Letters

A calculation mistake?

I am a civil/structural engineer who does quite a bit of roof consulting. I have received and enjoyed Professional Roofing magazine for a number of years, and I appreciate its articles and advertising. It is one of the few trade journals I still find the time to read anymore.

As you may have noted, there is an error on page 23 of the November 2003 issue. The sidebar has in its conclusion that, "Therefore, 6 inches of snow created at least 30 pounds per square foot (psf) of additional weight." I believe the author, Conrad Kawulok, president of B & M Roofing of Colorado Inc., Boulder, meant 36 inches of snow rather than 6 inches.

Jim Danner P.E.
Denson Engineers Inc.
New Orleans

Mother Nature may be tough, but she is not as illogical as Kawulok, who concludes 6 inches of snow weighs 30 psf. Everything is OK until he writes "therefore." Fortunately, 6 inches of snow (using his numbers) weighs 5.2 psf or our roofs are really in trouble. If the error was intended to see who read the article, I did.

David J. Fielding, Ph.D., P.E.
Philadelphia

With good reason, Kawulok made a disclaimer at the start of his article. His story is good and thoughtful. His "cowboy engineering" is OK, but his arithmetic is wrong and confusing. If a building in Denver is indeed designed and built to carry a 30-psf load and the density of the snowfall is 1/6 that of water, the allowance snow depth is 35 inches: 30 psf x 6 (density of water/snow) divided by 5.2 (mass on 1 inch of water per square foot).

It appears most of the area had a snow accumulation of less than 3 feet, dangerous only to greater accumulation of drifting snow or buildings that do not meet the reported 30-psf live-load capacity.

Putting roofing workers on severely deflected roof systems (the 2-inch comfort zone) risks their lives unless a competent structural engineer has checked a building first. My compliments to Kawulok for a well-thought-out plan, but he should get help the next time a large snowfall occurs.

Werner Gumpertz
Simpson Gumpertz & Heger Inc.
Waltham, Mass.

Following is Kawulok's response to the letters:

"Thank you for your interest in my recent article. Per the careful observations, the following corrections are in order:

• In the sidebar under ‘therefore,' the reference to 6 inches was a typographical error; this should have been 36 inches.
  
  
• On page 24, my reference to additional weight should be more properly expressed as live load. In this example, the structures were experiencing live loads (in areas without snow drifts) of 30 psf to 40 psf. This amount of weight was at or greater than the building code design requirements for our area of the United States. Roof surfaces with snowdrifts 48 inches to 60 inches deep were subjected to live loads with serious weight considerations.
  
  
• On page 24, the conversions from pounds per square foot and inches to their metric equivalents contained several errors. My apologies for these conversion errors.

"Finally, the comments expressing concerns with putting roofing workers at risk on severely deflected roofs was one we shared, as well. We employed techniques to remove the snow weight methodically using access points aligned over primary structural elements initially as opposed to midspan locations, a method developed with the consultation of a structural engineer. Readers should note assistance, consultation and guidance from structural engineering professionals are a must in these types of critical situations.

"Thank you again for your comments and interest."

PIMA should take some responsibility

I wish to comment on the letter from Jared Blum, president and chief executive officer of the Polyisocyanurate Insulation Manufacturers Association (PIMA), which appeared on page 12 of the November 2003 issue. Blum's letter was in response to polyisocyanurate issues raised by Dick Baxter, president of CRS Inc., Monroe, N.C., in "A different look at polyiso," June issue, page 40. In particular, I wish to address Blum's admonition of roofing contractors to store and handle insulation properly on job sites by using protective tarpaulins or covers. There is no doubt proper storage and handling are necessary to protect insulation from damage by moisture. It also is evident the polyethylene shrouds supplied at the point of manufacture are not adequate to provide long-term protection.

Reputable roofing contractors accept they are responsible for providing the required protection when material is received on-site. However, what Blum and PIMA fail to recognize is the role of the distributor in the transport and storage of material. In a great many instances, the insulation does not arrive on-site directly from the plant but is channeled through a distributor. It is not unusual, in Canada at least, to find rows of insulation stacks stored outside and in distributors' yards protected only by factory shrouds.

PIMA should advise distributors about the need to provide proper storage and handling. In fact, PIMA members should show leadership by making it a requirement of their distribution agreements. Before chastising roofing contractors, PIMA and the polyisocyanurate industry literally should clean up their own backyards.

Peter Kalinger
Canadian Roofing Contractors Association
Ottawa, Ontario, Canada