Q. I’m doing a small strip center and my home at the same time. Both are being designed with flat roofs. The concern is that the roof areas are pretty large, mostly so I can divide the stores into different areas depending on how much a tenant takes. I also want large spaces for the house. I’m concerned about the weight of snow. I’ve experienced some creaking in another store, and had an engineer look. He confirmed that the structure wasn’t meant for heavy snow. He also said that the roof should have been at a greater angle. It was almost flat. Is there a rule of thumb for how flat a roof can be or shouldn’t be, and do you advise steeper roofs and maybe a different kind of roof? The house plans are done, but I’m concerned about the insulation, since it’s been so cold lately. I might as well add more. Does this also have an effect on the weight?
A. I often tell clients that if you have an insufficient foundation or roof, it really doesn’t matter what’s in between. I wish more clients were concerned about insulation and the pitch of their roofs. I avoid nearly flat roofs, given a choice, because standing snow, ice and rainwater cause many problems.
I once got a call from a checkout cashier on a day of heavy snow, who said she’d heard the ceiling “creaking and popping.” The head of the produce department was a client whose home we were renovating, and he told her to call me. I was looking at the snow coming down when she asked me how quickly I could be there to take a look. Instead, I told her to grab her coat and get everyone out of the store. She called me from her car to thank me, because everyone headed for the door, and within seconds, the roof collapsed right where she had be standing when she first called. It made the news that night.
I often see where loads for things like high wind or snow are assumed or even ignored in the field, based on common practice. In other words, the plans may have been thoroughly checked, engineers’ calculations done and connections detailed, but, like any part of any building, it’s only as good as the last hands that touched the construction. If field decisions altered any part or if someone took the opportunity to interpret for themselves something not spelled out completely, the performance in snow, or for that matter, in any extreme weather may be in jeopardy.
Safety factors for anything are a must. The safety factor for bridge design is 100 times the minimum strength, yet building structures are often barely over the minimum. When non-professionals offer their gut opinions without any math or science to back them up, you need to consider the source. Put in as much insulation, as much slope and as much waterproofing as possible. Be redundant and safe.
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