A typical curb-mounted residential skylight showing flashing integration with the surrounding roof surface.
Types of residential skylights
Two dominant installation methods are used in residential construction: curb-mounted and deck-mounted (also called self-flashing). Each has different performance profiles in cold climates.
Curb-mounted skylights
A curb-mounted skylight sits on a raised wooden frame — the curb — that protrudes above the roof deck. The curb height is typically between 100 mm and 150 mm. This design keeps the glazing unit further from meltwater and snowpack on the roof surface. In regions with significant snowfall, a higher curb reduces the risk of water infiltration during spring thaw cycles.
The curb itself must be built from preservative-treated lumber where it contacts the roof deck, and its top surface must be sloped slightly to shed water. Flashing details — step flashing on the sides, counter-flashing at the head, and a saddle at the upslope side — are critical. Poor flashing is the most common cause of water damage in skylight installations.
Deck-mounted skylights
Deck-mounted units integrate directly into the roof surface without a separate site-built curb. They are generally lower-profile and easier to install on pitches between 15° and 60°. On shallower-pitch roofs, particularly those common in bungalows across the Prairies, they are less suitable because water drainage at the perimeter is slower.
Most deck-mounted products come with proprietary flashing kits designed for specific roofing materials — asphalt shingles, metal standing seam, or membrane. Using the manufacturer's flashing system rather than custom-fabricated alternatives reduces callbacks related to leaks.
Structural considerations in Canadian climates
The National Building Code of Canada (NBC) provides ground snow load maps by region. Roof snow loads are derived from these with a shape factor that accounts for roof geometry. A flat or nearly flat skylight opening must be assessed for the accumulated snow load it will carry before the surrounding roof area sheds snow to ground.
In practice, this means the curb framing and the header and trimmer members around the roof opening require sizing by a designer familiar with the local specified loads. In many prairie and northern locations, design roof snow loads exceed 2.0 kPa. The glazing unit itself must also carry these loads; manufacturers publish structural tables listing the maximum load capacity for each unit dimension and glazing type.
Glazing options and thermal performance
- Double-glazed units with low-e coatings are the minimum practical specification for heated Canadian spaces. Centre-of-glass U-values around 1.8 W/m²K are common.
- Triple-glazed units lower U-values to approximately 1.0–1.2 W/m²K but add weight, which has structural implications for the curb and roof framing.
- Solar heat gain coefficient (SHGC) should be selected based on orientation. South-facing skylights benefit from higher SHGC in heating-dominated climates; north-facing units gain nothing from solar heat regardless of SHGC.
Condensation management
Condensation on skylight frames is a persistent issue in cold climates. When warm, humid indoor air contacts the cold frame surfaces at the interior perimeter of a skylight, moisture can condense and run down onto ceiling finishes or the top of the curb.
Several factors reduce condensation risk. First, the frame thermal performance matters: polyurethane foam-filled frames or thermally broken aluminium frames transfer less cold to interior surfaces than solid aluminium. Second, the interior air seal around the well must be continuous; gaps that allow air movement from conditioned space into the cavity above the ceiling plane will carry moisture to cold surfaces. Third, mechanical ventilation to manage interior humidity during winter is effective across the building, not just at the skylight location.
Some manufacturers supply an integral condensation channel that collects moisture and drains it through small weep holes to the exterior. This is a useful feature but is not a substitute for proper air sealing and thermal design.
Vapour barrier integration
The vapour control layer — typically a polyethylene sheet in Canadian residential construction — must be continuous across the ceiling plane, including around the skylight well. The well itself is essentially a shaft from the ceiling to the roof deck, and all surfaces of that shaft should be inside the insulated envelope.
Common practice is to build the well with continuous vapour barrier on the interior face, with all joints and penetrations sealed using compatible tape. The transition from the ceiling vapour barrier to the vertical faces of the well is a detail that framers sometimes skip; this creates a thermal and moisture pathway that can cause both condensation and frost accumulation in the roof assembly above.
A small skylight on a residential roof, illustrating how the unit sits relative to the surrounding roofing surface.
Orientation and positioning
In the northern hemisphere, south-facing roof surfaces receive the most solar radiation. A skylight on a south-facing slope captures direct sun for more hours per day in winter than one on a north-facing slope, which may receive only diffuse light for most of the heating season.
That said, direct sunlight through a skylight can create harsh glare and excessive localised heat gain in summer. Strategies to balance these effects include:
- Using a diffusing glazing layer — either a translucent inner pane or a prismatic diffuser — to spread direct beam radiation across a wider area of the room.
- Specifying a lower SHGC for south-facing skylights in cooling-season-significant climates such as southern Ontario or BC's interior valleys.
- Installing an interior blind or shade that can be closed during peak summer sun. Tubular motors allow this in units that are otherwise inaccessible.
Daylighting performance in practice
A single skylight with a clear aperture of approximately 1.0 m² can produce adequate horizontal illuminance — 300 lux or more — over a floor area of roughly 10–15 m² on a clear day at northern latitudes, depending on ceiling reflectance and well geometry. This is consistent with residential task lighting requirements.
The well itself affects how much light reaches the room. A splayed well — one with angled sides that widen towards the room — spreads light more effectively than a straight shaft. Painting well surfaces with a high-reflectance white finish (reflectance above 0.85) increases the amount of light that exits the well rather than being absorbed by the surfaces.