×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Snow-shelf structure

Snow-shelf structure

Snow-shelf structure

(OP)
I have a new roof that is approximately 3 metres higher than an existing roof. I have looked at many options to address the issue of accumulated snow load that would result on the existing low roof, but found that the most economical approach is to build a snow shelf. This structure is in Canada.

The snow shelf will be supported entirely by the new roof and will extend above the existing low roof by a distance of approximately 3.5m. I am adding a high parapet on the new high roof to reduce the required projection of the snow shelf to 3.5m.

What is the criteria for locating the height of the snow shelf?

Top of deck to match height of the basic snow load of the low roof?

Underside of shelf structure to match height of the basic snow load of the low roof?

RE: Snow-shelf structure

I've always thought your snow shelf must essentially act like a bridge roof such that the high roof doesn't cause buildup on the shelf, and the shelf doesn't cause buildup on the low roof.

RE: Snow-shelf structure

I was told that the profile of the shelf should be of the same slope as the snow drift that would otherwise be there, although not necessarily at the same elevation. The idea being that slope would be the one that, aerodynamically, would result in lamellar wind flow over the roof surface and no snow deposition. Frankly, the whole thing's always seemed like a bit of a theoretical stretch to me. I'd love to see some research on it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Snow-shelf structure

At least here in the US, ASCE 7 doesn't really suggest that an upper parapet affects the drifting below. Maybe in the Canada codes perhaps.

Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies

RE: Snow-shelf structure

Is there a horizontal separation between new/old buildings? If not, how do you get away from windward drifting? A sketch would be awesome since I have never heard of this before.

RE: Snow-shelf structure

If my memory serves right, per Canadian Code, height of upper parapet does affect leeward drift on the lower roof.
@GalileoG>>>> You can avoid leeward drift on the lower roof by building a new screen wall / parapet of sufficient height.
However, as BadgerPE posted above, you still need to deal with windward drift on the lower roof (because of higher adjacent new construction).
Does the existing lower roof have parapet? This may provide some relief.
Qualitatively, the build-up of the windward drift on the lower roof will be reduced depending on the projection of the new snow shelf and the reduced length of the lower roof(lu per ASCE 7) upwind of the drift.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources