Window Sill
The horizontal member at the bottom of a window opening.
Quick Answer
A window sill is the horizontal member at the bottom of a window opening. The rough sill is the framing piece under the window, the exterior sill sheds water outward, and the interior stool is the trim shelf inside. Estimators count sills each from the window schedule or by the linear foot of trim.
The Full Picture
The word sill covers more than one piece. In the framing, the rough sill is the horizontal member that closes the bottom of the rough opening, with cripple studs below it. On the exterior, the sill is a sloped surface under the window, built into the window unit or added in stone, precast, wood or metal, that sheds water away from the wall. Inside, the stool is the shelf-like trim piece, and the apron is the trim beneath it. In everyday language a window sill can mean any of them, so the estimator has to read which one the drawings and spec call for.
Where it sits in the assembly depends on which sill is meant. The rough sill is part of the wall framing and is counted with the wall framing, along with the header. The exterior sill belongs to the window unit or the envelope trade, and its slope and flashing details are drawn in the wall section. The interior stool and apron are finish carpentry or millwork, shown in the interior elevations and trim details and sometimes in the finish schedule.
The unit depends on the type. Rough sills are counted in linear feet with the framing. Window units with an integral sill are counted each from the window schedule, and the schedule gives size and mark. Trim stools and apron are carried in linear feet, usually the window width plus the horns, the extension past the casing on each side. Stone and precast sills may also be counted each, with length stated. Waste for trim is usually applied to stock lengths, and the amount varies with the layout and material.
Pricing mistakes come from mixing the pieces. An estimate may carry the window unit and omit the stool, or include the stool in finish carpentry while the window supplier also includes a sill, so it is counted twice. Other errors are forgetting the horns, ignoring the material change at wet locations, and missing the sealant and flashing at the sill, which affects performance. Matching each window mark to a clear scope line avoids the problem.
Real Examples
Common Misconceptions
People assume: A window sill is one piece.
Actually: A window can have a rough sill in the framing, an exterior sill, and an interior stool and apron. Each belongs to a different scope and unit.
People assume: Sill length equals the window width.
Actually: Trim stools extend past the casing as horns, so they are longer than the window. Exterior stone sills often extend past the opening as well.
People assume: The window price includes interior trim.
Actually: Window units commonly arrive without interior stool and apron, which are finish carpentry. Confirm it in the spec.
Does MeltPlan Solve This?
Partially — adjacentWindow counts, sizes and marks come from the plans and window schedule, and MeltPlan's takeoff service applies to counting from them: AI pulls quantities from plans and US-based experienced estimators review and adjust them. Deciding which trade carries the sill, stool and flashing stays with your estimator.
Get window counts and sizes taken off your plans →Frequently Asked Questions
What is the difference between a window sill and a stool?
In trim carpentry the stool is the horizontal shelf inside the window, and the sill is the sloped surface that sheds water on the outside. In casual use people call the stool a sill, so check the drawings.
How do you take off window sills?
Count the windows from the window schedule, then carry each sill each or in linear feet, using window width plus any horns for trim pieces. Keep rough sills with the framing and finish sills with millwork or the window supplier.
What is a rough sill?
It is the horizontal framing member at the bottom of a rough opening, which supports the window unit and sits on short cripple studs. It is counted with the wall framing.
Where are window sills shown in the drawings?
The window schedule gives the marks and sizes, elevations show exterior sills, wall sections show slope and flashing, and interior elevations or trim details show the stool and apron.