A skylight brings natural light into rooms with limited windows — bathrooms, kitchens, hallways, vaulted living spaces. Done well, it's a transformative feature. Done poorly, it's a leak point. The work involves cutting through the roof and ceiling, installing the skylight unit, and waterproofing — including specific flashing techniques that prevent water entry around the skylight perimeter.

This guide is an overview of skylight installation. Permit and licensed roofer recommended; not first-time DIY work.

Before you start

Three things to settle:

1. Fixed vs venting skylight. Fixed: cheaper, simpler, no operating mechanism. Venting (manual or motorized): can open for ventilation. Skylight + ventilation = bathroom or kitchen. Plan based on use.

2. Cutting through rafters or trusses. Most skylights are wider than rafter spacing (16 inch OC). Cutting one or more rafters requires headers — perpendicular framing pieces between intact rafters that support the cut rafter ends. Engineer or BCIN designer for new openings.

3. Permit required for new skylight. Cutting new skylight: building permit. Like-for-like replacement: typically no permit. Verify with municipality.

What you'll need

Tools

Reciprocating saw
Drill with bits
Tape measure
Roofing safety harness
Caulk gun
Pry bar

Materials

Skylight unit (matched to opening size)
Skylight flashing kit (specific to skylight model)
2× lumber for headers (sized per engineer)
Joist hangers (matching rafter dimensions)
Lag bolts and structural screws
Underlayment patch
Roofing cement
Drywall (for ceiling patch around skylight)
Shingles (matching existing roof)

Use the manufacturer's flashing kit

Generic flashing won't fit skylight properly.

Manufacturer's flashing kit: includes step flashing, head flashing, sill flashing — designed for that exact skylight.

Mandatory for warranty and weather-tightness.

Step-by-step instructions

Plan opening location and size

Identify roof location (away from chimneys, vents, valleys).

Match opening to skylight rough opening dimensions.

Verify rafter and ceiling joist locations.

Plan headers needed if cutting through rafters.

Mark from inside (attic or ceiling)

From inside, mark the planned opening on the ceiling.

Drill through corners of marked area to mark roof from below.

From outside, connect drill marks to define opening on roof.

Strip shingles around opening

Remove shingles 2-3 ft beyond planned opening on all sides.

Pull nails; expose sheathing.

Save undamaged shingles for re-use.

Cut sheathing and rafters

Cut sheathing along marked opening lines.

Cut rafters that cross the opening at opening edges.

Reciprocating saw with appropriate blade.

Support cut rafter sections during work.

Install header framing

Two headers: one above and one below the opening, perpendicular to rafters.

Joist hangers attach headers to intact rafters.

Cut rafter ends attach to headers via joist hangers.

Sized per engineer (typical 2× lumber matching rafter depth).

Install skylight unit

Lift skylight unit into opening.

Position with skylight's mounting flange against rough opening edges.

Drive screws through flange into framing.

Verify skylight is level and properly oriented (per manufacturer).

Install underlayment patches

Underlayment around skylight, replacing what was cut.

Tucks under existing surrounding underlayment.

Sealed around skylight.

Install manufacturer's flashing kit

Step flashing on sides (one piece per shingle course, alternating with shingles).

Apron flashing at downhill side.

Saddle/head flashing at uphill side.

Per skylight kit's instructions — every brand has slight variations.

Re-shingle around skylight

Continue shingling pattern around skylight.

Each shingle course interleaves with step flashing on sides.

Cut shingles to fit around skylight.

Apply roofing cement at exposed edges.

Finish ceiling around skylight

Ceiling drywall around opening (cut to fit).

Trim around skylight (matching room style).

Paint to match ceiling.

Skylight light shaft (if vaulted ceiling): drywall, tape, paint.

Test for leaks

Wait for rain or hose-test the skylight.

Check from inside for any drips.

Address any flashing issues immediately.

Common mistakes that cost you

Generic flashing instead of skylight kit. Doesn't fit, leaks. Always manufacturer's kit.
Cutting rafters without headers. Roof structure compromised. Headers mandatory.
Improper shingle pattern around skylight. Water entry. Each course interleaves with step flashing.
Skipping underlayment. Water enters between sheathing and shingles. Always underlayment.
Working without fall protection. Roof work requires harness. Don't skip.

Frequently asked questions

Fixed or venting skylight?

Fixed: the most economical option. For passive lighting only.

Manual venting: crank to open; adds moderate cost over fixed.

Motorized: remote-operated; the premium option, with rain sensors available.

Bathrooms and kitchens: venting recommended for moisture control.

Do skylights leak?

Properly installed: rarely. Modern skylight + manufacturer's flashing kit + careful install: 25+ year leak-free typical.

Improperly installed: leaks common.

Old skylights: seals degrade after 20-30 years; replacement may be needed.

Can I install a skylight DIY?

Possible but not recommended for first-time roofing/structural work.

Manufacturer's instructions are detailed but assume some experience.

A professional install costs more than DIY materials alone, but covers structural framing, roofing, and flashing — and protects the warranty.

First-time: hire pro.

When to call us instead

Skylight installation is part of attic conversions and modern home renovations. Our team handles structural, roofing, and finish work.

Book a free 60-minute site visit →

Related Ottawa services

If you're considering skylights for a renovation, our team handles design and install. Book a free 60-minute consultation.