Snow load thermal factor Ct

[shaneelliss]

I have always thought that the thermal factor Ct for determining roof snow loads should be taken as 1.1 for most homes. From what I understand Ct=1.1 when the roof is ventilated and the insulation between the roof surface and the heated area is greater than R=25. I live in Utah and the roofs typically have an insulation value of R=38 and have standard roof vents. But I review plans and nearly all the engineers in the area use Ct=1.0 when determining snow loads on these roofs. Am I wrong in assuming Ct should be 1.1? What am I missing?

 

[MiketheEngineer]

Check the IBC 2009 code - it is in there.

 

[shaneelliss]

MiketheEngineer, I am not quite sure what you mean or where you intend me to look. IBC 2009 Section 1608 just says to use ASCE7 for determining snow loads. ASCE7 says that ground snow load should be multiplied by several factors to determine roof snow load. Ct is one of those factors and that is where my original question comes from.

 

[DST148]

@shaneelliss: Ref ASCE 7-05 - The commentary throws more light on thermal factor, especially in reference to dwellings.
By the way, which is the governing code? IBC or IRC? I have not used IRC. But many times the load requirements are quite different in the two codes.
Given your location of Utah and assuming IBC is the governing code, R > 25, I would use Ct = 1.1.

 

[shaneelliss]

The IBC is typically the governing code. Very few houses I deal with are designed using the IRC.

Does anyone see any reason why so many engineers in the area would use Ct=1.0 for the conditions described above?

I had finally decided that those who use 1.0 are wrong and I was going to start being nit-picky about it but then thought maybe I should find out what others thought about it. In my county we have snow loads that exceed 200 psf in some places. So thanks for any comments.

 

[mbullism]

I use Ct=1.1 for attics, 1.0 for cathedral ceilings...

 

[shaneelliss]

Just FYI in case anyone in Utah had read this thread, the Utah building code amendments (section 201 #53) eliminates the intermediate value of Ct=1.1. It basically says that Ct can be taken as 1.0 for everything except unheated structures.

 

[focuseng]

I only use 1.0 if I have a "hot roof". No venting (at all) and the insulation is airtight. Otherwise I assume 1.1 even with cathedral ceilings. Also some risk in residential design that nothing will be insulated the way the architect drew it so 1.1 is good bet when the client gets the budget # from the contractor for spray foam insulation. My region has Pg = only 25-30 psf so it is not too big a deal. Use 1.2 on a garage or out building with no heat.

MAP