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Installing Skylights in Single-Family Homes

Many residential remodeling projects begin with the idea of brightening up the home, bringing in more sunlight and fresh air. Increasingly, homeowners also are choosing improvements with long-term value that help conserve energy and environmental resources.

Skylights offer homeowners the benefits of daylighting in the form of top-lighting, helping lower energy use and costs by reducing the need for electrically powered light. High thermal performance skylights also save energy by managing solar heat gain. In warmer climates and seasons, operable skylights also provide natural ventilation to reduce HVAC use. The U.S. Green Building Council’s LEED for Homes certification program recognizes these advantages, as well as the benefits of outside views of the sky.

Single-family homes’ skylights may be fixed or operable and glazed with glass or plastic. They are commonly framed in aluminum; vinyl; fiber-reinforced thermoset profiles (FRP); steel; wood; or wood clad in FRP, vinyl or aluminum. They may be offered in a breadth of aesthetic options and accessories and may meet a variety of performance requirements.

Qualified contractors and installers believe code compliance is not optional. It is essential to sustain the integrity and performance of the entire roof system. To ensure all warranties are honored, product selection and application must comply with the requirements and guidelines of the roofing system manufacturer. Following are points remodelers should bear in mind when installing skylights.

Pre-installation

The manufacturer may wrap skylight glazing in protective plastic. The unit should not be kept in direct sunlight because the protective plastic may melt and damage the product. Additionally, skylights should not be set directly on the roof or other exterior flat surface exposed to direct sunlight without allowances for ventilation because heat build-up may cause damage to the unit.

The skylight installation location needs to be clearly identified with the rough opening constructed in accordance with the manufacturer’s installation instructions. If a site-built curb is used, the top should be square and in the same plane. In all cases, the roof deck and curb must be structurally adequate and provide appropriate means for anchoring the skylight in place.

When installing a skylight, it may be necessary to cut a rafter. When roof trusses or rafters need to be cut, temporary support is required for those sections. Constructing additional structural framing is necessary to provide proper support beneath the skylight opening and redistribute loads to adjacent structural members. In some cases, the load shift to adjacent structural members will require additional members to bolster load-bearing capacity.

Before modifying the roof structure, consult a qualified engineer to comply with applicable codes and roof load requirements. AAMA’s website provides a list of links for energy codes and state codes (aamanet.org/general/2/113/state-code-web-links).

Skylight applications are categorized as curb-mounted or self-flashing.

  • Curb-mounted: A skylight frame design that enables installation and anchoring typically to a 1 1/2-inch-wide flashed wood curb, which is a permanent part of the roof structure. This type of installation is typical on a low-slope roof. Generally, curb-mounted skylights are installed after roof construction.
  • Self-flashing: A skylight base that mounts directly to the roof without requiring a curb.

 Curb-mounted 

Curb-mounted skylights are installed on a premanufactured or site-built curb, which is provided separate from the skylight. After the curb and roofing materials are prepared and installed per the roofing material manufacturer’s recommendations, the preassembled skylight is attached.

Remember:

  • Most curb-mount unit skylight dimensions are given as outside curb dimensions and inside curb dimensions. You must consider the thickness of the curb that will be used and how it attaches to the roof and cut the roof opening accordingly.
  • Some areas of the country, such as high-wind or hurricane areas, may require special attachments of curbs to the roof. Be sure the curb is square and of adequate height to set the skylight to the proper code-required height above the finished roof.
  • After the curb is properly attached to the roof, the roofing material should be properly terminated and/or flashed per the roofing material or skylight manufacturer’s instructions. Most roofing material suppliers have a preferred method of terminating their materials to maintain their warranties. Most skylight manufacturers design a clearance between the outside face of the curb and skylight frame of 1/2 to 3/4 inch to allow for the installation of roofing materials and flashing.
  • When running the roofing material up the side of the curb and lapping it over the top of the curb, avoid excessive lapping because this uneven surface could prevent the skylight from properly sealing to the top of the curb, allowing excessive air infiltration and possibly water leakage. However, most skylight manufacturers do not recommend lapping the roofing material over the top of the curb.
  • A 3/8-inch nominal diameter bead of sealant should be applied on top of the curb. Hold the sealant back from the outside edge 1/2 inch. Select a sealant that remains flexible and meets AAMA sealant standards. Always check the compatibility of the sealant with the roofing material and skylight framing material. Follow the sealant manufacturer’s guidelines with regard to application temperature, surface preparation, application technique, compatibility, sealant location and dimensions, and movement capability.
  • The appropriate type of corrosion-resistant fastener needs to be thoughtfully selected for the substrate. Fasteners should conform to one of the requirements in the “Fastener Requirements” table, page xx.
  •  Place the fasteners in all the prepunched mounting holes to prevent the unit from blowing off in high winds. Look for indications on the skylight showing which side of the unit is to be located down-slope for proper drainage.
  •  Preassembled unit skylights do not require additional sealants and using them could block weep systems and create leaks. Do not attach roofing or apply sealant to any part of the skylight unless specifically instructed by the skylight manufacturer.

Self-flashing

A self-flashing skylight is supplied with its own pre-attached curb. These curbs can be single- or double-wall, insulated or uninsulated, and can be purchased in various heights. Prepare the roof opening per the skylight manufacturer’s dimensions.

Remember:

  • Most self-flashing skylight dimensions are given as inside curb dimensions or actual roof-opening dimensions.
  • If mounting the unit to a solid surface, such as wood decking, apply a 3/8-inch nominal diameter bead of sealant on top of the decking. Hold the sealant back from the inside edge 1/2 inch.
  • Properly attach the self-flashing unit to the roof deck or roof structure through the flange of the curb or mounting bracket using a corrosion-resistant fastener (see Fastener Requirements table, p. xx) for the substrate.
  • Place the fasteners at the spacing recommended by the skylight manufacturer to resist the design-uplift pressures for the region. High-wind- or hurricane-prone areas may require special attachments of self-flashing units to the roof. Be sure the unit supplied is of adequate height to set the skylight at the proper code-required height above the finished roof.
  • After the self-flashing unit is attached to the roof, the roofing material should be properly terminated and/or flashed per the roofing material skylight manufacturer’s instructions. A positive seal of the roofing materials to the skylight curb is critical for a watertight installation. Ensure the roofing material is not installed so it blocks the weep system of the skylight. These systems should always be above the top of the self-flashing curb.

Special Considerations

Remember for a low-slope roof:

  • Special consideration should be given to curb height. Snow, slush or rainwater build-up may require a 6-inch or higher curb. Also, building codes require skylights to be a minimum height above the finished roof, nominally 4 inches.
  • Most glass manufacturers require a minimum slope (approximately 3-in-12) at which they will warrant their glass products. Tapered curbs may be required.

 

Remember for a steep-slope roof:

  • Special consideration should be given where the skylights are placed with large areas of roof above them. These placements can lead to excessive amounts of water and/or sliding snow accumulating above the skylight. Diverters move this water and/or snow around the unit so it will not pool above the unit.
  • A higher curb than the minimum required by code may be needed for some steep-slope applications.
  • When possible, place the skylight on the roof with the short side running perpendicular to the slope to avoid larger areas where water and/or snow may accumulate above the unit.

 

Post-installation

Even after a qualified contractor has properly installed a reliable skylight, a homeowner may still think the unit is leaky. Identifying the source of the leak is key to quickly resolving a complaint. If possible, observe the leak closely while it is occurring.

  • If the leak occurs below the skylight frame on a curb-mounted skylight, the leak is likely associated with the roof and/or skylight installation. If the leak occurs at the skylight’s condensation gutter, the skylight should be checked.
  • If the leak occurs below the mounting flange or flashing on a self-flashing skylight curb, the leak may be associated with the roof and/or skylight installation. If the leak occurs from the condensation gutter and/or between the skylight frame and its pre-attached curb, the skylight should be checked.

 

If in either case the skylight is believed to be leaking, consider:

  • Broken glazing material can cause skylight leaks.
  • In older skylights, check if the weep holes are free to drain. This may require looking at the skylight from the inside or even disassembling the unit. Be sure to contact the skylight manufacturer before performing any disassembly.

 

Water Testing

When water testing a skylight for leaks, always begin at the lowest point of intersection between the skylight curb and roof and work upward. Flood the roof around the skylight or run water at the base of the skylight for about 15 to 20 minutes; some roof leaks take time to emerge. After completing the roof test, move to the skylight. Skylight leaks will generally show up almost immediately. Always have a spotter inside the building with access just below the skylight so the source of the leak can be specifically detected.

 

Negative Pressure

Negative pressure in a residence is a condition in which the home’s exhaust system or air-handling unit is removing more air from the space than can be replaced through normal openings. This forces air and water to be pulled into the building through every available hole or crack, including the weep system of the skylight. If the negative pressure is high enough, it can actually prevent water that infiltrated the skylight from draining out through the weep system, as well as pull water in through the weep system, thus causing the skylight to leak over its condensation gutter.

To prevent skylight leaks caused by negative pressure, the pressure within the home must be reduced or eliminated by providing alternative sources of air infiltration or adjusting the home’s air exchanges.

Final Reminder

Skylights are designed to withstand typical environmental conditions. Unless specifically designed for it, standard skylights are not intended to withstand human impact or falling objects. Although some skylights are more impact-resistant than others, never walk on them and always exercise caution near them. Access should be restricted to authorized individuals who have been adequately cautioned. Protective guardrails or screens around the skylights may be a consideration for those that are easily accessible.

 

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