Return to index: [Subject] [Thread] [Date] [Author]

Re: Question for the snow gurus

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Dan

A number of the other options for retaining snow on the roof include
'snowclips' which are more common on metal roof systems.  These have
produced very mixed results.  Most standing seam concealed fastener roof
systems have a continuous sheet metal angle running accross the slope
that is attached to the standing seam with a friction connection.  These
have not performed as well as desired.  Several sites that I have
visited in the spring find the clips laying on the ground along with the
snow that shed from the roof.  Some others are plastic clips that are
bonded to the roofing surface.  These likewise have not been successful
for various reasons.  The added concern with using 'snowclips' to retain
snow on a metal roofing system is the attachment of the metal roofing to
the structural deck.  As you have found the loads are significant.  Most
metal roofing is only rigidly attached at one end of the roofing sheet
with the rest of the connections being a slip connection so the panels
can thermally expand and contract.  Therefore all of the load you just
calculated needs to be resisted by the metal panel and the screws at the
fixed end.

I have designed clip angles specificly for retaining snow using screws
into the upper rib.  Even in moderate snow loads these ended up being 2
to 3 feet on center up the roof slope.

Changing the roof slopes relative to exit locations or adding gables
over the exits are better ways to avoid snow shedding on that expensive
lawyer.

Ice and water shield is good.  In this area it is not uncommon to run it
the full roof slope as a back up since it self seals around screws and
nails.

Dan Goodrich wrote:
> 
> This is pretty close to what is obtained using the UBC formula.  Now
> you have me curious.  I'm wondering if the other systems I've seen
> are being designed, or just put on randomly by the contractor.  They
> don't look stout enough to contain the kind of forces I'm obtaining.
> 
> The architect has specified ice and water shield from the eave up to 3 ft.
> from the ridge under the shingles.  Any experience with this causing a
> problem?
> 
> Thanks,
> Dan Goodrich, P.E.
> Utah
> 
> > Dan,
> >
> > You should design for the weight of snow to be contained times the sine
> > of the roof slope (5/13 in this case) because, under critical thaw
> > conditions, all of the snow will try to slide off at once.
> >
> > A word of caution: don't do this unless you are using a membrane
> > roofing.  If you have shingles you can expect the contained snow to
> > cause an ice dam which in turn will cause water to back-up under the
> > shingles and leak large volumes of water into the building.
> >
> > Regards,
> >
> > H. Daryl Richardson,
> > Calgary
> >
> > > Dan Goodrich wrote:
> > >
> > > I've designed a ski lodge where the snow load is 150 psf on a
> > > 5/12 pitch roof.  The owner has asked about putting log railings
> > > on the roof to help keep the snow on the roof, and not on the ground
> > > in front of the windows and doors.  I am trying to determine what
> > > force
> > > to design the connection to the roof system.  UBC Appendix 1648
> > > gives a force to design for vertical obstructions.  However, this
> > > seems
> > > very excessive, especially when compared to what I've observed on
> > > other roof systems.  Anyone know of any other guidelines to follow,
> > > or procedure?
> > >
> > > Thanks,
> > > Dan Goodrich, P.E.
> > > Utah
> > >
> > >
> >
> > ******* ****** ******* ******** ******* ******* ******* ***
> > *   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
> > *
> > *   This email was sent to you via Structural Engineers
> > *   Association of Southern California (SEAOSC) server. To
> > *   subscribe (no fee) or UnSubscribe, please go to:
> > *
> > *   http://www.seaint.org/sealist1.asp
> > *
> > *   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
> > *   send to the list is public domain and may be re-posted
> > *   without your permission. Make sure you visit our web
> > *   site at: http://www.seaint.org
> > ******* ****** ****** ****** ******* ****** ****** ********
> >
> 
> ******* ****** ******* ******** ******* ******* ******* ***
> *   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
> *
> *   This email was sent to you via Structural Engineers
> *   Association of Southern California (SEAOSC) server. To
> *   subscribe (no fee) or UnSubscribe, please go to:
> *
> *   http://www.seaint.org/sealist1.asp
> *
> *   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
> *   send to the list is public domain and may be re-posted
> *   without your permission. Make sure you visit our web
> *   site at: http://www.seaint.org
> ******* ****** ****** ****** ******* ****** ****** ********

-- 
++++++++++++++++++++++++++
Forrest T. Braun, P.E.
BBFM Engineers, Inc.
Ph (907)274-2236
Fx (907)274-2520
Anchorage, Alaska
http://www.bbfm.com
++++++++++++++++++++++++++

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********