Waterproofing skylights on roofs: key flashing details to prevent leaks

Skylight waterproofing on roofs is one of the most common sources of leaks. In most cases, the skylight itself (glass/polycarbonate) isn’t the problem—the failure is in the perimeter flashing and terminations (laps, upstands, sealants, and junctions with tiles, metal, or parapets). If you’re seeing stains around a skylight, this guide explains where leaks start and how to fix them properly.

Why skylights leak (usually at the perimeter)

Typical weak points include:

• Insufficient laps of the roof membrane (bitumen/EPDM/TPO/PVC) onto the skylight upstand.
• Missing or poorly installed termination bar/reglet to clamp the membrane on the vertical.
• Aged sealants due to UV exposure (wrong silicone, unprotected mastic).
• Ponding water from poor falls or nearby drainage issues.
• Through-fixings (screws) without proper sealing washers or with cracked grommets.
• Tile/metal junctions solved with “just sealant” instead of proper step flashing.

A skylight can be in perfect condition and still leak because the flashing detail 2 cm away is wrong.

Quick diagnosis: how to pinpoint the entry point

Before repairing, confirm the source:

• Leaks after wind-driven rain often indicate side/top junctions.
• Leaks after long steady rain often point to ponding or a low-side lap.
• Check the inside corners of the upstand/frame—leaks commonly start there.
• Look for cracked sealant and loose termination bars.

If uncertain, a controlled hose test by zones (without excessive pressure) can help isolate the failure.

Recommended solutions by roof type

The key is to rebuild the termination system—not to “re-silicone” the outside.

Flat roofs with membranes (bitumen, EPDM, TPO/PVC)

A typical professional fix:

• Remove failed details and prepare the substrate.
• Form a proper perimeter upstand and fillet/cove at the angle.
• Turn the membrane up the vertical and secure it with a termination bar + compatible seal.
• Add counterflashing/capping (metal flashing) to shed water and protect the seal.

Tiled roofs

This is where patching fails most often:

• Install a bottom apron flashing and step flashings on the sides integrated with the tiles.
• Provide a top detail that blocks wind-driven rain (a proper head flashing).
• Ensure water is not being guided into the opening; check nearby valleys.

Metal roofs or sandwich panels

Common critical points:

• Fasteners and panel joints.
• Perimeter flashing with butyl tape and correct profiles (not foam alone).
• Allow for thermal movement: use compatible elastic sealants for metal.

Common mistakes (and how to avoid them)

• Smearing sealant without rebuilding the detail: short-lived and can worsen drainage.
• Mixing incompatible products (sealants that don’t bond or damage membranes).
• Leaving sealant exposed to UV: it cracks quickly.
• Insufficient upstand height: debris, snow, or ponding pushes water in.
• Ignoring the root cause (falls, drains, buildup): the leak returns.

What a skylight leak repair quote should include

To compare proposals, ask for:

• The flashing/termination system and specific materials.
• Corner and junction treatment.
• Check of nearby falls/drains.
• Final verification or watertightness check.
• Written warranty.

Apartment buildings in Madrid: when to intervene

In Madrid, many shared roofs have skylights over stairwells, courtyards, or common areas. For recurring leaks, the most efficient approach is usually:

• Repair the entire perimeter detail, not isolated “spots”.
• Inspect all skylights on the same roof—details are often identical.
• Schedule works before autumn/winter to avoid damage to ceilings, plaster, and services.

Need help with a leaking skylight?

If your skylight is leaking, Impermyna can inspect the flashing, identify the entry point, and propose a long-lasting solution tailored to your roof in Madrid or Toledo. Get in touch for a no-obligation visit.