Snow loads. Code minimum or do you modify?

[jpranch]

I would like to know how many of you out there "bump-up" or modify your roof loads for snow beyond the code minimum. Do you allow load reductions? I do know that there are many that believe the codes are far too liberal in this regard with roofs going as low as 20psf.

 

[mtlogcabin]

The state allows the local jurisdictions to determine the snow loads. The study done by the University of Montana for the state is for roof loads not ground snow. Using the ground snow produces a roof snowload about 8 pounds less then what we have historically issued in the past plus we have historical data of roof collapses during the 96/97 winter of building that where designed to 40 lbs.

In short our ground snow is 51 but the roof must be designed to a 46 lb minimum snow load.

 

[jpranch]

mt, would you happen to know where I could get a copy of that report? I would think that Montana and Wyoming would be similar except for the latitude.

 

[mtlogcabin]

http://www.coe.montana.edu/snowload/docs/MTSnowloads112105noMaps.pdf

http://www.coe.montana.edu/snowload/

This is specific to each state. Wyoming has additional links

http://www.fs.fed.us/t-d/snow_load/states.htm

This is based on ASCE 7 and you have to have an account

http://www.groundsnowbyzip.com/

 

[TheCommish]

In Massachusetts the state has a list ground snow load for each town, which is different for commercial building and one or two family buildings (the explanation is difficult). For 1&2 family the number is it, look at the rafter span chard and come up with the answer.

For commercial building the RDP starts with the ground snow load and then applies the modification factors in ASCE 7 and allows for snow drift according to the tables

 

[jpranch]

Sorry to all. Left out my intent / question / concern. What about flat roofs? Less than 1:12. Seen way to many of them on the ground the past 25 years or so.

 

[TheCommish]

The ASCE 7 modification factors take into account roof slope, insulation value, and indoor air temp.

One issue we have seen is the increase snow on roofs as the IECC requires or the customer increases the R value of the roof and less snow melts off that had in past on some of the buildings.

 

[Papio Bldg Dept]

For commercial building the RDP starts with the ground snow load and then applies the modification factors in ASCE 7 and allows for snow drift according to the tables

it is the same here too.

 

[codeworks]

in vermont, the state has a drawing for the snow loads in given areas. one issue experienced there, and other places i would think is winter rains. snow can hold an excessive amount of water, and this needs to be considered in determining loads. we don't worry about it much in south texas, though.

 

[Big Mac]

Snow loads here range from 25PSF minimum roof design snow loads to about 450PSF for a fire/ranger station. We get some pretty high hills around here and virtually always have a rain on snow surcharge before it melts.

The 25PSF roof design snow load is minimum and we allow no reductions from that. If the snow load exceeds 25PSF then we can have a discussion about what reductions might be appropriate. There can be reductions for slope and exposure.

Reductions are less likely for those unheated structures where snow is likely to stay on the roofs longer.

 

[Alias]

Roof snow load is 30# with a minimum 4 and 12 slope recommended for residential. Snow load for the city is more restrictive than the CA code due to our location/elevation and city code reflects the change..

Commercial is what the RDP submits.

Sue