Glass design considerations for sloped glazing
January 22, 2021
The use of skylights and sloped glazing systems continues to grow in popularity as builders and homeowners seek to incorporate natural daylight in homes and commercial buildings. Sloped glazing is defined as inward or outward angled fenestration installed 15 degrees or more from vertical. Architects and engineers face unique challenges when designing sloped glazing systems.
6 Technical challenges of sloped glazing systems
1. Retention of Broken Glass Fragments
Appropriate glass selection is the most important consideration in sloped glazing design due to the life-safety issue of potential glass fallout after breakage. Overhead glazing has a higher chance of falling from the opening when it breaks compared to vertical glass. The International Building Code mandates the use of heat-treated laminated glass lites where sloped glazing is installed over occupied spaces below (2018 IBC Section 2405 Sloped Glazing and Skylights).
- Laminated glass provides protection from broken glass falling from the opening. In insulating glass units, the lite facing the occupied space is the laminated lite, while the other lite can be tempered or heat-strengthened. The minimum interlayer thickness is 0.030 inch, or 0.015 inch for residential skylights less than 16 square feet, provided the highest point of the glass is 12 feet or less above a walking surface.
- Monolithic heat-strengthened and fully tempered glass installed as skylights must have mesh screening capable of supporting twice the weight of the glazing to capture broken glass pieces.
- Annealed glass is limited to areas where the walking surface below the glazing is permanently protected against the risk of falling glass, and in certain greenhouse applications.
2. Effective Drainage
Water and condensation control can be a challenge in sloped glazing system designs. Exterior water in sloped glazing pockets cannot drain as quickly as vertical glazing. Higher relative humidity levels in the interior air near skylights may form condensation on the glass. Designers specify internal gutters and channels to drain water to the exterior of the building. Condensation may be collected in evaporation trays where the moisture can be evaporated back into the building space.
Midpoint deflection of the glass may cause water to pool in the middle of a lite, potentially leading to visible exterior staining if the glass is not sufficiently sloped to drain. Exterior caps can impede water drainage, but lower profile caps can reduce the blockage and provide a means to redirect water around them. Alternatively, systems are available with structurally glazed horizontal joints to allow for unimpeded water flow down the face of the glass.
3. Mitigation of Thermal Stress
Unlike vertical fenestration, sloped glazing systems often have more direct orientation to the sun, producing a challenge to balance bringing daylight into the building interior with the impact of increased surface temperature and heat radiating into the building. Thermal properties can be improved using high-performance glass and advanced warm-edge insulated glass spacers. Glare and direct sunlight can be mitigated using frits or patterns on the glass to offer a degree of shading and can be combined with low-emissivity coatings to increase thermal efficiency.
4. Analysis of Dead Loads and Live Loads
Exterior skylights and sloped glazing can be subjected to a unique combination of loads, including dead load, snow load and wind uplift. The building codes specify various load combinations that skylights and sloped glazing must be designed to meet. In northern climates, snow load is often the limiting factor, while wind is more likely to be the limiting factor in a warm, sunny climate, particularly in a coastal location. 2018 IBC section 2404 gives equations for calculating loads for sloped glass, wired sloped glass and sloped patterned glass. The load requirements of ASTM E1300 Standard Practice for Determining Load Resistance of Glass in Buildings must also be followed.
As with wind load, the geographic location of a building is only one factor in determining the design snow load. IBC requires design snow loads to be determined in accordance with ASCE/SEI 7 Minimum Design Loads for Buildings and Other Structures, as it does for wind load. Snow drift loading must be included in the calculation as it can often be several times the magnitude of the ground snow load.
Dead loads and snow loads are examples of long-term loads that may be imposed on the glass. When designing laminated glass for sloped glazing, the interlayer properties for the appropriate load durations should be used. Calculation of the design loads on skylights and sloped glazing should be performed by the building designer. AAMA GDSG-1 Glass Design Guide for Sloped Glazing and Skylights provides guidance in applying the complex combination of design loads.
5. Impact Resistance
Sloped glazing and skylights are more susceptible to impact from falling objects and thrown objects than vertical glass. Sloped glazing systems can be designed with increased resistance to water and air infiltration for threats from windstorms and from forced entry.
Incorporating thicker glass substrates with laminated glass and using structural glazing to retain the glass in the framing if breakage occurs can allow sloped glazing to perform in severe weather or forced-entry events. Consult the NGA with GANA Glazing Manual and Laminated Glazing Reference Manual for additional laminated glass strength considerations.
6. Installation Methods
Architectural glass is typically supported by capturing the edges of the glass inside perimeter framing. For installations where improved transparency with reduced visible framing is desired, glass can be point supported using fittings directly connected through holes in the glass.
NGA Glass Technical Paper FB07-05 (2019) Point Supported Glazing gives additional design considerations. For point supported sloped glazing, consult an engineer who is familiar with the use of glass as a structural material to determine the glass thickness required for the size of glass lites and the applicable code-designated design loads.
Reference standards and technical resources
- ASTM E1300 Standard Practice for Determining Load Resistance of Glass in Buildings
- ASCE/SEI 7 Minimum Design Loads for Buildings and Other Structures
- AAMA GDSG-1 Glass Design Guide for Sloped Glazing and Skylights
- NGA with GANA Glazing Manual (downloadable) *
- NGA with GANA Laminated Glazing Reference Manual (downloadable) *
- NGA Glass Technical Paper FB07-05 (2019) Point Supported Glazing (free download) *
*Resources from the National Glass Association are available at the NGA Store. Please login or create an account when prompted to purchase resources as well as to access any free downloads.