# High Slope vs. Low Slope Roof?



## jpranch (Feb 9, 2010)

I'm looing for a definition for high slope vs low slope roofs. Where is the cut off? 2:12, 3:12, 4:12???


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## Mule (Feb 9, 2010)

Re: High Slope vs. Low Slope Roof?

Is this it???

R905.3.3.1 Low slope roofs. For roof slopes from two

and one-half units vertical in 12 units horizontal

(21/2:12), up to four units vertical in 12 units horizontal

(4:12), underlayment shall be a minimum of two layers

underlayment applied as follows:


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## jpranch (Feb 9, 2010)

Re: High Slope vs. Low Slope Roof?

That helps but I didn't put enough information in my OP. I'm working on snow load requirements for code adoption. My target is really low slope of flat roofs in the commencial world.


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## Mule (Feb 9, 2010)

Re: High Slope vs. Low Slope Roof?

Here's something for "not smaller than 3:12"

Maybe this will get you something that you can work with. No? Yes?

R802.10.2.1 Applicability limits. The provisions of this

section shall control the design of truss roof framing

when snowcontrols for buildings not greater than 60 feet

(18 288 mm) in length perpendicular to the joist, rafter or

truss span, not greater than 36 feet (10 973 mm) in width

parallel to the joist span or truss, not greater than two stories

in height with each story not greater than 10 feet

(3048 mm) high, and roof slopes not smaller than 3:12

(25-percent slope) or greater than 12:12 (100-percent

slope). Truss roof framing constructed in accordance

with the provisions of this section shall be limited to sites

subjected to a maximum design wind speed of 110 miles

per hour (49 m/s), Exposure A, B or C, and a maximum

ground snow load of 70 psf (3352 Pa). Roof snow load is

to be computed as: 0.7 pg.


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## mtlogcabin (Feb 9, 2010)

Re: High Slope vs. Low Slope Roof?

Why does the slope of the roof mattter? It is accounted for in the design equation. We use 1607.11.2.1 to NOT ALLOW A REDUCTION in roof loads below what has been previously established by a state university study. Example historical 50 year study says 46 lbs minimum ICC would allow 39 lbs minimum in our area. We require 46 lbs minimum no reductions period.

1607.11.2.1 Flat, pitched and curved roofs.

Ordinary flat, pitched and curved roofs are permitted to be designed for a reduced roof live load as specified in the following equation or other controlling combinations of loads in Section 1605, whichever produces the greater load. In structures where special scaffolding is used as a work surface for workers and materials during maintenance and repair operations, a lower roof load than specified in the following equation shall not be used unless approved by the building official. Greenhouses shall be designed for a minimum roof live load of 12 psf (0.58 kN/m2).

Lr = Lo R1R2	(Equation 16-27)

where: 12 £ Lr £ 20

For SI: Lr = Lo R1R2

where: 0.58 £ Lr £ 0.96

Lr	=	Reduced live load per square foot (m2) of horizontal projection in pounds per square foot (kN/m2).

The reduction factors R1 and R2 shall be determined as follows:

R1  = 1 for At £ 200 square feet (18.58 m2)	(Equation 16-28)

R1 = 1.2 – 0.001At for 200 square feet < At < 600 square feet	(Equation 16-29)

For SI: 1.2 – 0.011At for 18.58 square meters < At < 55.74 square meters

R1 = 0.6 for At > 600 square feet (55.74 m2)	(Equation 16-30)

where:

At	= 	Tributary area (span length multiplied by effective width) in square feet (m2) supported by any structural member, and

R2 = 1 for F £ 4	(Equation 16-31)

R2 = 1.2 – 0.05 F for 4 < F < 12	(Equation 16-32)

R2  = 0.6 for F > 12	(Equation 16-33)

where:

F	=	For a sloped roof, the number of inches of rise per foot (for SI: F = 0.12 ´ slope, with slope expressed as a percentage), or for an arch or dome, the rise-to-span ratio multiplied by 32.


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## GHRoberts (Feb 9, 2010)

Re: High Slope vs. Low Slope Roof?

"My target is really low slope of flat roofs in the commencial world."

Those are flat roofs. They get the usual snow loads with appropriate adjustments.


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## Robert Ellenberg (Feb 9, 2010)

Re: High Slope vs. Low Slope Roof?

"I'm working on snow load requirements for code adoption"

I took a quick look on the internet and there is excellent information on both explaining the variables and even some local code calculations.


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## jpranch (Feb 9, 2010)

Re: High Slope vs. Low Slope Roof?

All good stuff. Thanks to all. Robert, could you copy and paste a link?


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## Robert Ellenberg (Feb 9, 2010)

Re: High Slope vs. Low Slope Roof?

http://www.bgstructuralengineering.com/ ... E70803.htm

http://www.rwdi.com/cms/publications/16/t05.pdf

http://www.nysboc.com/forms/Snow%20Loads.pdf


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## skipharper (Feb 10, 2010)

Re: High Slope vs. Low Slope Roof?

Y'all may also find this interesting.  

http://www.groundsnowbyzip.com/


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## jpranch (Feb 10, 2010)

Re: High Slope vs. Low Slope Roof?

Thanks to all. Been very busy. The next 2 days even more. I'll be off the board for about 10 days starting this Sunday. Be down in Texas warming my bones. jp


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## TJacobs (Feb 11, 2010)

Re: High Slope vs. Low Slope Roof?



			
				skipharper said:
			
		

> Y'all may also find this interesting.   http://www.groundsnowbyzip.com/


These kind of things are nice but I'll bet they don't cover amendments.


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## mtlogcabin (Feb 11, 2010)

Re: High Slope vs. Low Slope Roof?

Agree with TJ. Our 51 lbs ground snow equates to about a 39 lb roof snow load after deductions. That's why we do not allow less than the 46 lbs roof snow load previously established by historical data.


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