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RE: GSBN:Loadbearing sb with moisture damage



<x-charset windows-1250>Hi,

Chris' detail is a good illustration of the questions about waterproofing
fenestration, so I'll be so bold as to Chris as a guinea pig  (do guinea
pigs build straw houses, or is that another kind of pig?).  Chris' rant
against pans shows the problems that might be associated with them:

<I'm not a big fan of pan-style flashings under the windows. Those pans
always have seams or lumpy bits where they are folded, and these always seem
vulnerable to me. Also, if they are to effectively allow water to leave the
wall, there must be a gap between the underside of the wooden sill and the
pan. If there isn't, then water is just going to sit under the wood and keep
it soaked. If water can run free of this pan, that means that there is also
a gap for air to infiltrate under the window. If the pan is properly bent
and sealed, this air won't make it right into the home, but it will make the
window base very cold (at least here in Canuckland) and very prone to
condensation on the inside. Also, water under this sill will be prone to
freezing.>

But we are called upon to do two things that are contradictory: (1) Seal the
assembly from the outside to prevent water from getting in and (2) collect
any water that leaks around the window and let that water get to the
outside.  Chris' explanation of his kerfed sill doesn't show how both of
these are satisfied:

<My preferred method (submitted to the CASBA details) involves a window buck
in which the bottom sill is made from stock 2-inches wider than the sides,
and is notched into the uprights, bevelled and has a drip kerf cut into the
bottom. By notching the sill into the uprights (and then gluing the joint),
water cannot go through or around the sill, but follows the bevel away from
the wall and rolls free at the kerf. >

This is an assumption that, according to the Third Pig's Law of Water
Dynamics, might be unwarranted.  The Third Pig's law is: "Water flows
downhill, except not always."  Water, driven by wind, flows uphill, sideways
and all kinda ways, even filling up Lake Ponchitrain (sp) from behind.  So
we need to address that in detailing, and Chris' explanation doesn't give
all the details:

1. If water is blown over the top of the sill, what's to prevent it from
going under the window and then inside the wall and/or collecting there and
rotting wood, freezing, etd.?
2. What happens at the side of the sill/buck, on the horns, which could
provide an entry into the wall, behind the plaster.
3. What happens if/when water leaks in along the jamb, and falls down to the
sill, behind the plaster.  How will that water get out and/or be prevented
from going into the wall?

Generally this is accomplished by layering, for instance with a sill over a
pan which leaks out under the sill, where the sill provides a mechanical
barrier to water intrusion, and the pan collects water and, protected by the
sill, lets it drain to the outside, and I haven't figured out a way for that
to happen otherwise.

John "except not always" Swearingen



John Swearingen
 SKILLFUL MEANS
design and construction
 HYPERLINK "www.skillful-means.com"www.skillful-means.com


-----Original Message-----
From: GSBN [HYPERLINK
"<a  target="_blank" href="mailto:GSBN@..."mailto:GSBN@...";>mailto:GSBN@..."mailto:GSBN@...] On
Behalf Of Chris Magwood
Sent: Wednesday, March 08, 2006 9:42 AM
To: GSBN
Subject: Re: GSBN:Loadbearing sb with moisture damage


David and all,

This is great to be addressing these issues!

I think that there is some hope that the CASBA details will end up including
several good strategies for making good window and door openings. I don't
think there's just one way to do it well, because the kind of window, its
materials, the trim scenario and the position on the wall all have an affect
on the chosen strategy.

I agree with David that relying on caulking is a bad idea. However,
caulkings and/or glues do tend to last a very long time when the materials
they are joining are making fairly full, flush contact with one another and
are not exposed to UV or weather. In the window cutting arrangement, we had
a very flat, straight edge on the cut plaster, and made the wooden frame
inserts so they fit tightly against that cut plaster. The caulking that
seals the two is then behind the wooden window trim, which is in turn
caulked to the face of the plaster. Water must get behind the trim, and then
through the tight, caulked joint. I felt very confident with this (although
I've never repeated it exactly).

I'm not a big fan of pan-style flashings under the windows. Those pans
always have seams or lumpy bits where they are folded, and these always seem
vulnerable to me. Also, if they are to effectively allow water to leave the
wall, there must be a gap between the underside of the wooden sill and the
pan. If there isn't, then water is just going to sit under the wood and keep
it soaked. If water can run free of this pan, that means that there is also
a gap for air to infiltrate under the window. If the pan is properly bent
and sealed, this air won't make it right into the home, but it will make the
window base very cold (at least here in Canuckland) and very prone to
condensation on the inside. Also, water under this sill will be prone to
freezing.

My preferred method (submitted to the CASBA details) involves a window buck
in which the bottom sill is made from stock 2-inches wider than the sides,
and is notched into the uprights, bevelled and has a drip kerf cut into the
bottom. By notching the sill into the uprights (and then gluing the joint),
water cannot go through or around the sill, but follows the bevel away from
the wall and rolls free at the kerf. This means that my "rough" buck sill is
actually visible, so I use nice wood stock that is treated on all sides for
moisture resistance (or some owners metal clad this sill). Lately I've taken
to making this an intentional feature of the home, and have been using thick
slab hardwood for these sills, making them heavy and distinctive in the
wall.

Another thing that I've been doing lately is cutting plaster kerfs into my
frames (or anywhere where plaster will meet wood). A decently deep kerf will
mean that the plaster is not just resting on the surface of the wood (where
it's pretty easy for air and water to get
behind) but is filling the kerf and slowing (not stopping, since the plaster
will shrink a bit) this tendency.

Chris
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For instructions on joining, leaving, or otherwise using the GSBN list, send email to GSBN@...HELP in the SUBJECT line.
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