How much would it cost to install a pre-cast stone sill and parge underneath basement windows?

Installing a pre-cast stone sill and parging the foundation wall beneath each basement window typically costs $350–$800 per window in the GTA, depending on the sill size, material, wall condition, and accessibility. For a home with 4–6 basement windows, you're looking at $1,500–$4,500 for the complete project including all materials, surface preparation, and finishing.

The pre-cast stone sill itself is a manufactured concrete or reconstituted stone piece that sits on top of the foundation wall at the base of the window, creating a finished ledge that sheds water away from the wall. Pre-cast sills range from $40–$120 each depending on length, profile, and finish. Natural stone sills (limestone or granite) run significantly higher at $150–$400 per piece. Installation involves setting the sill in a bed of Type S mortar, ensuring a proper slope away from the window (minimum 15-degree pitch) so water drains outward rather than pooling against the frame. The mason will also apply a bead of polyurethane caulk where the sill meets the window frame and the foundation wall to create a watertight seal — critical in the GTA where freeze-thaw cycling will exploit any gap where water can enter.

Parging the foundation wall beneath the window involves preparing the existing concrete or block surface, applying a concrete bonding agent, and then trowelling on one or two coats of parging mix to create a smooth, weather-resistant finish. For these smaller sections, polymer-modified parging ($12–$18 per square foot installed) is the smart choice because it offers superior freeze-thaw resistance and flexibility compared to traditional Portland cement parging. Each window section is typically 3–8 square feet of parging area, so the parging component runs $50–$150 per window.

The labour cost is where the numbers add up, because each window requires individual attention — surface prep, sill fitting, mortar work, parging, and caulking. A skilled mason can typically complete 3–4 windows per day depending on conditions and the state of the existing foundation surface. If the existing parging beneath the windows is crumbling or delaminated, removal of the old material adds $50–$100 per window. If the concrete block or poured concrete beneath the windows has cracks, those should be addressed before parging — crack injection runs $300–$800 per crack if structural, or can be patched with hydraulic cement for $20–$50 in materials if the cracks are hairline settling cracks.

Timing matters significantly for this project in the GTA. Parging and mortar work require temperatures consistently above 5 degrees Celsius day and night for at least 7 days after application. The practical window for this work is mid-April through mid-October. Booking in summer rather than peak spring or fall season may yield slightly better pricing as parging contractors are often fully booked in April–May and September–October.

This is a project where hiring a professional mason makes sense even though the individual sections are small. Proper sill installation requires precise levelling and slope, correct mortar bedding, and waterproof caulking at all joints. The parging beneath windows is highly visible from the street and receives heavy exposure to rain, snow, and salt splash. A poorly installed sill that slopes inward will direct water against the window frame and into the basement — exactly the opposite of what you want. If you're planning this work, ask your contractor about adding a penetrating concrete sealer ($3–$7 per square foot) to the finished parging for added protection against salt damage and moisture absorption.