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Re: GSBN:Loadbearing sb with moisture damage


All good points and although we may have some differences of opinion on
some bits, I think you bring up both some good solutions and some
issues for more discussion. It's worth thinking about this issue of
pans from a conceptual standpoint as much as from the practical issues
you raise. This is not always the case - since concepts and realities
are often at odds with each other, but I say this because what I think
is most essential here is that we are thinking about devising ways to
accomplish multiple goals in the most effective and hopefully,
environmentally responsible, efficient and aesthetic ways we can.

What we are trying to do is to ensure the longevity of the walls around
openings by minimizing the ability of water to get in and maximizing
the ability of water that does get in to get out. We have to consider
things like thermal bridging and condensation, freeze-thaw cycles,
capilarity, sequencing of installation, ease of ensuring reasonable
dependability both in installation and in service over time, including
maintainability. And, I think we should assume certain things. Windows
leak. Plasters and wood can crack and do move and what I like to see is
thinking that goes beyond (or behind or under) the first layer of
protection for when that happens. Like your plaster kerfs that minimize
the problem by design and detailing.

While I don't want to scare people about these things, I also have
heard too many people dismiss them as not being such a big problem.
Dealing with a few buildings and the unhappy and unfortunate owners of
them when they do fail has just reinforced in me the need to pay
attention to them all the way through, from initial design to the final


-----Original Message-----
From: Chris Magwood cmagwood@...
Sent: Wed, 8 Mar 2006 12:42:07 -0500
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.

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