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Construction glossary · Envelope and finishes

What is window sillin construction?

A window sill is the horizontal bottom member of a window opening that supports the unit and sheds water away from the wall below. On the exterior it is often a sloped metal or stone sill with a drip; on the interior it is a stool or finished sill. The estimator counts sills off the window schedule and converts them to linear feet of sill stock for material and labor.

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Updated August 2026 · Reviewed by the Ruh construction team

Pan slope 5 to 15 deg outFunction Sheds water outIncludes Sill pan + stool + apronTakeoff Each opening

Window sill section at rough opening

Window unit + nailing finSloped sill pan flashingSill plate / framingInterior stool + apronSloped 5-15 degBack dam at sill
The definition

Window sill, explained.

In commercial construction the window sill is the bottom of the window assembly and its associated trim, and it does two jobs: it carries the unit and it directs water out of the opening. Exterior sills are typically formed metal pans, precast or cut stone, or a sloped masonry course, all detailed with a positive slope and a drip edge so water runs to the face and not back into the wall. Interior sills (the stool) are finished material, often solid surface, stone, or wood, with an apron below. As an estimator you read the window schedule for the count, the type, and the rough opening width of each mark, then go to the sill detail in the wall section to confirm depth, slope, end dams, and material. Sills are quantified two ways at once: a count by window mark, and the linear footage of sill stock those counts produce. Both feed the price book, because some sills price each (precast units) and some price by the foot (extruded metal or stone slab).

How it is measured

Sills are taken off the window schedule by mark and count, then cross-checked against the wall section sill detail for material, depth, and slope. The governing dimension is the rough opening width per mark, which becomes linear feet of sill stock once you add end-dam returns and a cutting allowance. Stone and precast sills are often carried as each by size; extruded metal and slab sills are carried by the linear foot. Record material, finish, and end conditions per mark. Sum the lf across all marks for ordering, and keep the count for unit-priced items and for labor, which usually tracks the number of openings.

Worked example

A worked window sill example.

Worked example

From the window schedule, say there are 18 windows on mark W1 at 4 ft 0 in rough opening and 6 windows on mark W2 at 6 ft 0 in, all getting an extruded metal sill. Convert each opening to sill stock with a 2 in end-dam return on both ends, so add 4 in (0.33 ft) per sill. W1: 18 x (4.0 + 0.33) = 18 x 4.33 = 77.9 lf. W2: 6 x (6.0 + 0.33) = 6 x 6.33 = 38.0 lf. Total net = 115.9 lf. Add 8 percent for cutting and miters: 115.9 x 1.08 = 125.2 lf, round to 126 lf. At an illustrative US installed rate of 18 dollars per lf, 126 lf x 18 dollars = 2,268 dollars. The 24 sill count also carries to labor, since each opening is a separate set-and-seal operation regardless of width.

How Ruh handles it

How Ruh handles window sill.

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Window sill: frequently asked questions

Should I take window sills off the schedule or the elevations?+

Start from the window schedule for an accurate count and rough opening widths by mark, then verify the sill detail in the wall section for material, slope, and end dams. Elevations confirm location but the schedule drives the quantity.

Why carry both a count and linear feet for sills?+

Because pricing splits two ways. Precast and stone sills often price each by size, while extruded metal and slab sills price by the foot. Labor usually tracks the count of openings, so you need both numbers.

What do I add to the bare opening width when figuring sill stock?+

End-dam returns on each end and a cutting or miter allowance. The drip and the turn-ups at the jambs consume stock beyond the net rough opening, so the ordered footage runs longer than the sum of widths.

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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.

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