The window stool, also known as the window sill or interior sill, is an important part of any window assembly. It is the horizontal shelf that sits at the base of the window on the interior side. The window stool serves several key functions.
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Purpose of the Window Stool
The main purposes of the window stool are:
- Provide a resting place for the window sash and frame
- Cover up the rough window opening in the wall
- Provide a water drainage plane to direct water to the exterior
- Add decorative detail to interior windows
Without a window stool, the bottoms of the window sashes and jambs would be exposed. The stool gives them a clean finished edge. It also covers up the rough framing opening and any gaps or cracks. This gives a smooth finished look to the interior of the window.
The stool is angled slightly to the exterior to allow any moisture that enters the window assembly to drain to the outside. This is an important water management detail that protects the wall cavity from water damage.
Finally, window stools can add aesthetic appeal with decorative profiles, materials, and trims. They are one of the few parts of a window that are visible inside and can complement the interior design.
Construction of Window Stools
Window stools are typically constructed from wood or wood composites. The most common materials are:
- Wood – Paint or stain grade solid wood like oak or poplar
- MDF – Medium density fiberboard with a veneer or laminate finish
- PVC – Cellular PVC cut in a wood profile shape
- Marble/Granite – Solid natural stone slab
- Concrete – Poured and molded concrete
Wood and MDF are the most common since they are affordable and easy to manufacture in complex molding profiles. PVC is a great low maintenance option. Natural stone and concrete add high-end luxury but at a much higher cost.
The stool is cut in the profile shape needed, typically with a slight slope to the exterior. Common profiles are rectangular, beveled, ogee, and bullnose shapes. The length is cut to fit tightly between the side jambs.
How the Window Stool Attaches
The main methods used to attach a window stool are:
- Nailed through the bottom edge
- Glued to the sill plate or wall framing
- A cleat provides support underneath
- Silicone adhesive caulk
The simplest attachment is through the bottom front edge. Finish nails or pneumatic brad nails are used to secure the stool to the framing beneath. This provides a fast and secure connection.
Construction adhesive is another common method. A quality adhesive like Liquid Nails is applied to the back side and the stool is pressed into place. This can provide a cleaner finish with no visible nail holes.
For extra support, a wood cleat can be installed under the rear edge. This provides a sturdy ledger for the stool to rest on. The cleat is screwed into the framing and adhesive applied between it and the stool.
Finally, silicone caulk can adhere the stool in place. This works for lightweight materials like PVC. But is not the most secure method by itself.
The installation method can vary based on the stool material chosen and personal preference. Many carpenters will use a combination of mechanical fasteners and adhesive to ensure a durable bond.
Stool Attachment Points
There are two main framing elements that window stools can attach to:
- Sill Plate – The horizontal board at the base of the wall framing.
- Rough Sill – A board that extends into the window opening.
Sill Plate Attachment
The sill plate is always present and provides a continuous mounting surface:
Attaching the stool to the sill plate with adhesive and/or nails through the bottom edge is simple and effective. No additional framing needs to be added.
Rough Sill Attachment
A dedicated rough sill offers more support but requires extra framing:
The rough sill extends into the window opening and provides a solid surface for the stool to mount to. The stool can be nailed through the bottom or a ledger added behind.
Interior Sill Considerations
Some other details to consider for interior window sills:
- Slope the stool 1/4″ per foot minimum to the exterior to encourage drainage
- Provide expansion gaps at the corners if using solid wood
- Miter joints at the corners for a clean seamless appearance
- Use backer rods and caulk under the stool for better sealing
- Add trim pieces like quarter round for decorative details
Keeping the stool angled down and allowing for wood movement will help prevent water issues. And the trim can really add that finishing touch to the interior view.
Exterior Sill Options
For completeness, here are some notes on the exterior sill options that align with the interior stool:
- Brickmould – Extends past the window frame to cover the gap between the jambs and wall.
- Nail Fin – Flange around the window for attaching directly to wall sheathing.
- Block Frame – Heavy frame sits directly on the rough opening.
- Receptor – Built-in drainage slope underneath the window.
These exterior sill styles then interface with the interior window stool. The most common combination is either a brickmould or nail fin window with a simple interior stool.
Conclusion
In summary, the main points about what a window stool attaches to include:
- Provides interior covering and trim for bottom of window
- Typically constructed from wood, MDF, or PVC
- Main attachment points are the sill plate or rough sill
- Nails, glue, caulk used to adhere stool to framing
- Should be sloped slightly down to exterior
- Pairs with exterior sill styles like brickmould and nail fin
Understanding how the window stool mounts and integrates with the surrounding structure helps ensure a secure and well-sealed installation. It’s a small but important piece of the overall window system. With attention to detail on the stool attachment, you can achieve both functional and beautifully trimmed window openings.
