Volume 34, Number 1, January 1999

 

Cloudy Skies Above

Preventing Problems That Cloud Skylights

by Robert C. Sampson

When the crew from Warner Moore & Co. showed up to erect an elaborate glass skylight at the main entrance of an East Coast shopping mall, they found a problem: a massive signpost protruding up through the space where the skylight was to go.

They modified the shop-built custom skylight in the field to accommodate the obstruction, obtained additional glass, and flashed around the pole with machine-formed sheet metal and silicone sealant.

"That pole was not supposed to be there," recalls Richard Midgett, president of the Virginia Beach, Va., skylight and architectural specialty firm. "Nobody ever thought about it," he says.

While that’s an extreme example, surprises come with the territory for skylight contractors: many architects, general contractors, and subs are still in the dark about overhead glazing.

Here are some suggestions to help prevent or minimize problems that cast shadows over skylights.

*Enlighten the architect. "We try to educate them on what’s practical to build, easy to erect, and easy to service," Midgett says. "We tell them to design the skylight with units of glass not to exceed 25 square feet each. That way, if glass fails or breaks, I can climb up with a couple men to replace it. Larger units need a crane."

*Develop a preference for prefab. When jobsite layout and material handling requirements permit, use skylights with frames that arrive as factory-preassembled weldments. You’ll get faster installation and fewer errors. It beats an erector set of loose parts that have to be assembled on the job.

"The more prefab that can be done at the factory, the better off everybody is," Midgett says.

Carol Caspell, president of All Seasons Products, Dahlgren, VA, in the Washington, D.C., area, likes one manufacturer’s double-pitched and structural ridge units. Each half of the frame is a prefabricated aluminum grid — "like a ladder," she says — with rafters welded to ridge and sill. The two halves interlock at the ridge. "It’s a nice, simple, brain-dead skylight. Almost anybody can install those."

Welded assemblies provide continuous, seamless condensation gutters and other moisture-control devices free of mechanical splices and sealant. Some designs include welded dams at the locations where one assembly fastens to another to form a longer skylight. Each module’s weep system is self-contained; water never flows across splices.

Prefabricated "apron flashing," used between the skylight’s base frame and the supporting curb, is another plus. It comes preformed where risk of air and water infiltration is greatest with fully welded, mitered corners. When not prefabricated, sheet-metal pieces have to be bent and "gunked" by the installer.

*Curb communication problems. A roof curb constructed improperly is probably the most common problem glaziers encounter. Prior to installation, Midgett recommends holding a "coordination meeting" with the general contractor, the roofer, and the subcontractor responsible for building the curb. Make sure that shop drawings get to the field, not just to the file. Subs must understand that the curb is built to the requirements of the skylight, not vice versa. The curb must be square, true, level, and the right height. You can’t pound on a skylight and wrestle it into submission.

"My biggest concern about doing the project correctly is the superintendent," Caspell says. "Very few general contractors know about skylights," particularly the exacting requirements for roof curbs.

"If the curb is right, that’s the sign of a good contractor," she says. A specialist in skylights and canopies, Caspell vows there are some builders she won’t work with again. "Their projects aren’t profitable, and they have people who don’t know what they’re doing."

*Preview the project. Review shop drawings with your foreman and crew so they understand how the product goes together and how the finished result should look.

Installers should understand the location and function of the skylight’s weep systems for routing condensation from the glass to the outside. These must be kept clear of sealant and debris. Some skylights also have passages to drain the moisture that collects under aluminum retainers and other exterior components. You’ll choke off the flow by caulking the openings at the low side of the retainer near the sill, over-tightening the retainer fasteners, or bending the retainer.

A small but important detail is installation of glazing blocks or stops — aluminum and rubber bumpers inserted in the frame — to support the bottom edges of the insulating glass panels. Skylight glazing is at an angle; one edge is higher than the other. If stops aren’t positioned accordingly, the glass can shift and put undue stress on the edge seal.

Plan on using the sealant recommended by the manufacturer — or an approved substitute. For a given application, some sealants are superior to others. Skylight makers have compiled a considerable body of knowledge on sealants and how to apply them. When installation instructions call for a "bed of mastic," don’t apply a bead of sealant.

It’s critical to use the proper, corrosion-resistant hardware for anchoring the skylight to the curb. Typically this means using a specified size of stainless steel lag bolt. Wind and snow loads combined with the skylight’s dead weight can exert upwards of 2000 pounds of sideways force on the anchors. Without adequate anchoring by the installer, this lateral thrust is enough to push the skylight off the curb.

The logistics and material handling for skylighting are different than those used in curtainwall or storefront work. First, you have to get the materials to the roof. Roofing stones, gravel, curbs and concrete pads are part of an alien lunar landscape that glass handlers must navigate.

Despite the pitfalls, skylight contractors say they enjoy their work. They have a marketable specialty that gives them a business edge. And they feel they’re providing a visible, architecturally dramatic element that’s a source of professional pride — as well as profit.

"I like doing skylights because it’s not a commodity product," says Midgett. "I enjoy watching it go up, then driving by with my children and saying, "Hey, your dad did that skylight."

Robert C. Sampson is technical and marketing director of Wasco Products, Inc., Sanford, Maine, a manufacturer of commercial and residential skylights for more than 60 years.


USG

Copyright 1999 Key Communications, Inc. All rights reserved. No reproduction of any type without expressed written permission.