Skylight installation, and when not to

Site it from inside, then check it from the roof

Where the light actually lands depends on the roof orientation, the pitch, the rafter bay you have to work between, and the angle of the shaft through the attic. A shaft splayed wider at the ceiling end throws light further into the room than a straight box does. All of that is decided from below, then transferred up and checked against what is on the roof — vents, valleys, the plumbing stack, and the way water runs down the slope.

Watch for: A position picked purely for the convenience of the rafter spacing. It is the one decision you cannot revise later without patching a roof, and it is the decision that determines whether the room is pleasant or glaring.

Establish orientation and specify the glazing for it

North-facing gives even, cool light all day. South and west face the sun at its strongest, and in an already-warm room that becomes a heat source you will end up covering. The response is glazing specification — low-emissivity coatings and tinted or laminated glass reduce heat gain — plus, in the harder cases, an integral blind or an exterior shade specified up front rather than bought in frustration the following July.

Watch for: Being sold on brightness with no mention of solar gain. Overheating and glare are the most common regrets with skylights, comfortably ahead of leaks, and they are the ones nobody warns you about because they cannot be blamed on the installer.

Cut and frame the roof opening

The deck is opened and the cut rafters are supported with headers at the top and bottom of the opening, and often doubled trimmers at the sides. This is structural work: the roof load that used to travel down the severed rafter now has to travel around the hole. Most jurisdictions permit it, and many want the framing seen before it is covered.

Watch for: Rafters cut without headers, or headers sized by eye. If the crew talks about the framing as an afterthought rather than as the part that needs a plan, stop. An unheadered opening quietly overloads the rafters either side of it.

Set the unit — deck-mounted or on a curb

A deck-mounted unit sits directly on the deck with an integral frame and gives the lowest, cleanest profile. A curb-mounted unit sits on a built-up wooden curb, which raises it above the roof plane and is the normal choice on low-slope roofs and for large or custom openings. The choice is driven by roof pitch and by the unit, not by preference.

Watch for: A curb built from untreated material, or built short. On a low-slope roof the curb height is what keeps standing water and snow away from the seal, and a curb that saves an inch costs you the unit.

Flash it with the kit made for that unit and that roof covering

This is the step the whole job turns on. The manufacturer supplies a flashing kit specific to the model and to whether you have asphalt shingle, tile, metal or a low-slope membrane — a sill piece at the bottom, step pieces that interleave with each course up the sides, and a head flashing at the top with a deflector that sends water around rather than into the unit. It is installed as the covering is laid up around it, in a fixed order.

Watch for: Sealant used as the waterproofing rather than as a supplement to it. A bead of caulk around a skylight is a leak with a delay on it — sealant has a service life of a few years and flashing has decades. Improvised flashing also voids most manufacturer warranties, so the failure arrives uninsured.

Build, insulate and air-seal the light shaft

Where there is attic space between roof and ceiling, a shaft connects the two openings. It is framed, insulated to the same standard as the surrounding ceiling, and — the part that gets skipped — air-sealed on the warm side so household moisture cannot reach the cold surfaces inside it.

Watch for: Insulation stuffed in with no air barrier. Warm humid air reaching a cold shaft condenses, runs down and drips, and the homeowner reports a leaking skylight in a week with no rain. This is the single most common false leak call in the trade.

Finish the interior and hand over the controls

The shaft is drywalled and finished, trim goes on, and any blinds, motors or rain sensors are set up and demonstrated. On a venting unit that means the opening mechanism, the remote or wall control, and the sensor that closes it when it starts raining.

Watch for: No demonstration of the rain sensor and no note of the model number. You want the model and the flashing kit recorded on your paperwork — it is what you need years later for a seal, a blind or a warranty claim.