The skylight industry installation guidelines published by AAMA are organized into two major groupings: skylight installations on steep-slope roofs (3:12 pitch or greater, about 15 degrees from horizontal up to 75 degrees or 15 degrees from vertical) and installations on low-slope or flat roofs (less than 3:12 pitch).
Installations on steep roofs with asphalt shingles, wood shake or slate, tile or metal are covered; in the case of low-slope or flat roofs, built-up, modified bitumen and single-ply roofing systems are discussed. In general, skylights are usually installed on steep-slope roofs after the roof is finished. For low-slope or flat roofs, the skylight typically is installed before the roof is finished.
Most installations are deck mount, in which the skylight attaches directly to the roof deck, or curb mount, in which the skylight utilizes a curb consisting of 2- by 4-inch or 2- by 6-inch nominal lumber or prefabricated metal. The curb is anchored to the roof deck or framing into the structural supporting rafters to become a permanent part of the roof structure, raising the skylight above the roof deck surface and reducing its exposure to water and snow accumulation.
Sometimes a separate roof curb is used, providing a means of flashing the roof penetration created by a skylight by integrating it with the roofing system. Roof curbs can also be used to modify the slope of the skylight relative to the roof. Curb installation must be verified as level and square.
Roof curbs can be constructed on the job site or prefabricated—often by the skylight manufacturer as an accessory. Steep-slope roofs generally warrant lower roof curbs. Low-slope and flat roofs sometimes require higher roof curbs as water levels on the roof surface increase.
Some unit skylights will have an integral flashing flange that contains and directs water that runs off the slate shingles/wood shakes. Sealant prevents water from traveling beneath the roofing material and underlayment and the top surface of the flange onto the roof deck. Some designs include a specially configured flange that provides this function and eliminates the requirement for sealants.
GENERAL INSTALLATION CONSIDERATIONS
The rough opening must first be cut into the roof deck and checked that it is properly sized, level and square.
When a skylight is to be installed in an existing roof and roof trusses or raf- ters need to be cut to accommodate the position and size of the unit, it may be necessary to temporarily support those sections. Additional structural framing must redistribute transmitted loads to adjacent structural members. In some cases, this will require additional members or reinforcement to bolster load-bearing capacity.
The underlayment or felt is cut back around the perimeter of the opening. For deck mounting, replace the underlayment and bring it up to the edge of the rough opening. For curb mounting, bring it to the base of a curb and up the sides of the curb. A cant strip, which is a beveled support, is often used at the intersection of the roof deck with vertical surfaces so that bends in the roofing membrane to form base flashings can be made without breaking the felt. Using separate pieces of underlayment around the skylight saves the hassle of trying to cut full rolls around the skylight and makes for a more watertight installation.
Adhesive-based underlayment can be more effective in many cases, and some skylight manufacturers require this. Wrapping the skylight with an ice and water membrane instead of the standard underlayment is a good idea and may be required in some areas.
Good skylight design follows the principles of good watertight design. Proper installation essentially means doing it right the first time to ensure the integrity of the roof’s water-resistive barrier.
The following guidelines and other publications are available through the Publication Store:
AAMA 1607-14, “Installation Guidelines for Unit Skylights”
AAMA 501.2, “Quality Assurance and Diagnostic Water Leakage Field Check of Installed Storefronts, Curtain Walls and Sloped Glazing Systems”
AAMA 503, “Voluntary Specification for Field Testing of Newly Installed Storefronts, Curtain Walls and Sloped Glazing Systems”