Caulk-free
columns: The Foundation to Durable Exterior Trim
by Gary M. Katz
Close your eyes and imagine that
your store has no computer system; that you're still
writing all sales receipts, purchase orders, and invoices
longhand; that you're still keeping track of profit
and loss and payables and receivables on legal-sized
ledger paper. Just remembering those days probably makes
you want to close your eyes.
New building materials have had
the same fast-forward effect on contractors and installed
sales programs. We can no longer build houses they way
our fathers and grandfathers built homes. Architects,
contractors and carpenters need to be educated in new
installation techniques; certification programs must
be introduced to provide continuing education for our
industry. And another difference between yesterday and
today, is that the best report card from any construction
education program should have Five D's.
Five Straight D's
DESIGN. Failure or success occurs before cutting the
first board or driving the first fastener. The longevity
of exterior cladding, trim, and paint is no longer a
given. Modern materials require more care than old growth
lumber. Designing exterior cladding systems that include
Deflection, Drainage, and Diffusion is the only way
to ensure a long-lasting, claims-free job, and a long-lasting
reputation.
DEFLECTION. Excessive water absorption
in exterior cladding and wall systems leads to mold,
mildew, rot, and ruined reputations. Always design exterior
cladding and trim details to shed water and deflect
moisture quickly. As Norm Slavik from Norcon Forestly
LTD is fond of saying: a 5 degree slope on window sills
is a thing of the past. 10 degrees works better, but
15 degrees gets the water off fast. The same approach
applies to all exterior trim installations." Deflection
also means flashings. Every horizontal and raked or
angled surface— window and door heads, roof trim,
aprons, water tables and bands, —requires a flashing
that's integrated with the housewrap system and rapidly
deflects water away from the house.
DRAINAGE. Provide a means for
moisture to drain away quickly from wall and trim assemblies.
. Don't inhibit or block the potential flow of moisture.
Weep holes and drainage designs must be incorporated
into every cladding system.
DIFFUSION. This is probably one
of the most overlooked yet critical areas of construction.
Diffusion through evaporation is the means by which
moisture vapor and liquid water are encouraged to dry.
According to building scientists, a minimum 4 mm (3/16-in.)
space is required to break the surface tension of moisture
caught between cladding and drainage plane materials.
But a larger gap, some say as much as 10mm, and some
say it must be a true rainscreen wall—ventilated
to "neutralize the forces that draw water behind
the siding" (Caruso), is necessary to prevent capillary
action from driving liquid water through cracks, crevices,
nail holes, and up the backside of lap siding. I'm no
scientist. I'm only a carpenter. I like to keep things
simple. From experience I know that wood products need
breathing room. Diffusion is achieved by providing 'breathing
room' between exterior cladding and housewrap systems.
DURABLE EXTERIOR TRIM. Regardless
of which housewrap system you use—felt paper,
plastic housewrap, textured drainscreen, or Rainscreen
products—allowing exterior cladding an opportunity
to drain and dry is the secret to durable exterior trim
and long-lasting paint applications. Today's wood building
materials are cut from fresh growth with few dense winter
growth rings, mostly soft summer growth. These trees
haven't aged enough to convert sap wood to heartwood.
As Stephen L. Quarles wrote in a recent article in Fine
Homebuilding: "Because of the extractives in heartwood,
both mold and decay fungi prefer sapwood. …Today's
houses have more potential mold and decay food than
earlier houses built with…lumber cut predominantly
from the heartwood of softwood species."
Wood movement tears up trim
The following model for a Caulk-Free Column is based
on the Five D's. In my last article, we discussed wood
movement and how it effects miters. Because wood moves
more on the outside of a home than on the inside, miters
should be avoided on exterior trim. That explains why
nearly every manufacturer of exterior trim doesn't recommend
the use of miters with their products. But we should
also avoid another 'old fashioned' construction building
habit—cross grain laminations—stacking boards
or moldings on top of each other and creating peanut-butter-and-jelly
sandwiches. Walk through the simple steps required for
constructing a caulk-free column and you'll see the
Five D's in practice.
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| Designed for Failure |
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The
first stage in the design process is understanding
wood movement and building from the ground
up. Sandwiching two pieces of wood together
creates a breeding ground for mold and rot,
and when laying two pieces with the grain
running in opposite directions, the chances
of failure increase to 100%. Attaching finished
material directly to rough framing invites
excessive moisture content (MC) and chemical
treatment transmission directly into finish
materials, guaranteeing premature product
failure |
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The perfect
storm for product failure. Unless exterior trim Designs
provide Deflection, Drainage, and Diffusion, product
failure rates will continue to soar. The plinth detail
on these columns failed for several reasons. Applying
exterior trim boards (MC = 8-10%) directly to the rough
post (MC = 20-25%), forced moisture migration into the
trim. The finish materials were installed too close
to the concrete. The column wrap boards swelled across
the grain after installation, violating the miters on
the plinth details and causing severe product failure.
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Five-D Design
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What's a Caulk-free Column? One that you don't have
to keep caulking months after the job is finished!
Always start columns, and all exterior cladding
projects, from the bottom, and work your way up,
just like shingling a roof. First isolate the bottoms
of posts from concrete foundations by setting them
well above grade (A). Protect the bottom of the
post with a membrane or weather resistant wrap that
provides for Drainage at the bottom of the post
(B). Don't sandwich the plinth on top of the finished
column wrap. Instead, install back-out blocks (C),
then install the plinth detailing. |
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| Flash the top
of the plinth to Deflect, moisture out from behind
the finish material and away from the base of the
column |
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Then install housewrap
on top of the flashing and around the rough post,
which will allow a high-moisture-content post to
dry out evenly, without checking, warping, or otherwise
ruining the finish boards. |
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| A rainscreen product,
like HomeSlicker,
is perfect behind columns or large areas of flat
trim, like pilasters and panels. Rather than using
blocking or stripping, this 1/4-in. thick nylon
matrix is mostly air, so it allows for maximum Diffusion
behind trim and siding. |
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Next install the Scotia molding.
Because this molding is not fastened directly to
the plinth or column wrap, and instead is isolated
and allowed to float freely, the miters are safe
from substrate wood movement. |
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| Install the column-wrap
boards last. Temporarily fasten the these butt joints
with clamps and stainless steel finish nails. |
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Then reinforce
each corner with Senco Weatherex#3 coated square-drive
trim-head screws or stainless steel screws. |
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Construction today is different than yesterday, but
that's a good thing for building product professionals.
Educated lumber and building material dealers and installers
are able to better serve their customers. And service,
not low pricing, is the key to long-term profits for
our industry.
Portions of this article will
appear in the new WindsorOne Exterior Trim Installation
Guidelines, to be released in 2007. In the writing of
this article I was assisted by information from many
specialists, including Bill Robinson, Norm Slavik, Steve
Easley, Jim Chestnut, and others. For additional information
see the sources below.
Sources:
Caruso, George. "Managing
Moisture in Residential Building Envelopes." White
Paper, www.Benjamin Obdyke.com, 2006.
Chown, G.A., W.C. Brown and G.F. Poirier. "Evolution
of Wall Design for Controlling Rain Penetration."
Construction Technology Update No. 9 Dec. 1997
Hazleden, DG and P.I. Morris.
Designing for Durable Wood Construction: The 4 Ds."
Vancouver, Canada, May/June 1999. Eighth International
Conference on Durability of Building Materials and Components.
http://www.durable-wood.com/pdfs/The_4_Ds.doc
Holladay, Martin. "Choosing
a Sheathing Wrap." Journal of Light Construction,
August, 2000
Lstiburek, Joseph. "Understanding
Drain Planes." Building Science Corporation, 2005
(www.buildingscience.com)
Lstiburek, Joseph. "The
Mold Explosion: Why Now?." Fine Homebuilding, January
2007
Quarles, Stephen L. "Can
moldy framing lumber cause rot?." Sidebar in Joseph
Lstiburek, "The Mold Explosion: Why Now?"
Fine Homebuilding, January 2007
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