Skylight Types Explained: Curb-Mount, Deck-Mount and Tubular

Step by Step

1. 1Step 1: Identify your roof type and pitch

   Deck-mounted skylights are designed for pitched roofs. Manufacturers publish a minimum and maximum pitch, commonly starting around 3/12 and going up to very steep, but the exact range depends on the model, so confirm it in the installation instructions before buying.

   Below that minimum pitch, a curb-mounted unit is the right answer, because the curb lets the roofing membrane turn up and run water around the unit instead of depending on a flashing kit that was never designed for a near-flat surface. See how to install a skylight on a flat roof.

2. 2Step 2: Decide between deck-mount and curb-mount

   Deck-mounted units have a low profile and come with a matched flashing kit. They look cleaner from the street and are simpler to flash on shingles. The frame sits on the deck and the kit laps under and over the shingles.

   Curb-mounted units sit on a box frame, the curb, that raises the glass above the roof surface. A common minimum is at least 4 inches of curb above the finished roof surface on a pitched roof, and about 8 inches on a flat membrane roof, where the membrane's flashing height decides. Confirm it with both manufacturers. The height matters because it keeps snow, pooled water and debris below the glass edge. Curbs are covered in how to build a skylight curb.

   Tip: On a shingle roof, a curb-mounted unit is sometimes chosen for a wide or heavy unit, or where a site-built curb is easier than fitting a kit to an odd location.

3. 3Step 3: Choose fixed or venting

   A fixed skylight does not open. It has fewer moving parts and fewer gaskets, so it is the most reliable for keeping rain out. It is the usual choice over hallways and living spaces that have other ways to ventilate.

   A venting skylight opens by hand crank or electric motor, and some have rain sensors that close them. Venting units help clear moisture from bathrooms and warm air from upper floors, but they add seals that age. Electric and solar models need a power supply or a solar panel and wiring run to them, which is also part of planning.

   Venting units are not a substitute for an exhaust fan. A bathroom still needs a proper duct to outside, as in how to vent a bathroom fan through the roof.

4. 4Step 4: Compare glazing for heat, condensation and noise

   Most residential skylights use sealed double-pane insulated glass with a low-e coating and gas fill. A second glazing layer reduces heat loss and raises the interior glass temperature, which directly cuts condensation on cold mornings.

   Building codes commonly require the inner pane to be laminated or otherwise safety glazed on skylights over living areas, so falling glass stays in the frame. Laminated glass also cuts rain noise, which matters in heavy Fraser Valley downpours. Plastic domes are lighter and cheaper but have poorer insulation and scratch more easily.

   Ask for the U-factor and solar heat gain numbers in the manufacturer's data sheet. South facing skylights over living spaces in a valley with hot summers benefit from lower solar heat gain.

5. 5Step 5: Size the skylight to the framing

   Rafters or trusses are commonly spaced 16 or 24 inches on centre. Many skylights are made in widths that fit between two rafters with small gaps, such as about 14-1/2 or 22-1/2 inches of rough opening. A unit that fits between rafters only needs short headers, called trimmers and headers, above and below it.

   A wider skylight means cutting a rafter or truss. That is a structural change. It needs proper double headers and doubled trimmers to carry the load, and may need an engineer, a building permit or both. Trusses must never be cut without written direction from the truss engineer.

   Watch out: Never cut a truss member to fit a skylight. Trusses are engineered as a system and cutting one can overload the rest.

6. 6Step 6: Think about the light shaft and the room below

   On a shallow attic, the shaft can be a simple boxed opening. In a deeper attic, it is a longer tunnel. Splayed, or angled, sides spread the light better than straight shafts, and sides angled toward the light give the brightest room.

   The shaft walls must be insulated and air sealed. An uninsulated shaft is a cold surface in a warm house, which is the most common source of condensation and staining that gets mistaken for a leak. See how to fix a leaking skylight for telling the two apart.

7. 7Step 7: Consider a tubular skylight where a full unit will not fit

   A tubular skylight has a dome on the roof, a flashing base, a reflective tube and a ceiling diffuser. The round hole, commonly 10 to 14 inches for residential units, often fits between rafters without any framing changes, so it is a simpler install than a full skylight.

   It lights a room well but gives no view and no ventilation. Some units include an optional fan or light kit. See how to install a tubular skylight.

8. 8Step 8: Time the decision with the roof

   Skylight flashing is interwoven with the shingles and underlayment, so it works best installed during a roof replacement. A skylight added to an existing roof means tearing up a patch of shingles and trying to integrate the flashing with old material.

   Old skylights on a roof that is about to be replaced are nearly always better swapped than reflashed. See how to replace a skylight during a reroof.

Quick Comparison of Skylight Types

Fraser Valley Notes

Common Mistakes

• Buying before measuring rafter spacing. A unit that does not fit between rafters turns a simple install into a structural cut.
• Using a deck-mounted unit on a roof below its minimum pitch. The kit was not designed for near-flat surfaces and will leak.
• Choosing a venting unit where a fixed one would do. Every seal and moving part is something that can wear out.
• Ignoring the light shaft. A cold, uninsulated shaft makes condensation that looks like a roof leak.
• Placing it in a valley or right under a chimney. Concentrated water flow at those spots overwhelms skylight flashing.
• Adding a skylight separately from the reroof. The flashing ends up patched into old shingles, which is where most skylight leaks start.