Defining the Red Line for a Potential Roof Collapse

Collapsed roof

Roofing systems can suffer damage in several ways including falling branches, being blown off by high winds, and being damaged by debris that blows across the roof’s surface. While any of these events can be considered as catastrophic, many of the most devastating incidents are roof collapses caused by excess weight bearing from accumulated snow or water. Here are several measures than can define the “red line” for trouble and help to avoid a potential roof collapse.

1) Develop an understanding of the weight bearing capability of your roof – Generally speaking, an undamaged roof should be rated to support at least 20 pounds of snow or water per square foot. Areas with higher amounts of snowfall may require roofs that are rated for much heavier weight loads. The requirements for weight bearing capacities in your area can determined by consulting local building codes or by calling local roofing companies.

2) Learn the dynamics of how efficiently your roof sheds water – Snowmelt and rainfall are shed most efficiently by roofs with a pitch ratio of 3 to 12 or higher (an increase in slope of 3 inches for every 12 inches measured on a horizontal axis. There is a direct relationship between increasing pitch ratios and the efficiency of drainage off of the roofing system. The risks of inefficient drainage increase as the pitch ratio of the roof decreases from a 3:12 ratio, with flat roofs having the highest propensity for accumulated snow and/or ponding water.

3) Know the approximate weight that your roof is bearing at any given time – Water at 1 inch deep weighs approximately 5 pounds per square foot, meaning that trapped water that is more than 4 inches deep will be pushing the red line for weight bearing capability. A foot of fresh snowfall also weighs about 5 pounds per square foot, which would put the red line at an accumulation of 4 feet. Packed snow is 3 to 4 times heavier than fresh snow, stressing the roof at a depth of one foot. The weight of ice on a square foot basis is roughly equivalent to water, so a thickness of anything more than 4 inches can put stress on the structure.

4) Take action if you see accumulations at levels that may be approaching the red line and stressing the structure – The most common cause of pooled water is clogged drains, so clearing them may provide a quick fix. If accumulated snow is the issue you may be able to remove it using an extendable snow rake.

Knowing where the weight bearing red line is for your roof can lead to fast actions that can prevent a catastrophic collapse. Be aware that going on the roof to deal with weight bearing issues is extremely dangerous, so if you can’t reach drains when standing on a ladder or the snow rake is too short to cover the whole roof, bring in professional roofers to lighten the load immediately.