Tag Archives: flat roof snow load

Torpedo Heaters

Michael Perry is a Wisconsin author, humorist and intermittent pig farmer. Below I have excerpted from his November 10, 2014 article in the Wisconsin State Journal, “Weather just might call for a torpedo heater”. For more about Michael Perry’s work see www.sneezingcow.com

“Nothing captures the changing seasons like that moment when your cousin the carpenter calls and says he wants his torpedo heater back.

I borrowed the heater deep in the heart of last winter, when the snow atop my pole barns had accumulated to a depth I feared might exceed the load-bearing properties of both structures, to say nothing of the benevolence of my insurance agent. I broke out the roof rake and clawed around the edges, but this amounted to little more than a trim, and day after day the snow continued to fall. I contacted a friend who has a roof-clearing service. He and his partner came out and climbed the roof only to dismount and report that not only was the snow several feet deep, it had compacted and hardened to the point that they simply couldn’t budge it.

Now I was really fretting. I called my brothers Jed and John. They understand things like rafters and physics, whereas I tend to “intuit” things like rafters and physics, which often leads to “surprises” and “disappointment.” Both of my brothers have also built their own pole barns. Yah, they agreed, it had been a heckuva winter, but I probably didn’t need to worry, as my shed had been built by an established company and likely had to meet certain standards. I relaxed for a day or two. Then the forecast began to call for another snow dump, and I was back to fretting.

torpedo-heaterWhen my buddy Mills told me he had heard of someone running a torpedo heater in a pole barn until the roof warmed and shed the snow, I thought it worth a try. I didn’t own a torpedo heater, but it’s a rare Wisconsin carpenter who doesn’t own one, and so I gave my cousin a call. Sure enough, he had one and was willing to lend it, along with a can of kerosene. I hauled the heater home, fired it up, and the trick worked a treat: after a few hours, the snowpack calved off the roof of each barn like chunks of sheet cake down a playground slide. I was deeply relieved. (And later that week when the snows came and my brother’s pole barn caved in, I tried very hard not to be deeply smug — it didn’t seem the time to tell him I have a sort of intuition for these things.)”

Michael’s brothers Jed and John may understand things like rafters and physics, however they were taking a huge leap of faith with, “….but I probably didn’t need to worry, as my shed had been built by an established company and likely had to meet certain standards.”

Sadly, just because a construction company is established, does not mean they are meeting certain standards. In many portions of the country, pole barns (more technically post frame buildings) are not required to have building permits, or the permit process is so lax it does not require a plan review or field inspections.

Although Michael’s pole barns are in Wisconsin, here is a story about a Minnesota builder which tells the tale about an established builder: https://www.hansenpolebuildings.com/blog/2014/10/ag-buildings/

In snow country? Concerned about a big snow on the roof of a pole building which may or may not be able to support it (if it didn’t require a permit with inspections, you had better be concerned)? Check out the local The Home Depot® or similar retailer who should be able to set you up with a highly rated kerosene torpedo heater for about $200. It is far less than most people’s insurance deductibles.

Things You Never Wanted to Know About Snow Load

Yes I know, it is white (at least it starts out that way).

From a design standpoint there are lots of things to know about snow loads.

Cautionary Warning: The information contained herein is fairly technical in nature. We use ALL of this information in the design of your new Hansen Pole Building. Some clients will think this is all very cool, for others, it may cause your head to explode. I’ve been waiting three decades to pass along this information to a client, as I’ve always felt the understanding of it is pretty impressive.

1.  GROUND SNOW LOAD (otherwise known as Pg). This is based upon a once in fifty year (probability of event greater than design loads happening is 2% in any given year). The use of unrealistically high Pg values causes issues with the design for drifting snow.

The International Code specifies design snow loads are to be determined according to Section 7 of a document called ASCE 7. This document provides for all roof snow loads to be calculated from ground snow loads, however not every Building Department follows this procedure. When discussing snow load with anyone, it is crucial to have a clear understanding as to if the load is a ground or flat roof snow load.

Pf is FLAT ROOF SNOW LOAD – If, as a consumer, your concern is snowfall and you want to upgrade the ability of your building to carry it, THIS is the value to increase. Often changes of five or 10 pounds per square foot result in minimal differences in cost.

Pg is converted to Pf by this formula:

0.7 X Ce X Ct X Is X Pg = Pf

2. Ce is the wind exposure factor for roofs.

For an Exposure B or C for Wind; Fully Exposed = 0.9; Partially Exposed = 1.0; for fully sheltered (e.g. nestled in tightly amongst conifer trees as an example) Exposure B = 1.2, Exposure C = 1.1 (how you could have Exposure C and fully sheltered is beyond me)

We use partially Exposed (Ce = 1.0 as a default)

3. Ct is the effect of temperature (building heating), where:

Ct = 1.0 for heated structures (climate controlled)

Ct = 1.1 for Structures kept just above freezing and others with cold, ventilated roofs in which the thermal resistance (R-value) between the ventilated space and the heated space exceeds 25h – ft^2 – degreesF/Btu

Ct = 1.2 Unheated

We use Ct = 1.2 as the default value

Most truss designers will use a Ct value of 1.0 or 1.1 in their designs. This results in a decrease in the ability of the roof to carry snow loads. These values should only be used when appropriate.

4. “Is” is the IMPORTANCE FACTOR

ASCE I is a structure which is a low hazard to life in the event of a failure. Is = 0.87

ASCE II residences and frequently occupied commercial buildings (a warehouse or storage building is probably ASCE I) Is = 1.0

ASCE III Is = 1.1

ASCE IV Is = 1.2 (these are “essential” essential facilities – police/fire stations, hospitals)

5. There is also a Minimum Roof Live Load (known as Lr) of 20 psf (defined by Code) (psf = pounds per square foot) which accounts for weights such as construction loads, when Pg values are very low.

Lr is adjusted based upon the area the roof member supports and can be as low as 12 psf, in cases where a roof member supports over 600 square feet of area.

Doing the math, it would be unusual, using the laws of physics, for Pf to be greater than Pg – however, some jurisdictions have established Lr values which defy the laws of physics (e.g. State of Oregon, where most of the state has a minimum Lr of 25 psf – exceptions being some locations along the coast, where it is 20).

From Pf, Pr (Pressure on the roof) values are calculated depending upon whether the roof is a slippery surface or not, whether building is heated or not and the slope of the roof.

The Top Chord Live Load (TCLL) of any roof trusses will be the greater of Pr or Lr.

6. Duration of Load (DOL) for Snow is typically 1.15. DOL can play a part in some snow areas, where the Building Official (BO) has made the determination snow will remain upon the roof for extended time periods. Some Examples of this include Higher elevations in Utah and Kittitas County, WA where the BO has declared DOL = 1.0. In areas with little or no snowfall (where Lr > Pr) DOL = 1.25.

Yes, I know this is a lot of stuff to carry around in your head.  Trust me, I know all too well, and my character analysis consistently reads “does not like numbers”!  All these numbers and “code requirements” are why we not only ask, but insist you must take the page of our quote with the Design Criteria to your building department to get their blessing on it, and ask if there is anything else they require.  With over 7000 building departments in the U.S., it would be the greatest feat on earth if we could keep up with all of them, and which ones change on any given day. My last caution is to be careful when asking your building department about snow load.  Be sure you keep “roof” and “ground” snow loads separate.  Because when it comes to getting your building designed, priced and finally plans signed off by your building department, there is a difference!