Snow Loads

[SteelPE]

My local building code states the following:

1608.1 General. Design snow loads shall be determined in accordance with Section 7 of ASCE 7 using the ground snow load values determined by Table1608.1, but the minimum flat roof snow load shall be not less than that determined by Table 1608.1. The design roof load shall not be less than that determined by Section 1607.

The code is a state modified version of IBC 2006. The building is double pitched at 1:12 slope.

Table 1608.1 Ground snow load (pg) = 30psf
Table 1608.1 Minimum flat roof snow load pf = 30psf

There is an argument in my office on how to interpret the section of the code.

1: Calculate the snow loads based off Chapter 7 of ASCE 7. If they are less than the minimum of table 1608.1 then use the minimum of 30psf. If they are greater then use the greater number you calculated in accordance with ASCE 7

2: Calculate the snow load based off Chapter 7 of ASCE 7 replacing the minimum snow loads in section 7.3 of ASCE 7 with the 30psf described in 1608.1.

The question then basically comes down to the definition of a low sloped roof in section 7.3.4.

Which method would the community interpret as being correct?

 

[dik]

Often for residential construction, a roof slope of 3:12 or less is considered 'low slope'. I don't know your application or local considerations.

Dik

 

[steellion]

ASCE 7 defines a "flat" roof as less than 5 degree slope and a "low-slope" roof to be 15 degrees or less. I would use the ASCE interpretation since you are doing an ASCE calc. It seems the local building code simply wants to prescribe a minimum snow load to use for the area.

 

[SteelPE]

Thanks for your replies,

Basically it comes down to method 1 yielding 30psf for a roof snow load and method 2 yielding 21psf for a roof snow load.

It appears what you guys are saying that the calculations should be performed in accordance with method 1 above and end up at 30psf?

I have a call into the local building official and to the state…... nobody is there. My tax money hard at work.

 

[DaveAtkins]

Your interpretation is correct.

But it is interesting that your local Code essentially ensures that ASCE 7-05 flat roof snow load will never control. The local (arbitrary) value of 30 psf will always control.

DaveAtkins

 

[SteelPE]

Finally talked to the guys who wrote the code and this question threw them for a loop. At first they couldn’t understand why there was a question. After laying down the argument they could see confusion.

In the end, they said the purpose/intent of the code was to use a minimum roof loading of 30psf.

 

[RBS01]

This local code minimum roof loading is vague as far as determining your unbalanced or drift loadings say on pitched roof building. Will you use the code min. value or the lesser calculated Pf or Ps value of 21 psf as in the example above?

I remember in some areas (Oregon?) they would allow you to use the calculated roof snow instead of specified minimums for checking drifts and unbalanced snow.

 

[SteelPE]

From what I was told by the people who wrote this section of the code, the intent was to have the designer use ASCE 7 to derive the required snow loads then check to see if this load was greater than the minimum required by the code. Not to replace the minimum snow loads in ASCE 7 with the minimum call out by the local building code.

When would you end up greater than the 30psf? How about when considering snow drift?