January 1, 2003

Don't Trust Lean-To Buildings

Excerpt from Ohio State University

Structures notorious for weakening or even collapsing under the weight of heavy snow are self-constructed "lean-to" structures built onto the sides of other buildings, says an Ohio State agricultural engineer.

Richard Stowell, an agricultural structures and environmental specialist, says lean-to roofs often bear the brunt of snow that drops, drifts, and melts off of taller, adjacent roofs. Another problem is that roofs of lean-to structures often have less slope, which tend to hold snow longer.

As for structural safety on all farm buildings, it's wrong to assume warming weather will automatically solve snow-loading problems, Stowell says. Low-sloped roofs are susceptible to ponding that can weaken them, causing leaks or even collapses. Ponding occurs when ice buildup prevents water runoff and when a weakened section sags under the buildup of snow, ice and standing water.

"For most situations, wind and warm temperatures make the situation less hazardous," Stowell says. "They get rid of the snow that falls on the roofs. But for those unique situations, wind and warm weather can make things worse."

The severity of snow-load problems on farmstead buildings also depends on the snow's moisture content. Six inches of dry, fluffy snow is less of a problem than the same amount of heavier, wet snow.

Builders should know the snow-loading requirements of the local areas, because snowfall patterns vary with geography, Stowell says. Establishing windbreaks to cut the effect of blowing snow can prevent snow loading and drifting problems, Stowell says. Another way is to reinforce weak structures; but that requires knowledge of structural-system dynamics. Beefing up on one part of a roof may shift the load to another area, possibly weakening it.

For more information on this consult your local or state building codes.