Where two roofs meet

-By Oliver Witte –

Proper engineering is needed to prevent potential collapse from snow load.

Consulting engineer William Lotz, P.E., Acton, Maine, tells the story of the unsuspecting contractor who put up a building 12 feet high. Everything was fine until the owner’s plans changed and a second contractor was called upon to attach a second building to the first. That might have worked fine in the South but these buildings were smack-dab in the middle of the Snow Belt—the heavy snow belt—and the second building was 20 feet high.

Then it starts to snow. By the time the storm clears, it has dumped four feet on snow on the buildings. The problem was that the taller building has shed its snow onto the metal roof of the shorter, adjacent building, which now has to support eight feet of snow—its own four feet plus the four feet of snow that has slid off the metal roof of its taller neighbor.

Pity the contractor who built the first building when its roof collapses and the owner comes looking for whom to blame. The easy target is the first contractor because, you know, it was his roof that collapsed.

Not so fast, Lotz said. The first contractor had no idea that a second building might be coming along.

Lotz pointed the finger at the contractor of the second building.

“If you’re attaching a taller building to a shorter building, this difference in elevation causes drifting of snow on the shorter roof, causing it to collapse,” Lotz said. “The second contractor, who put in the structural steel, should have gone back to the first building and substantially reinforced the structural steel supporting the roof of the first building.”

Building in snow country can be especially hazardous because of the tendency of snow to drift and apply non-uniform loads —perhaps double or triple what might be anticipated. The wind can whip a two-inch snowfall into a load of six feet. A design snow load around 70 or 80 psf would not be unreasonable.

Ice creates even more problems in snow country. As snow piles up on a roof, it melts and forms ice dams that can create ponds three inches deep or more.

“Metal roofs leak under these circumstances,” Lotz said.

Lotz cited the case of a building with a roof span of more than 200 feet. The architect designed a metal roof with a slope of one-quarter inch per foot. Come February, snow piled up and began to melt, ice dams formed and a pond three inches deep developed. The contractor, of course, was sued, but Lotz blamed an inadequate roof slope. A quarter-inch slope might work down south but leaks should be expected farther north.

“We cannot think of any circumstance where we would recommend a standing seam roof with a slope of less than one inch per foot,” Lotz said.

To add slope to an existing roof, Lotz suggested a retrofit roofing system like that offered by Roof Hugger and others, adding a sloped roof above the existing roof.

An alternative solution involves EPS insulation between the flutes, polyiso insulation over the top and a single-ply roof membrane over the insulation.

Condensation can pose additional problems – even in temperate climates. Dripping can result from condensation due to overlapped or rolled and stapled vapor barrier seams. When the fiberglass facer is only overlapped or folded and stapled and the relative humidity inside the building passes 30 percent, condensation and dripping should be anticipated, especially in buildings that require high humidity, such as printing plants.

“Moisture rises up and finds all the holes and joints in the fiberglass, goes through them, hits the cold metal roof and condenses,” Lotz said. “Then it starts raining inside.”

“The vapor barrier must be sealed and airtight in its entirety,” Lotz said. “The seams and penetrations must be sealed at the floor-wall juncture, at the wall-roof intersections, etc.”

But getting an airtight seal isn’t easy, Lotz recommends foam-insulated panels. Several pressure-sensitive tapes match the insulation vapor barrier facer.

One of the types of buildings in every township is the highway garage. They are almost always metal buildings, Lotz said, and almost always insulated with fiberglass 60-inch wide blankets with a white vapor barrier. Trucks plow snow all day (or night in severe storms) and come in to park in the heated building. The snow they bring in melts and drips down onto the floor. The heat from the floor slab evaporates the moisture from the melted snow and ice, the humidity rises and finds all the holes and joints in the fiberglass vapor barrier. Then it starts to rain.

Lotz’s example is not far-fetched. He tells this true story:

A new suburban garage in northeastern Pennsylvania was nearing completion when the director of public works noticed an electrician connecting a big piece of equipment near the ceiling.

“What’s that?” the director demanded.

“It’s for ventilation,” the sub replied.

Furious, the director demanded that the installer cease.

“You are NOT to finish connecting that,” the director ordered. “I don’t want that system. There is no need to ventilate fresh air here. We open the doors to let trucks in and out and that’s plenty of ventilation.”

“OK,” said the electrician, and everyone was happy—until the arrival of the first winter, and it started raining inside the building.

The usual litigation was filed against the usual suspects. The builder, as is typical, was first in the line of fire. The director accused the builder of incompetence in installing the insulation vapor barrier. The judge disagreed, and the only parties that came out ahead on this project were the lawyers and the expert witness/consultant.

Other problems include either missing or inadequate collar ties, rot and inadequate trusses. Collar ties keep a sloped roof from collapsing.

When asphalt shingles leak on an old building, rot develops at the eaves and ridge. A five-foot deep snow load might be sufficient to bring down the building. The fault probably is with the owner who has been ignoring the normal maintenance requirements and thus should not expect the insurance company to have to pay for the cost of replacing the roof, Lotz said.

Trusses will be adequate if the building code is followed but some builders will skimp and install trusses with a lesser rating. Combined with a two-level roof, snowdrifts of six or eight feet and the result is not pretty, Lotz said.

Lotz insists that building collapses, indoor rain and related problems are not inevitable with metal roofing in snow country. The solution is to anticipate the problems and solve them during the design phase.

“Metal roofing systems have a good track record of performance in harsh winter environments—as long as they are properly designed,” Lotz said. “When a roof fails—metal or otherwise—the first party to get blamed is the contractor, whereas the cause of the failure is generally the fault of the designer. The key factors are usually snow loading requirements, adequate slope and the use of snow guards when needed. If these issues are addressed during the design stage and the roofing contractor does his job, these systems will almost always perform as intended.” RB

Related Posts:

COMMENT