How to Install a Skylight on a Flat or Low-Slope Roof
On a near-flat roof, water does not run past a skylight, it sits at it. The curb, the membrane and the cricket are what keep it dry.
- Professional
- 1 to 2 days
- Updated
The short answer
On a flat or low-slope roof, mount the skylight on a curb that stands at least 8 inches above the finished roof surface, the common minimum flashing height for membrane roofs. Turn the roofing membrane up the curb as an upstand, fasten and seal it at the top, and cap it with counterflashing. Add a small cricket behind the uphill side of the curb so water drains around it, and insulate the curb to prevent condensation.
On a pitched roof, water runs past a skylight. On a flat or low-slope roof it does not. It gathers, slows and pools, often right at the skylight, and sits there for hours or days. That changes everything about how the skylight has to be installed: the glass must be raised above the water, the roofing must be carried up around it and the water must be given somewhere to go.
That is why flat-roof skylights are curb-mounted. The curb lifts the unit, the membrane turns up the curb as an upstand and the counterflashing protects the joint. On top of that, a small cricket behind the curb directs water to either side instead of letting it dam. Every one of these details has to be right, and the membrane work is usually a job for a roofer.
This guide covers the special points for flat roofs. The general installation of the unit is in how to install a curb-mounted skylight, and the curb itself is in how to build a skylight curb.
Tools
- Reciprocating saw
- Level
- Tape measure
- Drill and driver
- Hot air welder or torch, depending on the roofing membrane
- Hand roller
- Hook blade utility knife
- Caulking gun
Materials
- Curb-mounted skylight
- Curb lumber or factory curb
- Roofing membrane and flashing compatible with the existing roof
- Insulation for the curb
- Tapered insulation or lumber and plywood for a cricket
- Termination bar, fasteners and sealant per the membrane manufacturer
- Metal counterflashing or curb cap
Safety first
- Work tied off or protected by guardrails. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury, and a flat roof still has an edge.
- Torch and hot air welding work carries fire risk, so keep fire extinguishers and a fire watch where the membrane type calls for it.
- Cutting a flat roof opening often involves joist or beam changes. Get structural advice before cutting into framing.
Step by Step
Step 1: Check the roof structure and the drainage first
A flat roof still slopes, usually a fraction of an inch per foot, toward drains or scuppers. Find the direction and keep the skylight from sitting across the main flow. See how to plan flat roof drainage.
Flat roofs are often framed with joists that carry full load across a long span. Cutting one needs proper headers and may need an engineer and a permit.
Step 2: Frame the opening and build or set the curb
Follow the framing plan and the skylight's rough opening size. Then build or set the curb as described in how to build a skylight curb, allowing for the thickness of insulation and membrane so the curb finishes at least 8 inches above the finished roof. That is the common minimum flashing height for membrane roofs, and it is taller than the 4 inch minimum many skylight makers give for pitched roofs. Use the higher of the skylight and roofing requirements.
Check the curb is square and level, and fasten it into the framing, not just the deck.
Step 3: Insulate the roof and the curb
Cut the insulation tight around the curb. Air gaps let warm moist air reach the cold curb and the membrane, causing condensation. Seal the joints with spray foam or tape.
Insulate the inside of the curb with rigid foam or mineral wool. A cold, uninsulated curb is a major cause of staining below.
Step 4: Build a cricket behind the curb
On a low-slope roof the curb acts as a dam. A cricket is a small tapered section of insulation or framing, peaked in the middle, that sends water to either side. Make it slope at least 1/4 inch per foot toward the sides, or steeper if the space allows.
Without one, water and debris pile up against the uphill side. The cricket is covered with the same membrane as the roof. See the same idea for chimneys in how to build a chimney cricket.
Tip: Tapered insulation boards are commonly used to make crickets on membrane roofs. See how to install tapered insulation.
Step 5: Turn the membrane up the curb as an upstand
The membrane needs to go up the curb to at least the height the roofing manufacturer requires, commonly at least 8 inches above the finished roof, and be fastened and sealed at the top. On a torch-on or modified bitumen roof, this means a base and cap flashing layer, detailed in torch-on parapet flashing and related guides. On EPDM or TPO, the membrane is adhered or welded up the curb, see how to flash a roof curb.
Corners are the weak point. Use pre-formed corners or follow the manufacturer's fold and weld details. Never leave a fish-mouth or an unsealed corner.
Step 6: Set the skylight on the curb
Follow the manufacturer's instructions for the gasket or sealant, positioning and fasteners. Fasteners must not pierce the membrane upstand below the level of the counterflashing or cap.
Set the unit evenly on the curb with no twist, and fasten without overtightening.
Step 7: Add counterflashing over the membrane upstand
The upstand must not be exposed at the top edge. Install metal counterflashing or a curb cap lapped over the membrane, fastened above the membrane termination. Use sealant only as a supplement at the top edge, not as the main defence.
Check that the membrane is held down with a termination bar or per the manufacturer at the top, and that there are no openings at the corners.
Step 8: Test the roof and check the ceiling for condensation
Run water around the skylight for several minutes per side while someone checks the ceiling below. If you can, fill the area behind the cricket to check it drains freely.
Condensation is the most common false alarm on flat roof skylights. See how to fix a leaking skylight for telling it apart from a real leak.
Why Flat Roof Skylights Leak
- Low curb: ponding water and splash reach the seal and the top of the membrane flashing.
- No cricket: water and debris dam against the uphill side and find any weak point.
- Bad corners: membrane corners that are not formed or welded properly.
- Exposed upstand top: missing counterflashing or termination lets water behind the membrane.
- Condensation: a cold uninsulated curb and shaft produce drips that look like leaks.
Snow and Drainage in the Fraser Valley
Wet snow and rain both load a flat roof, and melt water has to get off it. Keep drains and scuppers clear and make sure the curb does not block the flow. See how to install a roof drain.
Choosing a Unit for a Flat Roof
Flat glass on a flat roof does not shed water or dirt. Rain dries in place, leaves a film, and needles and moss collect on the glass. Flat-roof skylights solve this in one of three ways: a domed or pyramid top that sheds water, a flat-glass unit made for flat roofs with the pitch built into its frame, or a curb built with a slope so a standard unit sits at an angle. Check which approach the product uses and the minimum pitch it needs.
Venting units on a flat roof need the same care as any opening: the hinge and seals sit closer to standing water and snow than they would on a pitched roof. Where the aim is only daylight in a hallway or bathroom, a tubular skylight with a flat-roof flashing kit puts a much smaller hole in the membrane.
Working Safely Around the Opening
An open skylight hole, and many installed skylights, will not hold a person's weight. Cover or guard the opening while you work, never step or kneel on skylight glazing, and treat the opening like a roof edge when planning fall protection. WorkSafeBC requires fall protection where a fall could be 3 metres (about 10 feet) or more, and a fall through a roof opening onto a floor below usually qualifies.
Common Mistakes
- Using a deck-mounted unit on a flat roof. The kit was not designed for ponding water.
- Setting the curb too low. Insulation and membrane eat the height and the seal ends up near the water.
- No cricket behind the curb. Water dams and debris collects there.
- Unformed or unsealed membrane corners. They are the leading leak point on any curb.
- Exposed membrane top edge. It lets water behind the upstand.
- An uninsulated curb. Condensation stains the ceiling and is mistaken for a leak.
- Flat glass with no slope. Water and grime sit on the glass, and the seals spend the winter wet.
When to Call a Professional
Flat roof skylights depend on membrane work, torch or welding skills and framing details, and a mistake is expensive to reverse. Hire a roofer who works with the roof's membrane type, and get structural advice if joists are cut.
If an existing flat-roof skylight leaks repeatedly, the fix is usually at the curb corners or the cricket, and is best diagnosed by someone who can see the membrane.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Skylights service or the towns we cover.
How to Install a Skylight on a Flat Roof: Questions
Can you put a skylight on a flat roof?
Yes, using a curb-mounted unit. The curb lifts the skylight above the roof surface, commonly at least 8 inches on a membrane roof, and the roofing membrane turns up the curb as an upstand. Flat roofs also need a cricket behind the curb to divert water. A deck-mounted unit is not designed for near-flat roofs.
Why do flat roof skylights leak?
Usually because water ponds against them. Low curbs, missing crickets, poorly formed membrane corners and exposed upstand edges all let water in. Some leaks are actually condensation on a cold, uninsulated curb or shaft. Checking whether the water appears only on cold mornings helps identify which it is.
What is a cricket behind a skylight curb?
A cricket is a small peaked or sloped area built behind the uphill side of the curb that splits water flow to either side. Without one, water and debris pile up against the curb, which acts as a dam on a low-slope roof. It is usually made from tapered insulation or framing covered with the roofing membrane.
How high should a flat roof skylight curb be?
On a flat membrane roof, commonly at least 8 inches above the finished roof surface, because that is the usual minimum height for membrane flashing at curbs and walls. Skylight makers often list 4 inches, but that figure is aimed at pitched roofs, so use the higher of the skylight and roofing requirements. Measure from the finished surface after insulation and membrane, and go taller where snow drifts or water ponds.
Related Guides
All Skylights guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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