Skylight Flashing Calculator

About the Skylight Flashing Calculator

Skylights are one of the most common sources of roof leaks when improperly flashed. Proper skylight flashing involves multiple components: sill (apron) flashing at the bottom, step flashing up both sides, head (saddle) flashing at the top, and often a cricket behind the skylight to divert water.

Most skylight manufacturers offer integrated flashing kits sized to the skylight model, but custom flashing or additional materials may be needed for unusual installations or steep pitches. This calculator estimates the flashing material for each zone around the skylight.

Proper skylight flashing also requires ice and water shield membrane extending at least 6 inches beyond the flashing in all directions. This provides a secondary seal layer if the flashing ever develops a gap.

When This Page Helps

Skylight leaks are the #1 complaint among homeowners with roof windows. This calculator ensures all flashing zones (sill, step, head, and ice shield) are properly sized, reducing the risk of leaks.

How to Use the Inputs

1. Enter the skylight rough opening width and height.
2. Enter the roof pitch (rise per 12 inches of run).
3. Review the step flashing, head flashing, sill flashing, and ice shield requirements.
4. Verify if a cricket (saddle) is needed at the top (required for skylights >30" wide on shingle roofs).
5. Order the manufacturer's flashing kit plus any additional materials.

Example Calculation

Tips & Best Practices

• Always use the skylight manufacturer's approved flashing kit for warranty coverage.
• Install ice and water shield under all skylight flashing extending 6" beyond each edge.
• A cricket behind the skylight prevents water and debris from pooling above the unit.
• On low-slope roofs (<4:12), use a curb-mount skylight with raised flashing for better drainage.
• Do not rely on caulk or sealant as a substitute for proper metal flashing.
• Flash the sill FIRST, then the sides (step flashing), then the head last — always work from bottom to top.

Deck-Mount vs. Curb-Mount Skylights

Deck-mount skylights sit directly on the roof deck with integrated low-profile flashing. They work best on slopes of 4:12 and above. Curb-mount skylights sit on a raised wooden curb (frame) and work on any slope, including nearly flat roofs. The curb raises the skylight above the water plane.

Flashing Sequence Detail

The critical principle is "layer like fish scales" — every piece must overlap the piece below it so water always runs over, never under. Start at the bottom with sill flashing, work up the sides with step flashing woven into each shingle course, and finish at the top with head flashing that covers the final step flashing pieces.

Condensation vs. Leaks

Not all moisture around skylights is from leaks. In cold weather, warm moist interior air can condense on the cool skylight frame and glass, dripping water that appears to be a leak. Proper interior ventilation, double- or triple-pane glass, and insulated frames minimize condensation.

Frequently Asked Questions

• Do I need a flashing kit or custom flashing?

  Use the manufacturer's flashing kit whenever possible — it's engineered for the specific skylight model and ensures warranty coverage. Custom flashing is needed for unusual sizes, very steep or low slopes, or non-standard roofing materials.

• What is a skylight cricket?

  A skylight cricket (or saddle) is a small peaked structure behind the skylight that diverts water around it. Crickets are required for skylights wider than 30 inches on shingle roofs and recommended for all skylights to prevent water pooling.

• When do skylights leak?

  Skylight leaks are usually caused by: worn/cracked flashing, missing ice and water shield, improper overlap sequence (head flashing under instead of over step flashing), or condensation (not an actual leak). Proper installation prevents all but condensation issues.

• How often should skylight flashing be replaced?

  When re-roofing, always replace the skylight flashing. Leading manufacturers recommend new flashing with every roof replacement. Between roof replacements, inspect the flashing annually for gaps, corrosion, or sealant failure.

• Can I install a skylight on a low-slope roof?

  Yes, but use a curb-mount skylight (not deck-mount) for slopes below 4:12. The raised curb provides better drainage and allows for taller flashing that prevents water intrusion at the low slope.

• What is the right sequence for skylight flashing?

  Always flash from bottom to top: 1) Sill (apron) flashing first, 2) Step flashing up both sides (woven with shingles), 3) Head flashing at the top (overlapping the top step flashing). This ensures every layer sheds water over the layer below it.