Skylight Installation + Replacement
Why this matters
Skylights are the highest-leverage daylighting upgrade - natural light improves perceived home value + customer enjoyment. Single skylight install. Replacement (no opening change). The risk: a leaking skylight is the #1 cause of roof + ceiling damage in homes that have one. Done correctly, modern skylights with proper flashing leak-free for 25 - 35 years; done poorly, they leak within 2 years.
Skylight types
Fixed skylight (non-opening)
- Most common; lowest price + simplest
- Tempered glass OR acrylic (glass is premium + clearer)
- Single-pane OR double-pane (double for thermal performance)
- Used purely for daylight
Vented skylight
- Crank-out OR motorized opening
- Adds ventilation; reduces attic moisture in well-managed installs
- Higher cost
- Premium: motorized + rain-sensor closing (Velux SmartShade)
Tubular daylight device (TDD / sun tunnel)
- Smaller diameter; uses tube + reflective walls to channel light
- Used where a full skylight won't fit (between rafters, irregular ceiling)
- Velux Sun Tunnel, Solatube major brands
- 10" - 21" tube diameter typical residential
Solar tube
- Synonym for TDD
Walk-on skylight + flat skylight
- Flat-roof skylight installations
- Specialty work; not the standard pitch-roof install
Skylight brands + product positioning
- Velux - dominant US brand; well-engineered flashing kits; expensive
- Fakro - Polish brand, growing US market; competitive pricing
- Marvin - premium residential; less common
- Pella - branded similar to Velux
- Solatube - solar tube market leader
- Sun-Tek - budget; quality varies by product line
For most residential, Velux is the safe choice. Premium installers stock Velux.
Pre-install assessment
Existing skylight (replacement)
- Identify make + model
- Confirm replacement-size availability
- Inspect roof + ceiling for damage (often the reason for replacement)
- Determine: full replacement OR re-flashing only?
New install
- Locate ceiling / room placement (which room benefits)
- Identify rafter / truss layout
- Verify roof structural ability to span between rafters (most fit between 24" OC rafters; engineered for wider)
- Check attic for obstructions (ducts, plumbing, electrical, structural members)
- Customer-side: light pollution, privacy, heat gain considerations
Sizing
- Rule of thumb: skylight area = 5 - 15% of floor area for good daylighting
- Larger skylight = more light + more heat gain + more potential leak surface
- Common sizes: 22.5×46.5", 30.5×46.5", 22.5×30.5" (mounting between standard 24"OC rafters with curb)
Roof slope compatibility
- Most pitch-roof skylights: 14° to 60° pitch (3:12 to 12:12)
- Low-slope OR flat: specific products
- Check manufacturer spec for slope range
Pre-install structural concerns
- Cutting a rafter (typical 22.5" wide skylight needs 1 rafter cut)
- Header above + below cut rafter
- Headers connect to adjacent uncut rafters
- Typically 2x lumber × the rafter dimension (2x6 header on 2x6 rafters)
- Heavy / wide skylight: engineering analysis required
Installation procedure (pitch-roof new install)
- Measure + mark: exact roof position; interior + exterior cuts; size matches kit
- Open the roof: remove drywall + cut/remove rafter section; frame headers; cut roof opening exactly per spec
- Install curb (curb-mount): 2x4 OR 2x6 lumber 4 - 6" above roof line; sheet metal wrap
- Install flashing kit (the critical step): underflashing first; step flashing alternating with shingle courses; top + bottom counterflashing; watershed cricket above wider/lower-slope skylights. Every shingle course laps over the flashing's down-slope edge.
- Install skylight: place + fasten per manufacturer; level + plumb
- Interior finish: drywall to skylight frame; tape + mud + paint; window-stop trim
- Test + customer: spray-hose water test; customer walk + photo documentation
Replacement skylight (existing opening)
If existing flashing condition is good + opening is correct:
- Remove old skylight (4 - 8 screws + caulk OR adhesive)
- Inspect curb + flashing
- Repair OR replace flashing as needed
- Install new skylight per the curb / opening
- Re-seal at perimeter
- Test for leaks
If old skylight has been leaking for years, the deck around the opening is likely rotted - full repair required + skylight installs over good substrate.
Common installation mistakes
Wrong opening size
- Oversized: skylight won't seal; shimming required
- Undersized: skylight won't fit; rework required
Skipping watershed cricket on wide skylight low slope
- Water dams above; eventually backs up + leaks
- Cricket diverts water around skylight
Mismatched flashing
- Aftermarket "universal" flashing rarely seals as well as factory-matched
- Manufacturer flashing kits are engineered for the specific skylight
Sealant on the wrong surfaces
- Some interfaces are designed to be DRY (let water shed)
- Sealing those = trapping water against the seal = future leak
- Follow manufacturer detail
No interior vapor barrier
- Skylight area: warm interior air meets cold glass = condensation
- Without vapor barrier at the interior shaft, condensation drips into the room
Skipping insulation in the shaft
- Heat loss from open shaft is significant
- Insulate the shaft walls + above shaft
NEVER install a skylight without a manufacturer-supplied OR equivalent flashing kit. Hand-built flashings around skylights are the #1 leak failure mode. Factory-matched flashing kits are engineered for the specific skylight + the typical roof pitch + shingle / membrane interface. Cost. Time saved: 2 - 4 hours. Leaks prevented: most of them. Always use the kit; never improvise around a skylight.
Energy + performance
- Double-pane low-e tempered glass: best thermal performance + reduces UV fade
- Single-pane glass: minimal performance; budget option
- ENERGY STAR rated: meets energy code in cold zones
- Cool roof glazing: reduces summer heat gain
References
- IRC R808 (skylights)
- Manufacturer install guides (Velux, Fakro, Marvin, Pella)
- NFRC (National Fenestration Rating Council) ratings
- Energy Star Skylight ratings
- Manuall internal: Metal Roofing Installation