A skylight can change a room more than almost anything else you can do to a house. A windowless middle room in a Philadelphia row home gets daylight for the first time. A primary bathroom on the Main Line stops needing the lights on at noon. And a badly installed skylight can become the single most persistent leak on the roof.
Our owner trained at VELUX's training center in Maryland and is VELUX Pro Partner Academy Trained. Here's what that training reinforced about why skylights leak — and how to install them so they don't.
Why Skylights Leak
When a homeowner tells us a skylight leaks, the cause is usually one of these:
- Flashing that relies on sealant. The most common failure. The skylight was set in a hole and the joint to the roof was sealed with caulk or roof cement. Sealant cracks as the roof heats and cools, and the leak starts a few years in.
- Flashing that fights the roof. Water on a roof runs downhill in layers. A flashing that isn't woven into those layers — tucked under the shingle or panel above, lapped over the one below — sends water behind the roofing instead of over it.
- Aged units. Old acrylic domes craze and crack, glazing seals fail, and curb framing rots. Past a certain age the unit is the problem, not the install.
- Condensation mistaken for a leak. Warm, humid bathroom air hitting cold glass drips in winter. That's a ventilation question, and a venting skylight can be part of the answer.
VELUX Flashing Is the Whole Game
A properly installed VELUX skylight uses a flashing kit made for that unit and that kind of roof — shingle, metal, tile, or low slope. The kit's pieces interlock with the roofing around them, and an underlayment collar seals the opening beneath them. Nothing depends on a bead of caulk to stay dry.
Think of the skylight as part of the roof system, not an object sitting on top of it. The roof sheds water around the skylight exactly as it would if the skylight weren't there.
That approach is also what VELUX's warranty is built around. In its 2026 residential catalog (document XUS 20183-1025), VELUX states that its deck- and curb-mounted skylights carry a 10-year installation warranty when installed with VELUX flashing, plus 20 years on glass seal failure, 10 years on the product, and 5 years on shades and controls. VELUX also publishes a 10-year hail warranty on its Standard Safety Glass, which is laminated for protection in out-of-reach places. Read VELUX's current warranty terms for the exact conditions before you buy; they apply to the product you choose, not to a contractor's promise.
That's VELUX's promise on the product. Our own workmanship warranty on a skylight we cut into a new opening is 10 years, and on a skylight we replace on its own it is 2 years — and when a skylight goes in as part of a new roof, it's covered under that roof's workmanship warranty.
- Installed with VELUX flashing.
- Covers the skylight unit and its flashing only. The existing roof around it is excluded.
- Skylight in a new opening only: void if the roof is replaced, or the skylight flashing is disturbed, by anyone other than ALFE Roofing. If ALFE Roofing later replaces the roof, the skylight carries on under that roof's warranty.
The Re-Roof Rule: Never Put an Old Skylight Back on a New Roof
If you're replacing your roof and you have skylights, this is the most important paragraph in this article.
During a tear-off, the old flashing around each skylight is torn out along with the old roofing. At that moment you have two choices: reinstall the old skylight with new flashing, or put in a new skylight with its matching flashing kit. The first choice looks cheaper. It usually isn't:
- The new roof will outlast the old skylight. A new roof is built to last decades. A skylight that's already 15 or 20 years old will fail long before that.
- Replacing it later means opening the new roof. When that old unit fails, someone has to cut back the roofing you just paid for to get at it.
- The hard part is already done. With the roof open, the opening, framing and underlayment are right there. Swapping the unit now costs a fraction of doing it as its own job later.
That isn't just our opinion. VELUX tells installers the same thing in its own service letter on skylight cladding (document V-USA-1-0-10): “Replace the existing flashing anytime a new skylight is installed or whenever the roof is replaced around an existing skylight.” VELUX's reasoning is that pulling an old flashing apart and putting it back leaves nail holes through it and rebuilds it without the instructions — which, in VELUX's own words, “often results in future roof leaks around the flashing.”
The same document is blunt about the shortcut some crews take. VELUX states that using site-made flashing on its skylights is “not advisable as it will void any existing warranty and likely allow water penetration at the skylight.” If a quote doesn't name the matching VELUX flashing kit, that's what you're being sold — and the manufacturer's warranty goes with it.
VELUX also gives a plain rule for storm damage: replace the whole skylight any time it's been hit by hailstones larger than 2" in diameter, because the seal holding the glass to the cladding can be compromised where you can't see it — and it's a factory sealant that can't be re-applied on a roof.
The Types, in Plain English
| Type | What it does | Where it fits |
|---|---|---|
| Fixed | Daylight only. Doesn't open. | Hallways, stairwells, living rooms, vaulted ceilings you can't reach. |
| Fresh Air venting (manual) | Opens with a crank or rod to let hot, humid air out. | Bathrooms and kitchens within reach. |
| Solar-powered Fresh Air venting | Opens and closes by remote, powered by a small solar panel — no wiring. A rain sensor closes it automatically when it starts to rain. | High ceilings, primary baths, anywhere you'd never reach a crank. |
| Sun Tunnel | A small dome on the roof and a reflective tube that carries daylight down to a ceiling diffuser. | Interior rooms, closets and halls with attic space above. |
Mounting matters too. Most pitched roofs take a deck-mounted skylight that sits low in the roof plane. Low-slope and flat roofs — and some metal roofs — need a curb-mounted unit raised on a frame so water can't pond against it.
Philadelphia Row Homes: Daylight for the Middle Room
A classic row home is long and narrow, with windows only at the front and back. The middle of the house — a bathroom, a stairwell, a second bedroom — often has no daylight at all. Most row homes also have flat or low-slope roofs.
That combination usually points to a curb-mounted skylight over a stairwell or bath, or a Sun Tunnel where there's a small attic or ceiling cavity to run through. Either one has to be integrated with the flat roof membrane properly — the same care we take with the parapet coping on your party walls .
Main Line Homes: Wayne, Merion and the Towns Between
The homes in Wayne, Merion, Bryn Mawr, Gladwyne and Villanova are a different project. Larger houses, steeper roofs, and rooms built around light: primary bathrooms, kitchens with vaulted ceilings, upstairs landings, and third-floor rooms tucked under the roof.
- Primary baths are where a venting skylight pays for itself, clearing steam and humidity that otherwise sits under the ceiling.
- High and vaulted ceilings put skylights out of reach, which is exactly where the solar-powered venting models make sense: open them with a remote, and the rain sensor closes them if a storm comes through while you're out.
- Mature trees are part of what makes these streets beautiful, and they shade a lot of roofs. We check how much sun the roof actually gets before recommending a solar-powered unit, and we look at where leaves and debris collect around each skylight so water and debris shed cleanly.
- Slate, cedar and metal roofs are common on older Main Line homes, and each needs the flashing kit made for it.
How We Install Skylights at ALFE Roofing
We install VELUX skylights the way VELUX trains installers to: with the VELUX flashing kit made for the unit and the roof, integrated with the underlayment and roofing around it, to VELUX's published instructions.
- Skylights are part of our 27-point roof inspection — the unit, the flashing, the curb and the ceiling below.
- On every re-roof, we quote replacing the skylights in the same visit and show you the cost side by side with keeping them.
- We recommend the type for each room — fixed, venting, solar venting, or Sun Tunnel — based on reach, humidity and how much sun the roof gets.
