Design and snow loads: an engineer’s perspective
As a firm, we're working on project number 10,000. We started at 500. I always get a chuckle when someone jabs us with, "You're an engineer, what do you know about construction?" As building inspection engineers, we know a lot, and we see the errors made by generations of those in the construction industry. Our insight is tremendously valuable. Take advantage of it. In years past, building design was typically simpler. When nontraditional designs were contemplated, a qualified architect was engaged. Sadly - particularly in residential construction - too many homes are designed by non-licensed professionals and serious errors result. A client just forwarded me the construction plans for their new home. They raved to me about their builder and the builder's commitment to quality. While that commitment may be true, there are myriad design errors that are evident in the plans. As examples:• The septic design indicates a SHWT (seasonal high water table) of 48 inches below grade. That's not unusual for New Hampshire. As is not uncommon, the home is designed with a basement floor that is nearly 8 feet below grade. There is no provision in the design documents to address this. Sounds like a recipe for a wet basement, right?• The same plans are essentially an overgrown cape. There are obvious errors in the roof framing design that fail to meet current snow load requirements. While I don't expect a catastrophic failure, recent 2-foot dumps in central New Hampshire show that we can have some good snow loads. Further, failure to design properly will at least result in cosmetic damage (drywall separation, for example). Again, such matters are easily prevented.• Vapor control. We've come a long ways in this arena. While lots of the builders and insulators seem to understand the right way to build, I still see too many basement slabs being placed without vapor barriers, and the design submitted by my client has extremely poor detailing of the envelope.For too many years, we've omitted good vapor separation between the living space and the attic which results in mold factories in attics. Too many real estate transactions have been spoiled by this problem. It's a tricky matter, since the moisture capacity of the interior of a building is a function of many factors. All of these matters point to a simple truth. Get a professional on your construction team. I understand that the South Carolina home plans may be pretty, but at least have your trusty architect and engineer do a review to make sure that it will work here in the Granite State. Snow on the roof Our phone starts to ring as snow loads build up. The callers ask: "How much snow is safe on our roof?" The answer is ... it's complicated! There is no generic answer for how much snow a roof can handle. The standard practice for a few decades in New Hampshire was to build our roofs to support 40 pounds per square-foot (psf) live snow load. That's about 4 feet of powder and as little as 2 feet of wet snow. Unfortunately, most buildings constructed during those years (1970-2000) do not even meet the 40 psf requirement. The science of requirements for structures in New Hampshire got a great boost in 2002 when the U.S. Army Corps of Engineers hired a team of engineers to determine what was really necessary. The document they produced gives specific requirements for towns throughout the state. As a result, answering the question of how much snow a roof can theoretically handle has now become town-specific. As an example, the current "ground" snow load requirement for Manchester is 70 psf. That means properly built buildings since 2002 should be safe to support about 7 feet of dry powder. Unfortunately, we regularly inspect buildings (new and old) in the area with roof frames having a safe loading capacity of below 20 psf. Often, this is the result of improper connections and detailing of the framing. As a result, when someone asks, "How much snow can my roof safely support?" The most accurate answer can only be provided by a licensed and qualified engineer. John P. Turner is president of Criterium-Turner Engineers, Goffstown. He can be reached at 603-497-3137 or through Criterium-Turner.com.