The water makes its way through the roofing material and sheathing and
down through the walls.  If it manages to freeze inside the wall, or
otherwise pass unnoticed to finding its level, you can develop rot,
mold, varmits, all kinds of challenges.  Usually the failure of plaster
or other interior finishes is the observed symptom.  Vapor barriers
complicate the whole situation, either by hiding the symptoms, or
creating complications through condensation and progressively ruined
insulation.

Ice and water shield, helps, of that I have no doubt.  But I choose
avoiding ice dams by keeping the roof surface cold and not allowing
snow to melt when the temperature is below freezing, and getting water
off the roof fast and routed to an appropriate place far away from
foundation walls and foot paths.  (Good thing you guys can't see the
icicle on my roof, nor have the opportunity to loose your footing on
the footpath where the gutter dumps its charge.)

HOWEVER, some historic buildings just naturally have ice dams.  Kind of
a design feature.  The challenge is to understand how the building has
managed to survive recurrent ice dams, and not screw that up with the
introduction of modern materials and systems.  Yes, I have advocated
preserving bad design details, when they are necessary to understand
the building, its construction, its constructors, or to antagonize
someone with an opposing opinion.

But Ralph's house is not a museum...yet.  And Ralph should do something
that will make his house easier to maintain when it is a museum and his
repair is a significant feature, worthy of preservation.

-jc



On Wednesday, February 26, 2003, at 03:19  PM, Becker, Dan wrote:

>> -----Original Message-----
>> From: J.A. Drew Diaz
>> Sent: Wednesday, February 26, 2003 1:48 PM
>>
>>
>> As I understand the problem the Ice dam alone isn't really
>> causing the problem, the problem is with capiallary action
>> which is wicking water back up under the shingles and
>> attacking the roof sheathing,
>
> A really impressive ice dam involves more than capillary action. They
> can form to be a significant number of inches tall, trapping enough
> water behind them that it literally submerges the entire face of
> shingle
> units, allowing water to freely flow into the structure. Imagine trying
> to hold water in a v-shaped trough with slate shingles as one of the
> faces of the trough, the ice is the other face. Can't be done. Asphalt
> 3-tab shingles don't do too much better.
>
> And it gets really good and deep when it happens at the bottom of a
> valley....
>
> _________________________________________________________
> Dan Becker,  Exec. Dir.,   "Help me, Mr. Wizard!  I don't
> Raleigh Historic            want to be here anymore!"
> Districts Commission                 - Tooter the Turtle
> [log in to unmask]
> 919/890-3678
>
> --
> To terminate puerile preservation prattling among pals and the
> uncoffee-ed, or to change your settings, go to:
> <http://maelstrom.stjohns.edu/archives/bullamanka-pinheads.html>
>

--
To terminate puerile preservation prattling among pals and the
uncoffee-ed, or to change your settings, go to:
<http://maelstrom.stjohns.edu/archives/bullamanka-pinheads.html>