Window Installation Guide

Complete guide to window installation including rough opening calculations, flashing sequence, level/plumb/square checks, shimming techniques, and finishing

Rough Opening Calculations

Critical Rough Opening Formula

The rough opening (RO) must be precisely sized to allow for shimming and adjustment while ensuring proper fit and operation of the window.

Standard Formula: Rough Opening = Window Size + 1/2 inch shimming gap (1/4" each side)

Rough Opening Size Chart

Window Size (Width × Height)	Rough Opening Width	Rough Opening Height	Shimming Gap
24" × 36"	24-1/2"	36-1/2"	1/4" each side
30" × 48"	30-1/2"	48-1/2"	1/4" each side
36" × 60"	36-1/2"	60-1/2"	1/4" each side
48" × 48" (Picture)	48-1/2"	48-1/2"	1/4" each side
60" × 48" (Bay/Bow)	60-3/4"	48-3/4"	3/8" each side
72" × 60" (Sliding)	72-3/4"	60-3/4"	3/8" each side

Measuring Window Size

Replacement windows: Measure existing window frame at 3 points (top, middle, bottom), use smallest dimension
New construction: Measure rough opening and subtract 1/2" for window size
Check manufacturer specs: Some require 3/8" gap, others 1/2"
Large windows: May need 3/4" total gap for easier installation

Rough Opening Requirements

Level sill: Bottom must be perfectly level
Plumb sides: Jack studs must be perfectly vertical
Square corners: Must be 90 degrees (check diagonals)
Diagonal check: Diagonals should be equal within 1/8"

Header Sizing for Rough Openings

Up to 3 feet wide: Double 2x6 header (load-bearing walls)
3-5 feet wide: Double 2x8 header
5-7 feet wide: Double 2x10 header
7-8 feet wide: Double 2x12 header or engineered beam
Over 8 feet: Consult engineer, likely needs LVL or steel beam
Non-load-bearing: Can use single 2x4 flat header for any width

Flashing Sequence & Waterproofing

Critical Flashing Order

Proper flashing installation is the most important factor in preventing water intrusion. Flashing must be installed in the correct sequence to create a shingle effect where each layer overlaps the one below.

Correct Order: Sill First → Sides Second → Head Last (water flows down and out)

Flashing Types & Applications

Flashing Type	Location	Material	Purpose
Sill Pan Flashing	Bottom of RO	Flexible membrane or metal	Creates water-tight sill, first line of defense
Side Flashing	Jambs (sides)	Adhesive-backed tape or metal	Directs water down to sill pan
Head Flashing	Top of window	Metal drip cap or tape	Diverts water away from head, over side flashing
Back Dam	Back of sill	Foam or wood block	Prevents water from running inside under window
End Dams	Ends of sill pan	Folded membrane corners	Contains water on sill, prevents edge leaks

Step-by-Step Flashing Installation

1.Install back dam: Place 1/2" foam backer rod or wood strip at back edge of sill
2.Sill pan membrane: Install flexible flashing membrane on sill, up sides 6", seal back dam
3.Create end dams: Fold corners up at 45° to create water-tight corners
4.Side flashing: Install jamb flashing tape from sill up to 1" past head, overlap sill pan
5.Install window: Set window in opening, secure temporarily
6.Head flashing: Install metal drip cap or head tape, overlapping side flashing by 2"
7.Integrate with WRB: Install housewrap or building paper over head flashing

Flashing Best Practices

Always follow "shingle principle" - upper layers over lower
Use quality adhesive-backed flashing tape (not cheap housewrap tape)
Warm tape with heat gun in cold weather for better adhesion
Roll all tape edges firmly with J-roller or hand pressure
Slope sill slightly outward (3-5 degrees) for drainage

Common Flashing Mistakes

Installing head flashing before side flashing (water gets behind)
No back dam on sill (water runs under window into wall)
Forgetting end dams on sill pan (water leaks at corners)
Using caulk instead of proper flashing (caulk fails over time)
Not integrating with building wrap properly

Level, Plumb & Square Verification

Critical Tolerances

Level (horizontal): Sill must be within 1/16" over full width
Plumb (vertical): Sides must be within 1/8" over full height
Square: Diagonals must be equal within 1/8"
Manufacturer tolerance: Follow specific requirements in installation guide

Checking Level

Tool: 24" or 48" level (longer is better for accuracy)
Check sill: Place level on bottom of window frame, check bubble
Check head: Verify top is also level
Adjustment: Shim under low corners until perfectly level

Checking Plumb

Tool: 6-foot level or plumb bob
Check jambs: Place level on both side jambs, check bubble
Check both faces: Verify inside and outside faces of jamb
Adjustment: Add shims behind jambs to push in/out as needed

Checking Square

Diagonal method: Measure corner to corner both ways
Equal diagonals: If equal within 1/8", window is square
If not square: Longer diagonal needs to be pushed in at corner
Operation test: Open/close window - should move smoothly

Final Verification

Sash alignment: Verify sash fits evenly in frame with consistent gap
Lock test: Window lock should engage smoothly without forcing
Open/close: Sash should slide or swing freely without binding
Gap check: Even reveal all around sash when closed

Shimming Techniques & Locations

Critical Shimming Locations

Proper shimming prevents window frame distortion and ensures smooth operation. Shims must be placed at specific structural points to support the window without bowing the frame.

Required Locations: At hinges, at latch/lock, at corners, maximum 12" spacing on all sides

Shim Placement Chart

Location	Shim Placement	Purpose	Spacing
Bottom corners	Under each corner of sill	Support weight, establish level	Both corners
Hinge side	Behind each hinge location	Support hinge stress, prevent sag	At each hinge
Latch/Lock side	Behind latch and lock points	Prevent jamb flex when locking	At each lock
Top corners	Behind head at corners	Prevent head sag on wide windows	Both corners
Intermediate jambs	Every 12" max on sides	Prevent jamb bow, maintain plumb	12" on center
Head (wide windows)	Every 12" on windows over 4' wide	Prevent head deflection/sag	12" on center

Shimming Best Practices

Use composite shims (won't compress or rot like wood)
Insert shims in pairs (opposite directions) for stability
Shim snug but not tight (don't bow frame)
Screw through shims into framing (not just into shims)
Score and snap shims flush after fastening
Recheck level/plumb/square after each shim adjustment

Common Shimming Mistakes

Over-shimming (bows frame, binds operation)
Under-shimming (allows frame movement, gaps develop)
Missing shims at critical locations (hinges, locks)
Spacing shims too far apart (allows frame flex)
Screwing without hitting framing behind shim
Not testing window operation after shimming

Low-Expansion Foam Application

Critical: Use ONLY Low-Expansion Foam

Regular expanding foam will bow and distort window frames, causing operation issues and warranty voidance. Only use foam specifically labeled "low-expansion," "window & door," or "minimal-expansion."

Rule: Fill gaps 50-75% full, foam will expand slightly to fill remaining space

Foam Application Procedure

1.Wait to foam: After window secured, shimmed, and screwed in place
2.Clean gaps: Remove debris, dust, and loose material
3.Dampen surfaces: Lightly mist with water for better adhesion/cure
4.Apply foam: Fill 50-75% of gap depth in continuous bead
5.Work in sections: Do one side at a time, allow to expand
6.Monitor expansion: Check in 15-30 minutes, trim excess

Foam Types & Properties

Low-expansion foam: Expands 30-50%, won't distort frames
Latex foam: Water-based, less expansion, easier cleanup
Cure time: 1-2 hours to tack-free, 24 hours full cure
R-value: R-3.5 to R-4.5 per inch (good insulation)
Air sealing: Excellent air barrier when properly applied

Foam Application Tips

• Warm can to 70-80°F for best flow and expansion (run under warm water)
• Shake can vigorously for 30-60 seconds before use
• Apply in multiple thin layers rather than one thick bead for large gaps
• Use backer rod for gaps over 1 inch before foaming
• Trim cured foam flush with utility knife or oscillating tool
• Protect foam from UV light with interior trim or exterior caulk
• Wear gloves - uncured foam is difficult to remove from skin

Step-by-Step Installation Process

Verify Rough Opening & Prepare

•Measure rough opening: should be window size + 1/2 inch
•Check RO is level, plumb, and square (within 1/8 inch)
•Inspect framing: headers, jack studs, and sill must be sound
•Remove any protruding nails or debris from opening
•Install back dam on sill (1/2 inch foam rod or wood)
•Dry-fit window to verify fit before proceeding

Install Sill Pan Flashing

•Apply flexible flashing membrane to sill extending 6 inches up sides
•Seal membrane to back dam at rear of sill
•Fold corners up at 45 degrees to create end dams
•Ensure no wrinkles or bubbles in membrane
•Roll with J-roller for good adhesion
•Optional: Add metal drip edge at front of sill for extra protection

Install Side Jamb Flashing

•Apply flashing tape to jack studs from sill to 1 inch above header
•Overlap sill pan flashing by 2-4 inches at bottom
•Extend past top of opening for head flashing overlap
•Press firmly and roll edges for good seal
•Cut X-slits for any protrusions, seal around them
•Do not install head flashing yet (comes later)

Set Window in Opening

•Have helper assist with larger windows
•Tip window into opening from exterior
•Center window in opening with equal gaps on sides
•Place temporary shims under bottom corners for support
•Push window flange tight to exterior wall sheathing
•Secure top corners with one screw each (temporary hold)

Level & Shim Bottom

•Place level on sill, check for level side-to-side
•Adjust shims under corners until perfectly level (within 1/16 inch)
•Add shim pairs every 12 inches along bottom
•Verify level doesn't change when pressing on sill
•Don't over-shim (just snug, shouldn't lift window)
•Screw through sill flanges into framing at shim locations

Plumb, Square & Shim Sides

•Check side jambs for plumb with 6-foot level
•Measure diagonals corner-to-corner (must be equal within 1/8 inch)
•Place shims behind hinge locations on casement/awning windows
•Place shims behind lock/latch points for smooth operation
•Add shims every 12 inches along jambs for tall windows
•Screw through jamb flanges into jack studs at each shim point
•Recheck operation: open and close window, should move smoothly

Secure Window & Install Head Flashing

•Fasten remaining flange screws per manufacturer (typically 8-12 inches)
•Don't over-tighten screws (flange should lie flat but not dimple)
•Install head flashing over top of window
•Overlap side flashing by 2-3 inches on each side
•Extend head flashing past corners for proper coverage
•Integrate head flashing with building wrap/housewrap above

Insulate Gaps with Low-Expansion Foam

•Wait until window fully secured and shimmed
•Lightly dampen gaps with water spray
•Apply low-expansion foam filling gaps 50-75% full
•Work from bottom to top to prevent drips
•Foam between shims, not on top of shims
•Allow 1-2 hours to cure before trimming excess
•Test window operation after foam cures

Exterior Finishing & Caulking

•Apply siding or trim to exterior per wall system
•Leave 1/4 inch gap between siding and window flange
•Apply quality exterior caulk between trim and window
•Do not caulk bottom flange (allows drainage)
•Caulk top and sides of flange for water seal
•Tool caulk smooth for clean appearance

Interior Trim & Final Touches

•Trim excess foam flush with framing using utility knife
•Install interior window trim (casing and stool)
•Maintain 1/8 to 1/4 inch reveal on jamb edges
•Miter corners at 45 degrees for clean appearance
•Nail trim to jack studs and header, not to window frame
•Fill nail holes with putty, caulk gap between trim and wall
•Remove any protective film or labels from window
•Final operation test: open, close, lock window multiple times

Best Practices & Common Mistakes

DO:

Follow proper flashing sequence: sill first, sides second, head last
Verify rough opening is correct size before ordering windows
Check level, plumb, and square multiple times during installation
Shim at all critical locations: hinges, locks, corners, every 12 inches
Use only low-expansion foam specifically for windows and doors
Test window operation before and after insulating
Use quality flashing tape, not cheap housewrap tape
Create proper drainage path with sill pan and back dam

DON'T:

Install head flashing before side flashing (causes water intrusion)
Use regular expanding foam (will bow frame and void warranty)
Over-shim or over-tighten screws (distorts frame, binds operation)
Forget back dam on sill (water runs under window into wall)
Caulk bottom flange (blocks weep holes, traps water)
Skip diagonal measurements (window may be racked/not square)
Install window without checking rough opening first
Rely on caulk alone for water protection (use proper flashing)

Required Tools & Materials

Essential Tools

• 6-foot level (for checking plumb)
• 24-48 inch level (for checking level)
• Tape measure
• Cordless drill/driver
• Hammer or trim nailer
• Utility knife
• Caulk gun
• J-roller or hand roller

Installation Materials

• Flexible flashing membrane
• Adhesive-backed flashing tape
• Composite shims (pairs)
• Low-expansion foam
• Exterior caulk (polyurethane or silicone)
• Backer rod (for large gaps)
• Screws (provided with window or #8 x 2-1/2")
• Interior trim (casing, stool, apron)

Safety Equipment

• Safety glasses
• Work gloves
• Dust mask (for cutting/trimming)
• Ladder or scaffolding
• Back brace (for heavy windows)

Optional But Helpful

• Oscillating multi-tool (trim foam)
• Miter saw (for interior trim)
• Pneumatic nailer (for trim)
• Spray bottle (for dampening)
• Window installation brackets
• Suction cups (for large windows)