For Home Owners

Adding a Skylight: What is Required?

Aug 08, 2019
Adding a Skylight: What is Required?

Skylights supplement your windows as a source of natural daylight. They brighten the parts of your home that windows can‘t, and reduce your reliance on artificial lighting. If operable, skylights vent moisture out of your bathroom and allow in mold-killing light.

Many view skylights as luxury items, ones that may score your home some points in the eyes of prospective buyers. Bright interiors are appealing, and a skylight‘s ability to illuminate an otherwise dark space may be an important selling feature of your home.

If you‘ve decided to add a skylight, you have homework to do. Skylight installation may entail penetrating your building‘s envelope, modifying the roof structure and allowing in additional solar heat. The location of your skylight, and the quality of installation are important factors in minimizing heat gain and safeguarding your home against the elements. It’s also vital to ensure compliance with the state and local codes that govern skylight additions, and check that the skylight and its opening are properly supported..

Skylights come in plenty of shapes and types, and some may be better suited for your home than others. You can determine suitability based on the skylight‘s mounting, material, shape, and operability.

The guide below highlights the factors that you should take into account as you plan to install a skylight in your home.

Mounting

Skylights can be either curb or deck-mounted. The former entails building a curb around the opening proud of the roof plane and fastening the glazing to its outer edge. This is the most common skylight configuration. It allows for an easy replacement of glass without disturbing the flashing and roofing membrane. It also affords the greatest variety of sizes, and helps avoid leaks. The downside of curb-mounting a skylight is the extra labor and materials involved in building the curb and flashing the opening.

Deck-mounted skylights are attached directly to the roof. This configuration is easier to install and offers a lower profile, which may be more aesthetically pleasing and energy efficient. The downside is a more challenging replacement procedure, as the entire assembly must be removed and a new one installed and flashed. Deck mounting also doesn‘t work with flat roofs, as rainwater can accumulate and submerge the skylight.

When deciding which type of mounting to proceed with, consider the following California Building Code requirements:

  • skylights set at an angle less than 45 degrees from the horizontal plane must be curb-mounted, at least 4 inches above the plane of the roof
  • deck-mounted skylights not allowed if installed on a roof where the pitch is less than 45 degrees, except in R-4 occupancies
  • in homes with two or fewer dwellings, deck-mounted skylights are permitted in roofs with a pitch steeper than 14 degrees
  • plastic skylights shall be curb-mounted, at least 4 inches above the plane of the roof, except on R-3 occupancy roofs with a pitch steeper than 25 degrees

Materials

Skylights may be glass or acrylic. Glass skylights are more energy efficient, less vulnerable to discoloration, better at canceling outside noise and are less likely to leak or allow in drafts. Acrylic skylights may discolor and get brittle over time; the thermal expansion and contraction they undergo also wears out the gaskets that seal them in the frame.

Poor fire resistance is another flaw of acrylic skylights. If you have your mind set on this material, make sure you take the following CBC requirements into account:

  • plastic skylights shall resist ignition when exposed to the edge of a flame
  • flat plastic skylights shall not slope less than 18.4 degrees
  • dome-shaped plastic skylights shall rise above at least 10% of the dome’s width but not less than 3 inches above the mounting flange
  • plastic skylight area within the curb limited to 100 ft², except in buildings equipped with automatic sprinklers
  • plastic skylights must be at least 4 feet away from each other, except in buildings with an automatic sprinkler system or in homes with two or fewer dwellings where multiple skylights serve the same room
  • skylights of any material shall be at least 6 feet away from fire rated walls

Operability

Skylights may be fixed or operable. Fixed skylights cannot be opened, but offer the best protection against leaks and drafts. They are also less expensive than their operable alternatives.

Operable skylights come with either automatic or manual operation. Manually operated units are cost efficient but require easy access to the closing mechanism. Automatic skylights offer the advantage of remote operation, but come with a substantial cost premium, and may experience more frequent breakdowns.

Operable skylights’ proximity to rooftop vents is governed by the California Plumbing Code. Per this ordinance, operable skylights should be at least 10 feet away from plumbing vents or vents 3 feet higher than the skylight, and at least 3 feet away from air vents that service stove fume hoods and bathroom fans. Operable skylights must also be 6’-0” from any fire-rated walls, which typically occur between units or near property lines.

Shape

There is a wide selection of shapes in skylight design. Glass skylights often come with a square or rectangular frame, but the shape of the light itself can vary. Pyramid, square, barrel vault, ridge, and other, custom shapes are available for your selection. Acrylic skylights are typically dome-shaped.

Tubular skylights are a cost-effective solution where you have to channel light from the roof opening to your ceiling below. With standard fixed skylights, you need to construct a light shaft from the skylight to a fixture in your ceiling. A tubular skylight comes with a reflective light tube, saving you the trouble of building such a light shaft.

Impact On the Envelope and Energy Efficiency

The roof is an integral part of your building’s protective barrier. Of all the envelope components, it is subject to the most abuse by rain, wind, and snow. Penetrations are inevitable during skylight installation, thus you have to ensure the roof continues to keep the elements out of your home after work is complete. This integrity is best preserved during the installation by bringing the roof membrane to the top of the curb, then cover the edges with metal flashing. It may seem like a DIY-type project, but if you want to keep water out of your home, let a qualified contractor install the skylight for you.

Title 24 sets out requirements aimed at energy conservation and green building design. These standards apply to a wide-range of building systems, including skylights, and govern the resistance to heat caused by sunlight (Solar Heat Gain Coefficient), the rate of heat loss (U-Factor), and the total area of glazing. They are as follows:

  • skylights < 16 ft² and replacement skylights - maximum U-Factor and Solar Heat Gain Coefficient (SHGC) are 0.55 and 0.30
  • skylights > 16 ft² - maximum U-factor and SHGC are 0.32 and 0.25
  • up to 3 ft² of tubular skylights with dual pane diffusers are exempt from U-Factor and SHGC limitations
  • total area of glazing, including skylights, limited to 20% of the building’s conditioned area
  • total area of glazing facing West, including skylights, limited to 5% of the building’s conditioned area
  • Climate Zones 1, 3, and 5 are exempt from the West-facing area and SHGC limitations

As these requirements impact the size, quantity and materials of your skylights, ensure you understand them before proceeding with your project.

Structural Implications

Your skylights will be subjected to many of the same loads as your roof. Per the CBC, the frame of the skylight should be strong enough to resist its tributary loads as well as meeting the requirements listed below:

  • aluminum panels in skylights not supporting the edge of glass shall not exceed a load deflection of 1/60
  • continuous aluminum structural members supporting the edge of glass shall not exceed a load deflection of 1/175 for each glass lite or 1/60 for the entire length of the member, whichever is more stringent

Another factor to keep in mind is the potential alterations to the roof structure. Your rafters may have to be modified if the skylight doesn’t fit between them, or the location may need to be moved if a structural beam is in the way. In either case, seek the advice of a structural engineer before you commit to the project.

How We Can Help

If you are thinking about adding a skylight to your home, we can help. Our expertise in structural engineering and remodeling can help you plan around all the variables of skylight additions. Call us at (877) 892-0292 for a FREE consultation and quote.

Sources:
[1]https://archive.org/details/gov.ca.bsc.title24.2016.02.2
[2]https://archive.org/details/gov.ca.bsc.title24.2016.05
[3]https://archive.org/details/gov.ca.bsc.title24.2016.06
[4]https://www.pge.com/myhome/edusafety/workshopstraining/pec/toolbox/arch/climate/index.shtml

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