backward), end caps or horns (extensions guiding water runoff), and below that, a flashing pan that collects any water bypassing the main sill and directs it safely to the exterior.

How does a window sill manage water and drainage?

The primary function of a window sill is water management. Rainwater and snowmelt must flow away from the building facade, not pool on the sill or seep into the wall structure. A properly designed sill slopes downward at 2-5 degrees, creating a gradient that encourages water to run off quickly. The slope is often achieved by angling the sill board itself or by using a sloped flashing pan beneath it.

The drip edge (a groove or rabbet cut 3-6 mm into the underside of the sill's front edge) is essential. Water running off the sill tends to cling to the underside due to surface tension and could be drawn back underneath by capillary action. The drip edge breaks this adhesive surface, causing water to drop clear of the facade instead of tracking back into the wall joint.

End caps or "horns" (small extensions at each end of the sill) prevent water from spilling sideways into the vertical reveals. Flashing details at the sides of the opening must overlap the sill to ensure water is guided toward the bottom, not trapped in corners where it could cause rot or damp.

What materials are commonly used for window sills?

Window sill materials vary by location (interior vs. exterior) and climate. The choice affects durability, maintenance, thermal performance, and cost.

Material	Exterior/Interior	Properties	Maintenance
Stone (granite, limestone, slate)	Exterior	Durable, low thermal conductivity, aesthetic appeal, heavy, can be porous	Periodic sealing recommended; avoid harsh cleaners
Concrete	Exterior	Cost-effective, durable, good load capacity, prone to cracking in freeze-thaw cycles	Seal to prevent water absorption and efflorescence
Cellular PVC	Exterior/Interior	Won't rot or split, low-maintenance, can be routed for drainage, paintable	Minimal; resists insects and water damage
Galvanized steel or aluminium	Exterior	Lightweight, corrosion-resistant, good drainage slope, check thermal expansion	Minimal; inspect fasteners and joints
Hardwood (oak, teak, redwood)	Interior	Aesthetic, warm feel, absorbs moisture, requires protection	Paint, stain, or sealer needed; monitor for movement

How is a window sill sealed and installed?

Installation and sealing are critical to long-term water management. The window frame typically sits on the sill with fasteners (nails or screws) driven through the frame's nailing flange or through the sill itself. The gap between the frame and sill must be sealed with flexible sealant, typically polyurethane or silicone, allowing thermal expansion and contraction without cracking.

Beneath the main sill, a flashing pan (often made of metal, rubberized membrane, or specialized flashing material) slopes toward the exterior and is integrated with the building's weatherproofing layers. If the building has brick masonry, the flashing may be bedded into mortar joints; in wood frame construction, it may be installed over house wrap before cladding. Water collected by the pan must drain to the exterior, either over a drip edge or through weep holes at the base of the reveal.

End caps are installed and sealed at the edges of the sill. Vertical flashing at the sides of the window ensures water from the reveal above is channelled across the sill, not allowed to run down inside corners where it could cause hidden rot or mould.

What thermal and moisture issues can occur at a window sill?

Window sills are common thermal bridge locations. The frame, sill, and wall reveal often form a direct pathway for heat to escape from the warm interior to the cold exterior. If materials like concrete, metal, or wood connect the interior to the outside without a thermal break, interior surface temperatures can drop significantly below room temperature.

When interior surface temperature falls below the dew point of indoor air, condensation forms. Over time, condensation promotes mould growth, especially in corners where air circulation is poor. This is particularly problematic in older buildings with poor ventilation or in cold climates where sill surfaces remain cold for extended periods.

The risk is greater where sills are made of highly conductive materials (metal, uninsulated concrete) and where there is a large temperature difference between interior and exterior. Mitigation strategies include using thermal breaks (insulating spacers between materials), increasing the thickness of exterior insulation to warm up the reveal, and improving indoor ventilation to control humidity.

Risk Factor	Description	Mitigation
Cold sill surface	Conductive materials allow heat loss to exterior	Install thermal break; increase insulation at reveal; use lower thermal conductivity material (cellular PVC, wood)
High interior humidity	Indoor moisture condenses on cold surfaces	Improve ventilation; use humidity control; ensure sill surface temperature remains above dew point
Poor air circulation	Stagnant air at sill allows condensation to accumulate	Ensure furniture does not block sills; promote air circulation with fans if needed
Failed sealant	Water seepage around joint allows water into framing	Inspect annually; reapply sealant as needed; ensure proper slope and drip edge

How does a window sill differ from a window reveal?

A window reveal is the entire opening created by the window penetrating the wall. It includes the horizontal sill at the bottom, the vertical jambs on the sides, and the horizontal header at the top. The window sill is the bottom component of the reveal assembly. Internally, the reveal is often called the "window well" and may have an interior sill or "stool" (a shelf-like horizontal board, often wooden for aesthetic reasons, that sits on the structural sill). Externally, the reveal is called the "opening" and is defined by the outer face of the wall. The structural sill bridges this opening and is the interface between the window frame and the wall, whereas the reveal is the entire three-dimensional space.

What maintenance is required for window sills?

Regular inspection and maintenance extend sill life and prevent costly water damage. Check sealant around the frame annually, especially after weathering or building movement. If sealant is cracked or missing, clean the joint and reapply with compatible flexible sealant. For exterior sills, inspect the slope and drip edge; any flat spots or debris should be cleared to ensure water drains freely. For stone or concrete sills, apply a sealer every 3-5 years to prevent water absorption. Cellular PVC and aluminium sills are low-maintenance but should be checked for fastener corrosion, thermal expansion gaps (especially in metal), and debris accumulation at joints and weep holes.