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Safety Cue
Task-specific safety guidance for ordinary work hazards.
Project 07
Install Window Stool, Apron, and Interior Casing
Install a complete interior window trim assembly — stool, apron, side casing, and head casing — with consistent reveal, a level stool, a centered apron, and unimpaired window operation.
Field Cue System
These cues appear at the point of work. They identify safety, code, manufacturer, and measurement issues before they become rework, callbacks, or hazards later in the book.
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Critical Safety Cue
Serious injury, fire, or major damage is possible.
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Field Safety Cue
Practical work-area safety: lighting, cords, footing, and housekeeping.
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Code Alert
Local requirements vary. Verify permit, inspection, and jurisdictional rules.
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Manufacturer Cue
Follow product-specific instructions for tools, materials, and equipment.
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Measurement Cue
Clearance, layout, spacing, and reference-point guidance.
Job Overview
This project qualifies you to install a complete interior window trim assembly: a stool that spans the opening and returns into the wall on each side, an apron that supports the stool visually and structurally beneath it, and side and head casing that frame the opening with a consistent reveal. The finished work product is one window with a level, securely fastened stool; a centered, securely fastened apron; and side and head casing installed with the reveal established in Project 01 and aligned to the stool horns.
You will verify the window is installed and operating correctly, establish the stool projection, horn length, casing reveal, and apron alignment from the approved trim schedule, cut and fit the stool to the window and side jambs including horns and jamb notches, fasten the stool without affecting window operation, cut and center the apron beneath the stool, mark and cut side and head casing to the established reveal, align the casing with the stool horns, and fasten the complete assembly.
This project depends directly on the Chapter 1 room survey and controlling-line layout, which recorded the casing reveal and the jamb depth needed to plan the stool and apron. It also depends on the window itself passing the Chapter 1 opening inspection for operation, frame alignment, and jamb depth. The techniques used here — establishing a reveal, cutting and fitting miters, and fastening casing to jamb and wall framing — are the same techniques used for door casing in Project 04 and are reused in every later trim chapter.
Skills You Will Demonstrate
- FC-037 — Measure the window opening, including jamb depth, jamb returns, and reveal-controlling dimensions.
- FC-038 — Cut the stool, including horn length, jamb notches, and wall scribes.
- FC-039 — Install the apron, centered beneath the stool and fastened to backing.
- FC-040 — Install side casing at the established reveal, aligned with the stool horns.
- FC-041 — Install head casing at the established reveal, with joints aligned to the side casing.
- FC-042 — Inspect window-trim alignment, including reveal, stool level, apron centering, and window operation.
Where This Skill Fits
Project 01 established the casing reveal for every opening in the room, including this window, and recorded jamb depth on the Opening Inspection Record. Project 03 taught the measuring, marking, and mitering techniques used throughout this project. Project 04 applied those techniques to door casing; this project applies the same reveal-and-miter method to a window opening, with the added steps of fitting a stool with horns and notches and installing an apron beneath it.
The finished window trim assembly in this project is inspected again in Project 18, where nail holes are filled, joints are caulked at designated paint-grade locations, and the assembly is prepared for paint or stain. It is also one of the trim assemblies integrated into the Chapter 20 capstone.
Scope and Intended Use
This project covers:
- verifying the window is installed, flashed, insulated, air-sealed, and operating correctly before trim work begins
- inspecting jamb depth, drywall plane, sill condition, reveal, and moisture at the opening
- establishing the stool projection, horn length, casing reveal, and apron alignment from the approved trim schedule and the Chapter 1 controlling-line layout
- measuring the window opening and jamb returns and transferring horn and notch dimensions to the stool blank
- cutting the stool ends, horns, and jamb notches, and test-fitting the stool against the window and side jambs
- scribing the stool for wall irregularities and fastening it to backing without affecting window operation
- measuring, cutting, centering, and fastening the apron beneath the stool
- marking the approved casing reveal on the side and head jambs, cutting side and head casing, and aligning casing joints with the stool horns
- fastening the head and side casing using the approved pattern and securing miters or butt joints as specified
- checking reveals, stool level, apron centering, casing plane, and window operation, and correcting open joints, a rocking stool, uneven horns, or binding
This project does not authorize the learner to:
- trim a window that has not passed the Chapter 1 opening inspection for operation, frame alignment, and jamb depth; refer an unresolved window defect to the responsible trade before proceeding
- alter window flashing, insulation, air-sealing, operating hardware, or weather seals in the course of installing trim
- reduce window operation, egress clearance, or accessibility clearance to achieve a planned reveal or stool projection
- conceal an open joint, rocking stool, or major gap with filler or caulk in place of correcting the fit; see Project 18 for the approved filling and caulking scope
Reference Conventions
Establish these references before any stool, apron, or casing piece is cut, and do not change them mid-window.
| Term | Definition |
|---|---|
| Stool | The horizontal trim piece that spans the window opening at sill height, sits on the sill or sill nose, and returns beyond the casing on each side as a horn |
| Horn | The portion of the stool that extends beyond the side jamb and beneath the side casing to the outer edge of the casing profile |
| Jamb notch | The cutout in the stool that fits around the side jamb and allows the stool to sit flush against the sill and wall |
| Apron | The horizontal trim piece installed beneath the stool, centered on the opening, that supports the stool visually and conceals the wall-to-sill transition |
| Controlling reveal line | The layout line marking the intended casing reveal on the window jamb, established in Project 01 and used here exactly as recorded |
| Long point / short point | The longer and shorter edges of a mitered casing piece, used to measure and align casing consistently with Project 03 and Project 04 methods |
Window Trim Record
Before cutting any piece, record the opening identity and controlling values directly on the work order:
Room: ______ Window ID (per window schedule): ______ Casing reveal (from Project 01): ______ Jamb depth: ______ Stool projection: ______ Horn length: ______
Prerequisites
- Completed Project 01 room survey and Opening Inspection Record for this window, confirming operation, frame alignment, and jamb depth
- Controlling-Line Layout from Project 01 recording the casing reveal for this opening
- Completed Project 02 cutting and assembly station, staged and ready
- Completed Project 03 measuring, marking, and mitering competency
- Approved trim schedule and finish specification identifying the stool, apron, and casing profile, dimensions, species or material, and paint-grade or stain-grade status for this window
Common Field Fundamentals
The stool is fitted first because it is the only piece in this assembly that must be notched around the jambs and scribed to the wall; every other piece in the assembly references the stool once it is set. The apron is centered beneath the stool rather than beneath the rough opening, because the stool's horns, not the framed opening, define the visible width of the finished assembly. The side and head casing then follow the reveal established in Project 01 and align their inside edges with the outer edges of the stool horns.
StoolMeasured, notched, scribed, and fastened first; controls the assembly
ApronCentered beneath the stool and fastened to backing
Side casingSet at the Project 01 reveal, aligned to the stool horns
Head casingSet at the Project 01 reveal, joined to side casing
Horn Length and Notch Fit
Horn length is the distance the stool projects beyond the outer edge of the side casing, measured from the approved trim schedule. The jamb notch depth equals the jamb depth measured at the opening plus any scribe allowance needed for wall irregularity identified in the Project 01 survey. Cut the notch after the horn length and overall stool length are established, and test-fit before committing to the final cut.
FC-037 – FC-042
Installation Qualification
Window Stool, Apron, and Casing Installation
A. Finished-Project Specification
Quantity: one complete stool, apron, side-casing pair, and head-casing piece per window, installed with the reveal, stool projection, and horn length recorded on the Window Trim Record, with stable stool level, centered apron, aligned casing joints, and preserved window operation.
B. Component Schedule
| Component | Quantity | Material | Finished Size | Preparation |
|---|---|---|---|---|
| Stool | 1 per window | Per trim schedule | Opening width plus two horn lengths, per Window Trim Record | Notched at both jambs; scribed to wall where required |
| Apron | 1 per window | Per trim schedule | Per apron alignment dimension on the trim schedule, centered beneath the stool | Square-cut or profiled ends as specified |
| Side casing | 2 per window | Per trim schedule | Head-jamb reveal to stool, per measured opening | Mitered or butted at head per the approved joint method; bottom end square or profiled as specified |
| Head casing | 1 per window | Per trim schedule | Between the long points of the side-casing miters, or per the specified joint method | Mitered or butted to match side casing |
C. Material and Documentation Takeoff
- Stool, apron, and casing stock: profile, species or material, and finish grade per the approved trim schedule and finish specification for this window.
- Fasteners: type, size, and quantity per the fastener schedule for stool, apron, and casing joints.
- Adhesive: used only at joints designated in the finish specification.
- Reference documents: the Project 01 Window Trim Record values for reveal, jamb depth, stool projection, and horn length.
D. Fastener and Hardware Schedule
| Joint | Fastener | Installation Notes |
|---|---|---|
| Stool to sill/backing | Per fastener schedule | Set below the surface without crushing the profile; angled away from glazing and sash |
| Apron to wall backing | Per fastener schedule | Centered beneath the stool; fastened into backing identified in the Project 01 Obstacle and Backing Map |
| Side and head casing to jamb | Per fastener schedule | Kept clear of fragile profile edges; consistent spacing along each leg |
| Side and head casing to wall framing | Per fastener schedule | Located at framing identified in the Project 01 Obstacle and Backing Map |
| Miter joints | Per fastener schedule | Pinned or fastened using the approved method; joint held closed during fastening |
E. Acceptance Standards
| Check | Standard |
|---|---|
| Stool level | Level within the tolerance stated in the finish specification, verified with a level across the full stool length |
| Horn symmetry | Horn length matches the trim schedule value within the stated tolerance on both sides |
| Apron centering | Apron centered beneath the stool within the tolerance stated in the finish specification |
| Casing reveal | Matches the Project 01 recorded reveal within the stated tolerance on both jambs and the head |
| Miter closure | No visible gap at any casing miter or butt joint under normal light |
| Window operation | Window opens, closes, and latches without binding, rubbing, or obstruction from installed trim |
| Fastener condition | All fasteners set below the surface without crushed or split profile |
F. Window and Opening Verification
- Verify the window is installed, flashed, insulated, air-sealed, and operating correctly. Operate the window through its full range and record the result before proceeding.
- Inspect jamb depth, drywall plane, sill condition, reveal, and moisture. Confirm these match the Project 01 Opening Inspection Record for this window; note and correct any discrepancy before continuing.
G. Layout Values
- Establish the stool projection, horn length, casing reveal, and apron alignment. Take these values from the approved trim schedule and the Project 01 Controlling-Line Layout, and record them on the Window Trim Record.
H. Stool Layout and Cutting
- Measure the window opening and jamb returns. Record the opening width, jamb depth at each side, and any wall irregularity noted in the Project 01 survey.
- Transfer horn and notch dimensions to the stool blank. Mark the overall length including both horns, and mark each jamb-notch location and depth.
- Cut the stool ends, horns, and jamb notches. Use the miter saw for straight cuts and a coping saw or jigsaw for the notch profile as specified.
- Test-fit the stool against the window and side jambs. Check notch fit, horn length on both sides, and full contact along the sill or sill nose.
- Scribe and adjust for wall irregularities. Use a block plane or sanding block to fit the stool to the actual wall condition without exceeding the scribe allowance recorded on the Window Trim Record.
- Fasten the stool to backing without affecting window operation. Operate the window again immediately after fastening the stool and before proceeding to the apron.
I. Apron Installation
- Measure and cut the apron. Use the apron alignment dimension recorded on the Window Trim Record.
- Center the apron under the stool and fasten it. Verify centering against the stool horns before final fastening, and fasten into the backing identified in the Project 01 Obstacle and Backing Map.
J. Casing Layout and Installation
- Mark the approved casing reveal on side and head jambs. Use the exact value recorded on the Project 01 Controlling-Line Layout for this opening.
- Measure and cut side casings. Measure each side independently from the reveal line to the stool.
- Measure and cut the head casing. Use the joint method specified in the trim schedule.
- Test-fit casing joints and align them with stool horns. Confirm the inside edge of each side casing aligns with the outer edge of its corresponding stool horn.
- Fasten the head and side casing using the approved pattern. Follow the fastener schedule for spacing and location.
- Secure miters or butt joints as specified. Pin or fasten using the approved method while holding the joint closed.
K. Inspection and Correction
- Check reveals, stool level, apron centering, casing plane, and window operation. Compare each measurement against the Acceptance Standards table.
- Correct open joints, rocking stool, uneven horns, or binding. Use the troubleshooting procedure in this chapter for any failed check.
- Prepare the assembly for filling, caulking, paint, or stain. Record the completed assembly on the Window Trim Record and hold the room for Project 18.
Common Cutting Inspection
Apply the following checks to every stool, apron, and casing piece before it is fastened permanently.
- Reference check: confirm the reveal marked on the jamb matches the Project 01 recorded value.
- Notch check: confirm the stool notch depth matches the measured jamb depth and permitted scribe allowance.
- Horn check: confirm horn length matches the trim schedule on both sides before the stool is fastened.
- Miter check: confirm side and head casing miters close fully in a dry-fit before any fastener is driven.
- Operation check: confirm the window operates correctly after the stool is fastened and again after casing is complete.
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Failure Points and Troubleshooting
Cleanup
- Remove offcuts, sawdust, and packaging from the work area and dispose of them per Project 02 waste procedures
- Return the miter saw, coping saw, planes, and clamps to the Project 02 workstation in working condition
- Store unused stool, apron, and casing stock flat or on approved racks, separated by profile and grade
- Record final reveal, stool level, apron centering, and window-operation results on the Window Trim Record
- File the completed Window Trim Record with the room file established in Project 01
Field Terms
- Stool
- The horizontal trim piece spanning the window opening at sill height, with horns that return beyond the casing.
- Horn
- The portion of the stool extending beyond the side jamb and casing.
- Jamb Notch
- The stool cutout that fits around the side jamb.
- Apron
- The trim piece beneath the stool that supports it visually and conceals the wall-to-sill transition.
- Controlling Reveal Line
- The Project 01 layout line marking the intended casing reveal, used exactly as recorded.
- Long Point / Short Point
- The longer and shorter edges of a mitered casing piece, used as the measuring reference.
- Scribe Allowance
- The permitted amount by which a stool or casing edge may be planed or sanded to fit an irregular wall or sill.
- Window Trim Record
- The field record of reveal, jamb depth, stool projection, and horn length for one window.
Skill Check
- Why must the stool be measured, cut, and fastened before the apron and casing are cut?
- What is the difference between horn length and stool projection, and where does each value come from?
- Why is the apron centered on the stool horns rather than on the rough opening?
- Why must the casing reveal be taken from the Project 01 Controlling-Line Layout rather than established by eye at the casing stage?
- What two checkpoints in this project require the learner to operate the window, and why does each occur where it does?
- What is a scribe allowance, and what happens if a stool edge is planed beyond it?
- Why must horn length be measured independently on each side of the stool rather than assumed symmetrical?
- What must the learner verify about the window opening before beginning stool layout?
- Why does a fastener schedule specify installation direction and location near the window rather than only fastener size?
- What must happen before this window trim assembly can proceed to Project 18?
Competency Rubric
| Competency | Independent Pass Standard | Not Demonstrated | With Assistance | Independently |
|---|---|---|---|---|
| FC-037 | Measures the window opening, jamb depth, and jamb returns accurately and records values on the Window Trim Record | |||
| FC-038 | Cuts the stool, including horns and jamb notches, to fit the opening within the stated tolerance without exceeding the scribe allowance | |||
| FC-039 | Installs the apron centered beneath the stool within the stated tolerance and fastened to identified backing | |||
| FC-040 | Installs side casing at the recorded reveal, aligned with the stool horns, with closed miter or butt joints | |||
| FC-041 | Installs head casing at the recorded reveal with joints aligned to the side casing | |||
| FC-042 | Inspects and confirms reveal, stool level, apron centering, casing plane, and unimpaired window operation, correcting any failed check |
Mark one column per competency. Only Demonstrated Independently awards the competency; Demonstrated with Assistance authorizes additional supervised practice but does not complete the requirement.
Inspection Checklist — Window Trim Worksheet
| Item | Checked (Y/N) | Defect Found (Y/N) | Correction Recorded (Y/N) | Reinspected (Y/N) | Result |
|---|---|---|---|---|---|
| Window operation before trim | |||||
| Stool level | |||||
| Horn length, both sides | |||||
| Apron centering | |||||
| Side casing reveal, both jambs | |||||
| Head casing reveal | |||||
| Miter/butt joint closure | |||||
| Fastener condition | |||||
| Window operation after trim |
Total open deficiencies: ____________ Total closed deficiencies: ____________
Window trim status: ______ (Approved / Conditional / Rejected) Date: ____________ Inspector name: ____________
Performance Check
With the assembly complete, review every measurement against the Acceptance Standards table and confirm the stool is level, both horns match the trim-schedule length within tolerance, the apron is centered beneath the stool, the side and head casing reveal match the Project 01 recorded value, every miter or butt joint closes without a visible gap, all fasteners are set below the surface without profile damage, and the window opens, closes, and latches without binding. Complete the Window Trim Worksheet and record the assembly status before moving to the next window or opening. If any item cannot be closed, record it as a condition and identify the correction required before Project 18 filling and caulking begins on this assembly.
Project Closeout
You have completed the window stool, apron, and casing installation project: measuring the window opening and jamb returns (FC-037), cutting the stool with accurate horns and jamb notches (FC-038), installing a centered, securely fastened apron (FC-039), installing side casing at the established reveal aligned to the stool horns (FC-040), installing head casing at the established reveal with joints aligned to the side casing (FC-041), and inspecting the completed assembly for reveal, stool level, apron centering, casing plane, and window operation (FC-042).
You now have one complete, secure, aligned window trim assembly with preserved window operation, ready for the filling and caulking work in Project 18 and for integration into the Chapter 20 capstone. The reveal, fastening, and joint-fitting methods used here carry directly into the baseboard work that begins in the next project.
File the completed Window Trim Record and Window Trim Worksheet with the room file established in Project 01. You will reference this assembly again in Project 18 and Chapter 20.