roofs, ceilings and skylights

When buying a skylight take into account the type and characteristics of your roof and ceiling, their support system, and the type of material and the pitch of it.

Roof materials and roof thickness

Pay also attention to the roof thickness, which depends largely on the materials it is made from: asphalt and fiberglass roofs are thin and demand a pre-engineered self-flashing (the flashing is the part of the skylight directly installed on the roof deck).

Roof shakes, slate, clay and cement tile (technically called high profile roofing materials) often require a site-build curb, over which the skylight is mounted.

See, for details on skylight flashing: Rectangular Skylights flashing

Roof slope

Roof pitch determines the type of flashing, and eventually the skylight model. Most Velux models, for instance, demand roof slopes from 15º (3:12) to 85º (137:12).

Roof support system

The roof support system is very important when installing a skylight. The easiness or difficulty to install the skylight depends largely on that system.

Roofs may involve different support systems (rafters, trusses…), but the most relevant when installing a skylight is the distance between joists. If the skylight dimensions are larger than the joist distance, joist reinforcing is almost always required. If the skylight fits between two roof joists, the installation will be easier and may dispense with joist reinforcing.

The type of ceiling

The type of ceiling dictates whether or not you need to use a shaft to direct the collected light into the room. A cathedral ceiling demands just a hole: the skylight is mounted right on the roof, and a hole is enough. But other more common ceilings demand either a flared or a straight shaft.

Shafts

Common ceilings demand a flared or a straight shaft, and two holes, one in the ceiling and one in the roof. If you need a large amount of light, choose a flared shaft. You may also use reflective shafts to increase and drive the light provided by the skylight.

Obviously, shafts put some problems and turn the skylight installation more complex, namely because they demand a careful insulation to prevent unwanted heat or heat losses.

Underlayment

For additional water protection, a self-sealing rubberized asphalt sheet is used as an underlay around the skylight.