Drainage around roof lights

Drainage is often treated as a roofing topic alone, but roof light installations fail when water is managed poorly at three levels: the roof deck around the opening, the glass surface itself, and any internal channels built into framed units. Standing water increases load, accelerates sealant ageing, carries dirt onto glass and masks the true source of leaks. This reference ties drainage principles to kerb detailing and installation practice.

Roof-level drainage

On flat and low-pitch roofs, falls (typically minimum 1:40 or 1:60 depending on roof system guidance) direct water to rainwater outlets. The roof light upstand interrupts the plane. Design so:

• Falls run away from the kerb on at least the longer sides where possible.
• No sump forms upslope of the unit where water collects with no outlet path.
• Outlet capacity suits the catchment area — moving an opening is cheaper at drawing stage than adding pumps later.

Where falls create unequal upstand heights, the roofer maintains continuous membrane turn-ups. The lowest point of the kerb is not an excuse to terminate membrane early.

Inverted (protected membrane) roofs place insulation over the waterproofing. Kerb detailing must preserve drainage at the wearing course and prevent insulation scouring at outlets near the opening.

Glass surface drainage

A horizontal glass roof is not a swimming pool. Rain, dust and pollen need a path off the external face:

• Fall on the unit — Some manufacturers specify a slight fall (for example 2–3°) achieved by shimmed bearing or curb geometry. Others allow level installation within tolerance if perimeter drainage is assured.
• Internal channels — Framed systems may drain water along a frame gutter to weep holes. Blocked weeps are a common call-back — protect during construction and blow clear before handover.
• Edge drip — External sealant and capping profiles should encourage water to drip free of the throat rather than run back under tiles or membrane.

Frameless units without channels rely on perimeter geometry and roof falls. Discuss requirement with the supplier when the deck is dead level locally.

Throats, flashings and water paths

At the upslope side of a kerb on a pitched or flat roof, wind-driven rain can push water up the intersection. A correctly formed throat sealant and cover flashing sheds water over the upslope face. This is drainage logic as much as weathering — water must have a defined exit.

Side aprons should extend far enough under adjacent covering that capillary draw does not pull water sideways into the build-up.

Ponding: causes and consequences

Ponding cause

Effect at roof light

Insufficient fall

Water laps kerb; dirt line on glass

Blocked outlet

Roof depth increases; loading rises

Deflected deck

Local sump at opening

Compressed insulation

Dip in membrane route

Ponding adds structural load beyond design assumptions for live load. It also keeps sealant joints immersed longer, accelerating cure shrinkage issues and debonding on some substrates.

After installation, observe the roof in heavy rain. Water should clear from the glass and kerb within a reasonable time. Persistent pools warrant survey — do not only re-seal symptoms.

Coordination with setting blocks and movement

Drainage detailing must not be defeated by installation choices. Setting blocks and internal gaps should not create dams. Sealant should not be tooled into a horizontal shelf that traps water on the external toe.

Thermal expansion joints at the perimeter (see ventilation and movement guidance) must remain open to function — sealant smeared across designed gaps blocks both movement and weep paths where applicable.

Maintenance

• Keep glass and kerb throats clear of leaves and moss — organic matter holds moisture against seals.
• Inspect weep holes on framed units annually.
• After roof maintenance, confirm nobody has damaged flashings or piled plant equipment blocking falls.

For designers

Show roof falls and outlet positions on plans relative to proposed openings. If a roof light must sit in a structurally dictated location with marginal falls, flag it early for manufacturer review rather than assuming site adjustment will suffice.

Made-to-order frameless roof lights are configured to your opening — drainage suitability is a site and roof design question that should be resolved before the kerb is built.

Every Vant Glass roof light is made to order in Britain, backed by a 20-year guarantee and free UK mainland delivery. Configure frameless or framed sizes in the online calculators or call 03330 902 592.