Skylight curb installation and flashing for low-slope and steep roofs

Skylights, and why the seam leaks before the glass

A skylight is a glazed opening in the roof that brings daylight into the space below, set either on a raised curb or directly on the roof deck. It is also the most reliable leak on a building, and the leak is almost never the glass. Water gets in at the flashing and the curb, where the roof had to be cut open and made watertight again by hand.

That is the one thing to carry into every skylight job. The pane sheds water fine. The seam around it is the question. A skylight is a penetration with a window in it, and like every penetration the field of the roof rarely fails while the detail around the hole does. Treat the glazing as the easy part and the flashing as the work.

The trade pattern is consistent. Call out a leaking skylight and most of the time you find a flashing that was short, a curb that was too low, a cricket that was never built, or a bead of sealant standing in for a flashing that should have been there. Now and then you find condensation that was never a leak at all. The skill is telling those apart before you pull anything off the roof.

Why do skylights leak?

Skylights leak because water finds the seam where the unit meets the roof, not because the glazing fails. The flashing and the curb are the leak path on the large majority of callbacks, the same way every other roof penetration leaks at its detail and not in the open field. Cut a hole in a roof and the edges of that hole are what you have to defend.

Rank the causes and you chase the right one first. A curb that sits too low, or no curb at all on a flat roof, lets water back up over the flashing. Flashing that was never tied into the roof membrane, so the lap runs the wrong way or stops short. No cricket on the uphill side of a wide unit, so water dams against the curb and tests the flashing all day. A skylight set on sealant alone with no real flashing under it. And condensation, which is not a leak but gets reported as one constantly.

The water test sorts it out, but the odds are set before you climb up. The detail somebody rushed at the end of a shift is where the building is wet.

The skylight types: curb-mounted, deck-mounted, TDD, and smoke vent

Skylights split into a few families, and the family decides how you flash it. A curb-mounted skylight sits on a raised curb and is the standard for low-slope and flat roofs. A deck-mounted, or self-flashing, skylight mounts low to the deck with an integral flashing flange and is the common residential unit on a shingled slope. The two are flashed by completely different methods, so step one is knowing which one is on the order.

Past those two sit the variants. A unit skylight is the factory-made box you buy and set. Structural or architectural glazing is an engineered glass system, a ridge, a pyramid, or a barrel vault, sized and detailed by the manufacturer and the design team rather than pulled off a shelf. A tubular daylighting device, the TDD or solar tube, is a small dome on the roof feeding a reflective tube down to a ceiling diffuser, flashed at the roof with the manufacturer's leak-proof kit like a small curb or boot. A smoke vent or roof hatch looks like a skylight but is a code fire-venting or access device with its own listing.

Each one is still a hole in the roof. The glazing and the operation change. The flashing logic does not.

Curb-mounted skylights on a low-slope roof

On a low-slope or flat roof the curb-mounted skylight is the detail that holds up, because it gets the glazing up out of the water. The curb is a raised frame, either site-built out of framing lumber or a manufactured insulated curb, that the skylight sits on top of. The roof membrane flashes up the outside of the curb as base flashing, the same as it does at any wall or equipment curb, and the skylight's own frame caps the top of that flashing.

The whole point of the curb is height. A flat roof ponds, drifts snow, and backs water up against anything standing on it, so the glazing has to sit well above the surface where that water sits. A skylight set flat to a low-slope deck, with the flashing turning up only an inch or two, is a leak waiting for the first hard rain or the first snowmelt that has nowhere to go.

This is the dividing line from the residential deck-mounted unit. On a steep shingled roof the water moves and a low-profile flashed unit works. On a low-slope roof the water sits, and the curb is what keeps the seam above it. Do not put a deck-mount detail on a flat roof and expect it to hold.

How high should a skylight curb be?

A skylight curb on a low-slope or flat roof should stand high enough to give the base flashing at least 8 in of turn-up above the finished roof, the same minimum the trade holds for base flashing at any wall or curb. That 8 in is about water depth, not looks. Water and snowmelt back up against the curb, and the flashing has to stay above whatever that backup reaches.

The code minimum and the practical minimum are not the same number, and that trips people. The building code for unit skylights, in the provisions commonly cited around R308.6.8, calls for a curb at least 4 in above the roof plane on slopes below 3 in 12 unless the manufacturer's instructions say otherwise. That 4 in is a floor, not a target. On a real low-slope membrane roof the common practice and most manufacturer details push the curb to 8 in or more so the base flashing gets its full height, and in heavy-snow country it climbs past that.

Confirm the number against the skylight manufacturer's installation instructions and the membrane manufacturer's detail, because those govern the warranted assembly and the section numbers shift between code editions. When the curb is too short, the margin the flashing was supposed to give you is simply gone.

Building the curb: framing, headers, and the load

The curb carries the skylight and the loads on it, so it is framing before it is flashing. A site-built curb is framed up from the roof opening with dimensional lumber, commonly 2x material doubled or sized to the span, sitting on or tied into the opening framing below. A manufactured curb arrives as a welded or molded insulated box you set over the opening and fasten down. Either way it has to be square, plumb, and fastened to the structure, because a racked curb will not let the glazing seat and seal.

The opening framing is structural. Cutting a hole in a roof deck interrupts the joists or rafters, so the opening gets headers at the top and bottom and trimmers at the sides to carry the loads around the hole, and on anything beyond a small unit that framing is the engineer's call, not a field guess. The curb then sits on that framed opening.

Insulate the curb. A bare framed curb is a thermal hole in the roof and a cold surface that sweats, so the curb gets insulated to match the roof assembly, and a manufactured curb usually comes insulated for that reason. The load matters too. Snow, wind uplift on the glazing, and anyone who steps on or near it all land on the curb and the framing, so size both for the real loads, not just the weight of the unit.

Flashing the curb on a membrane roof: base flashing and counterflashing

On a membrane roof the skylight is flashed exactly like any other curb, and the companion guide on roof penetration flashing carries the base-flashing and counterflashing logic in full. The roof membrane turns up the outside face of the curb as base flashing, runs up at least 8 in, and is fastened near the top with a termination bar so it cannot sag or peel down the curb. On a thermoplastic roof the curb flashing is the same chemistry as the field sheet and welds into one material, which is why the welded curb is the reliable version.

The counterflashing covers the top edge of that base flashing so water cannot get behind it. On most manufactured skylights the unit's own metal frame or a cap flange is the counterflashing, lapping down over the top of the membrane and shedding water onto the face of the flashing rather than into the seam. That overlap is the whole detail. The membrane carries water up the curb. The cap keeps it from getting in over the top.

The corners are where curb flashings leak, the same four corners that leak on every curb. Use the manufacturer's prefabricated molded corners where the system offers them, and treat any field-fabricated corner as the spot to inspect twice. A skylight curb has the same corner problem as a rooftop unit curb, and it gets the same answer.

Head, sill, and step flashing on a steep shingled roof

On a steep shingled roof a deck-mounted skylight is flashed with a stepped kit, and the sequence is what makes it watertight, not the sealant. Water runs down the roof, so every piece of flashing has to overlap the one below it like shingles do, with the upslope piece on top. Get the laps backwards and you have built a funnel.

The kit has four working parts. A sill or apron flashing at the bottom, downslope edge sheds water off the unit onto the shingles below. Step flashing up each side, woven in shingle by shingle, so each step laps the course beneath it and tucks under the course above. A head flashing at the top, upslope edge, with a saddle that carries water from above out and around the unit. And the unit's own cladding or cap that laps over all of it.

The step flashing is where the failures hide. Each step has to interleave with a shingle course, and a roofer who runs continuous flashing up the side instead of stepping it, or who caulks over a missed step, has built a leak that shows up a few seasons later. A wide unit on a low-pitch shingled roof also wants a cricket at the head, the same as a curb, to split the water before it reaches the top flashing.

Do you need a cricket behind a skylight?

You need a cricket on the upslope side of any wide skylight or curb, because a wide unit dams the water running toward it and that standing water tests the flashing constantly. A cricket, also called a saddle, is a small built-up ridge on the high side of the unit that splits the flow and steers it around both ends instead of letting it pond against the curb.

The common trigger is width across the slope. Code and trade practice commonly call for a cricket or saddle on the upslope side of any chimney or roof penetration wider than about 30 in measured across the slope, and a skylight curb is a penetration. The wider the unit and the flatter the roof, the more it matters. A narrow skylight on a good slope may shed fine on its own. A wide one on a low slope will pond on its uphill side until the flashing gives up.

Build the cricket to actually slope. On a membrane roof it is built from tapered insulation and flashed as part of the curb, and the drainage thinking is the same as for any curb or wall in the flow path. A cricket the membrane bridges instead of following, or one that does not really pitch, is decoration. The companion guide on rooftop supports and walkways covers the curb-and-cricket relationship from the equipment side.

Is my skylight leaking or sweating?

Before you tear off flashing, rule out condensation, because a sweating skylight gets reported as a leak constantly and the fix is the opposite of a flashing repair. Condensation forms when warm, humid indoor air hits the cold underside of the glazing or the frame and the moisture drops out as water, then runs down and drips off the frame or stains the lightwell. It shows up in cold weather, in humid rooms like kitchens and baths, and it has nothing to do with rain.

The tells separate it from a leak. Condensation appears on cold, clear nights and humid days, not with the storms. It tracks the indoor humidity, and it runs from the frame and the glass edges rather than from one point in the flashing. A real leak follows the rain and usually shows at a corner or a head where the flashing failed.

A skylight is built to handle some condensation by design. The frame has a thermal break that keeps the interior frame from going cold, and a system of condensation channels, small gutters in the frame, that catch the moisture and weep it to the outside through weep holes onto the roof. When those weep channels clog with debris, the gutters overflow and you get the stain that looks exactly like a leak. Clear the weeps before you condemn the flashing. And if the unit has no thermal break, an old acrylic dome being the classic case, expect it to sweat and manage the room humidity, because no flashing repair will stop it.

The glazing: glass, acrylic domes, and safety

The glazing is the part that almost never leaks, but it is the part that decides impact resistance, energy, and life safety. Two broad choices: a glass unit, usually an insulated glass unit with two panes and a sealed gas space, or a plastic dome in acrylic or polycarbonate. Glass holds its clarity and stays flatter for energy performance. A polycarbonate dome takes hail and impact that would break glass, which is why domes are common on industrial and high-hail roofs, though uncoated polycarbonate yellows under UV over the years. Acrylic is cheaper and more UV-stable, holding its clarity, but it is more brittle and prone to cracking under impact.

Overhead glazing is a safety-glazing question because anything overhead can fall on people when it breaks. The building code for sloped glazing and skylights, in the chapter commonly cited as IBC Section 2405, limits the glazing to laminated glass with a code-minimum interlayer, heat-strengthened or fully tempered glass, wired glass, or approved light-transmitting plastics, and it commonly requires a protective screen below heat-strengthened or tempered glass unless the unit uses laminated glass or plastic that stays together when it breaks. Confirm the requirement against the adopted code edition. The controlling concern is glass falling out of the opening after it breaks.

Energy is the other axis. The U-factor measures heat loss and the SHGC measures solar heat gain, and a skylight on a roof sees far more sun than a wall window, so a low SHGC matters on a cooling-driven building. Match the glazing to the climate and the energy code, and remember that an uncoated polycarbonate dome yellows over the years, and any dome sweats without a thermal break.

The skylight as a fall hazard

A skylight is a hole in the roof until something makes it not one, and people die going through them. A worker steps or falls onto a skylight, the glazing or an old acrylic dome gives way, and they drop to the floor below. OSHA treats an unprotected skylight as a hole, and the protection is required, not optional.

The numbers come from the OSHA fall-protection rules. In general industry, 1910.28 requires protection from falling through any hole, skylights included, at 4 ft or more above the lower level. In construction, 1926.501 sets the trigger at 6 ft. The accepted methods are a cover, a guardrail or screen around or over the opening, or a personal fall-arrest system. A skylight screen or cage that sits over the unit is the common permanent fix on a commercial roof, and a cover used as protection has to support at least twice the intended load and be secured so it cannot shift.

Do not trust the glazing to stop a fall. A skylight dome or pane is glazing, not a load-rated cover, and an aged plastic dome is brittle. Even units marketed as fall-rated have to meet a specific test load to count, so confirm the rating before you treat the skylight itself as the protection. On any roof people will service, the screen, the rail, or the rated cover is part of the install, not an afterthought.

Smoke vents and roof hatches

A smoke vent and a roof hatch look like skylights and flash like curbs, but they do a different job and carry their own listings. A smoke and heat vent is a code fire-safety device that opens automatically in a fire to let smoke and heat out, commonly held shut by a fusible link that melts and lets a spring throw the cover open. A roof hatch is an access opening for getting onto the roof. Both sit on a curb and get flashed into the roof the same way a skylight curb does.

The listings are what set these apart. Automatically operated smoke and heat vents are tested to UL 793, and their design and placement follow NFPA 204 for smoke and heat venting, with requirements that the vent open reliably without electrical power, reach full open within a set time under fire heat, and operate from a fusible link rated to release at a controlled temperature. Those are life-safety functions, so the device, its links, and its layout are engineered, not chosen in the field.

Flash the curb the same as any skylight, but never modify the operating mechanism or block the throw of the cover. A smoke vent that has been painted shut, fastened down, or buried by a re-roof that raised the surface around it is a code violation and a dead device when the fire comes. On a re-roof, confirm the vent still operates and the curb height still clears the new roof surface.

The install sequence, in order

The order is what makes a skylight watertight, and skipping or reversing a step is how the leak gets built in. The sequence below is the membrane-roof, curb-mounted version. The steep-roof deck-mount kit follows the same shingle-lap logic with its stepped flashing instead of a curb.

• Frame the roof opening with headers and trimmers sized to carry the loads around the hole.
• Set and fasten the curb square and plumb to the structure, insulated to match the roof.
• Run the roof membrane up the outside of the curb as base flashing, at least 8 in, and terminate it near the top with a term bar.
• Build the cricket on the upslope side of a wide curb from tapered insulation and flash it in.
• Set prefabricated molded corners, and probe or inspect every corner.
• Set the skylight on the curb on the manufacturer's gasket or sealant bed, seating it evenly.
• Fasten the unit to the curb on the manufacturer's pattern, snug, not overdriven.
• Lap the unit's cap or counterflashing down over the top of the base flashing so water sheds onto the face.
• Install the fall-protection screen, cover, or rail where people will service the roof.
• Water-test the finished assembly from the lowest detail up before closeout.

The sealant and the gasket: the flashing does the work

The single most common skylight mistake is making the sealant do the flashing's job. Sealant and gaskets are real parts of the assembly, but they are the secondary line. The flashing sheds the water. The sealant closes the small gaps the flashing leaves. A skylight set in a fat bead of caulk with no real flashing under it leaks the day the caulk fails, and caulk always fails.

Use the right products in the right places. The gasket between the skylight frame and the top of the curb is a compression seal that the unit's fasteners squeeze down, and it has to be the manufacturer's gasket or a sealant they specify, seated continuously so there is no skip. Lap sealant caps a termination bar and the cut edge of membrane flashing. The sealants have to be compatible with the membrane and the frame, because the wrong chemistry attacks the very thing it is supposed to protect.

Here is the rule that keeps you out of trouble: if the detail only works because of the sealant, the detail is wrong. Sealant has a service life measured in years and it is a maintenance item from the day it is applied. Flashing, lapped and fastened right, lasts the life of the roof. Build the assembly so it would still shed water with the caulk gone, then add the sealant as the backup it is meant to be.

How do you find a skylight leak?

You find a skylight leak by ruling out the three sources in order, condensation first, then the glazing seal, then the flashing, and you confirm it with a controlled water test rather than guessing. Most reported skylight leaks are flashing, a fair number are condensation, and a smaller share are a failed glazing seal, so sort them before you tear anything off.

Start dry. Check whether the water tracks the weather or the indoor humidity, because condensation that runs off the frame on a cold morning is not a flashing problem and clearing the weep channels may be the whole fix. If it follows the rain, it is a leak.

Then run water on the unit from the bottom up, one zone at a time, with someone watching inside. Wet the lowest flashing first and hold it. If nothing shows, move up to the sides, then the head and the cricket last, so when the water appears inside you know which detail let it in. Hitting the whole skylight at once tells you it leaks but not where, and wetting from below to above keeps you from chasing water that ran down from a higher detail. A failed insulated-glass seal shows as fogging between the panes and is a glazing replacement, not a flashing repair. The head flashing and the upslope corners are where the money is, because that is where the water load is highest.

Replacing an old skylight and re-flashing

Replacing a skylight is a re-flashing job, not a glass swap, and the most common retrofit leak is a new unit set into the old, failed flashing. The flashing is usually what wore out, or the old unit was leaking because the flashing was wrong in the first place, so reusing it just reinstalls the leak under a new dome.

Pull the unit and look at the curb and the deck. Years of a slow leak rot the curb framing and the deck around the opening, so probe the wood before you set anything new on it, and rebuild what is soft. On a low-slope roof a retrofit is the chance to raise a curb that was always too short and to add the cricket that was never there. On a shingled roof it means lifting the surrounding courses to weave in fresh step flashing, not caulking the new unit to the old shingles.

Match the new unit to the opening and the roof, and flash it with the manufacturer's current kit. An old curb-mount opening does not always take a new deck-mount unit, or the reverse, and forcing the wrong unit into the hole creates a detail nothing was designed for. Tie the new flashing into the existing membrane or shingles correctly, the same as new work, because the roof around a retrofit is exactly as good as the tie-in you make to it.

Daylighting and the energy tradeoff

A skylight trades free daylight against a hole in the thermal envelope, and both sides are real. Daylight from above is even and deep, it reaches the middle of a space a window cannot, and on a commercial floor it can cut daytime lighting load enough to matter, especially paired with controls that dim the electric lights when the sun is up. That is the upside, and it is why warehouses, big-box retail, and shops put skylights on the roof.

The cost is heat. A skylight has a worse U-factor than the insulated roof it replaces, so it loses heat in winter and, facing straight up, it can pull in serious solar gain in summer. On a cooling-driven building too much skylight area adds air-conditioning load that can swamp the lighting savings. The energy code caps skylight area as a percentage of the roof and sets U-factor and SHGC limits for that reason.

Size and place the glazing for the daylight you need and no more. The right amount of skylight, with a low-SHGC glazing and lighting controls that actually respond to it, pays for itself. A roof oversupplied with cheap, high-gain domes is a comfort and energy problem nobody budgeted for.

The commercial, industrial, and data center roof

On a big commercial or industrial roof the skylight count goes up and so does the exposure, because every unit is another curb to flash and another fall hazard to screen. A warehouse or plant roof may carry dozens of skylights and smoke vents laid out across the field, and at that scale the install discipline and the documentation are what keep the roof dry and the people safe.

A data center is the case where skylights mostly disappear, and for good reason. A critical facility cannot accept the leak risk, the fall hazard, the unwanted solar gain, or the security exposure of glazed roof openings over the white space, so the daylighting benefit loses to the risk and the roof stays solid. Where openings exist on those buildings they are smoke vents and access hatches driven by code, flashed and screened to the same standard as everything else on a high-value roof.

The pattern holds across the big roofs: the more the building has to stay dry and the higher the cost of a failure, the more every skylight has to earn its place. On a daylight-driven warehouse the skylights pay for themselves. On a facility that cannot get wet, the fewest openings win.

The manufacturer flashing kit and the roof warranty

Two warranties sit on a skylight job and they have to agree: the skylight manufacturer's and the roof membrane manufacturer's. The skylight maker warrants the unit and publishes the curb height, the flashing kit, the fastener pattern, and the gasket that the warranty depends on, and using their flashing kit instead of a shop-built substitute is usually a condition of that coverage. Their installation instructions govern the unit.

The roof warranty is the one the follow-on trades blow up. Adding or replacing a skylight is a roof penetration, and cutting into a warranted membrane without the membrane manufacturer's approval and their detail can void the roof warranty, the same as any other unapproved penetration covered in the rooftop equipment guide. The membrane manufacturer wants to be told before the opening is cut, often wants their approved applicator doing the flashing, and wants it built to their curb detail.

Get both approvals in writing and keep them with the job record. When a leak shows up at a skylight three years on, the question is whose detail failed and whether the work was approved, and the file you made on the roof is the answer. The skylight kit and the membrane detail are a matched set. Mixing a cheaper part into either one is how a covered roof becomes an uncovered one.

What to document

The skylight record is what defends the roof and the unit when a leak or a fall question comes up later. The flashing gets buried under the cap and the membrane the day the crew leaves, so the log made during the install is the only proof of how it was built. Capture each unit by location and tie the whole package to the manufacturer's details and the written warranty approvals.

Common mistakes

• Setting a skylight on a low-slope roof with no curb, or a curb too short to give the flashing its 8 in turn-up.
• Relying on a bead of sealant to do the flashing's job instead of building real flashing under the unit.
• Skipping the cricket on the upslope side of a wide skylight, so water dams against the curb and tests the flashing.
• Calling condensation a leak and tearing off good flashing, when the fix was clearing the weep channels or adding a thermal break.
• Leaving a skylight as an unprotected hole with no screen, cover, or rail where people service the roof.
• Running flashing that never ties into the roof membrane, or lapping it backwards so water runs behind it.
• Reversing the step-flashing laps on a shingled roof, or running continuous flashing where each step should interleave a course.
• Reusing the old, failed flashing when replacing a skylight, so the new unit leaks at the same detail.
• Burying, painting, or fastening a smoke vent shut on a re-roof so it cannot open in a fire.
• Setting the wrong glazing for an overhead opening, with no safety glazing and no screen where the code wants one.

Field checklist

Standards and references

The skylight manufacturer's installation instructions and the membrane manufacturer's curb detail govern the assembly, full stop. They set the curb height, the flashing kit, the gasket and sealant, the fastener pattern, and what the warranty requires, and the closeout inspection is run against those details. Confirm every figure in this guide against the actual product literature for the unit and the roof you are on.

The NRCA Roofing Manual is the trade reference for the curb, the base flashing, the counterflashing, and the cricket, and it is where the commonly cited 8 in base-flashing height and the penetration details come from. Treat its figures as recommended industry practice and confirm the current edition, because the manual is revised across cycles. The companion penetration-flashing guide carries those details in depth.

The codes apply by topic, and the section numbers shift between editions, so confirm them against the adopted edition and any local amendments before citing them. The building code covers unit-skylight curb height and slope, commonly in the provisions around R308.6.8, and the safety glazing for sloped glazing and skylights, commonly in IBC Chapter 24 around Section 2405, including the screening rule and the allowed glass and plastic materials. OSHA fall protection treats an unprotected skylight as a hole, at 1910.28 in general industry and 1926.501 in construction. Smoke and heat vents are tested to UL 793 and laid out to NFPA 204. FM Global data sheets add requirements on insured roofs, and the project specification ties it together and controls where it is stricter than any of these.

Units, terms, and synonyms

Skylight work uses a specific vocabulary, and the same part reads differently across a unit cut sheet, a roof detail, and the spec, so the terms are worth pinning down.

Curb-mounted means the unit sits on a raised curb. Deck-mounted, or self-flashing, means it mounts low to the deck on an integral flange. Base flashing is the roof membrane or metal turned up the curb. Counterflashing is the cap that covers its top edge. A cricket, or saddle, is the diverter on the upslope side. A TDD is a tubular daylighting device or solar tube. Glazing performance is given as a U-factor for heat loss and an SHGC, the solar heat gain coefficient, for solar gain, and an insulated glass unit, the IGU, is a sealed multi-pane assembly. The weep channels are the condensation gutters in the frame that drain to weep holes on the outside.

Curb-mounted skylight

A skylight set on a raised curb, the standard for low-slope and flat roofs

Deck-mounted / self-flashing

A skylight mounted low to the deck on an integral flashing flange, common on shingled slopes

Base flashing

The roof membrane or metal turned up the curb, commonly at least 8 in above the finished roof

Counterflashing

The cap or unit flange that covers the top of the base flashing so water sheds over its face

Cricket / saddle

A built-up diverter on the upslope side of a wide curb or skylight that splits water around it

Weep channel

A condensation gutter in the skylight frame that drains collected moisture to weep holes outside

TDD

Tubular daylighting device, or solar tube, a roof dome feeding a reflective tube to a ceiling diffuser

U-factor / SHGC

Heat-loss rate and solar-heat-gain coefficient of the glazing, the two energy numbers that govern selection

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