Penetrations
Skylights are sold as an amenity and owned as a liability. They bring daylight into warehouses and offices, reduce lighting load, and look good in a leasing brochure. They are also a deliberate hole cut through the most weather-critical surface on the building, surrounded by sealant joints and curb flashings that age faster than the roof around them. Across the buildings we advise on, skylights and the curbs beneath them generate leaks out of all proportion to the roof area they occupy. An owner who treats skylights as set-and-forget glazing rather than as active roof penetrations will eventually pay for that assumption, usually in interior damage to the tenant space directly below.
A Penetration Is a Leak Waiting for Time
Every skylight interrupts the continuous membrane that makes a low-slope roof watertight. To keep water out, the assembly relies on a curb that raises the unit above the waterline, a base flashing that ties the curb into the membrane, and a perimeter of sealant where the glazing or dome meets the curb frame. Each of those is a separate component with a separate service life, and the sealant joints in particular are the shortest-lived element on the entire roof. A field of TPO or PVC may carry a twenty-year warranty while the wet-sealed perimeter of the skylight beside it is degrading within five to seven years of exposure to UV, thermal cycling, and ponding.
This mismatch is the core of the problem. The roof can be young and sound while its skylights are already failing, because the weakest part of the assembly is also the part that ages fastest. Owners who budget around the membrane warranty and ignore the penetrations are budgeting around the wrong number.
Where Skylight Leaks Originate
When we trace a leak to a skylight, it almost never comes from the glazing itself cracking. It comes from the connections around it. The recurring failure points are consistent across building types.
- Perimeter sealant between the dome or glass and the curb frame, which dries, shrinks, and cracks under UV well before the roof membrane does.
- Curb base flashing, where the membrane turns up the curb and is terminated. Slumped, peeled, or low base flashing sits in ponded water at the upslope side of the curb.
- The upslope face of the curb specifically, where water that ponds against the obstruction has the most time and pressure to find a flaw.
- Condensation mistaken for a leak. Single-glazed or thermally broken units in humid interiors drip from interior condensation, sending owners chasing a roof defect that is not there.
- Fastener backout and gasket failure on the curb frame, common on older aluminum-framed units.
- Differential movement between the curb and the deck, which works sealant joints loose over repeated thermal cycles.
Because water entering at a skylight curb can travel under the membrane or down the curb framing before it appears inside, the interior stain is again an unreliable guide to the source. The skylight is usually the obvious suspect when a stain appears directly beneath it, but proving it requires inspecting the curb, not just looking up at the glass.
The Decision Owners Avoid
The hardest skylight conversation is whether to keep the unit at all. Many skylights on older industrial and retail roofs deliver marginal daylight value while generating recurring leak cost and ongoing curb maintenance. When a roof is being recovered or replaced, the owner faces a genuine choice: reflash and reinstall every skylight, replace the units with new curb-mounted assemblies, or remove some of them entirely and infill the openings with new deck and membrane. Reflashing the existing units is cheapest up front and frequently the worst value, because it preserves aging glazing and old sealant joints on top of a brand-new roof, guaranteeing that the first leaks on a fresh membrane will come from the penetrations.
We advise owners to make this decision deliberately at the time of any major roof work rather than defaulting to reflashing out of inertia. For a warehouse with adequate artificial lighting and a history of skylight leaks over inventory, infilling several units can remove a permanent liability for little more than the cost of flashing them back in. For a space where daylight genuinely matters, new curb-mounted units with current glazing and proper base flashing are worth the spend. What rarely makes sense is putting a new roof under tired skylights and calling the project complete.
Warranty, Safety, and Documentation
Skylights carry exposures beyond water. Many membrane warranties exclude the skylight units and their sealant joints, covering only the membrane and its termination at the curb, which means a leak originating in the glazing perimeter is the owner's cost even on a warranted roof. Skylights are also a recognized fall hazard. An aged, brittle, or unscreened skylight is a hole in the roof that a worker can fall through during routine HVAC or roof service, and addressing fall protection at these openings is both a safety and a liability matter for the owner. For both reasons, every skylight on a building belongs in the roof record as a tracked penetration, with its glazing type, curb condition, last reseal date, and known leak history documented alongside the membrane. Owners who inventory their skylights the way they inventory their roofs stop being surprised by them. They reseal curbs on a schedule before the joints fail, they make the keep-or-remove decision with eyes open at recover time, and they stop paying repeatedly for the same leak over the same tenant space.