DCI Products RafterVent User Manual
1
March/April 2007~CoastalContractor
I
In the Ocean State — where wind-swept
rains aren’t restricted to nor’easters, tropical storms, and hurricanes — I count on
a vented rain screen coupled with carefully detailed flashings to keep water out
of walls.
A rain screen is a cladding system with
a vent space (or a series of vent channels)
between the back side of the cladding and
the weather-resistive barrier. Openings
along the top and bottom of the vent
space let air flow freely. This vent space
provides a channel for any water that gets
past the cladding surface to drain out, and
the air flowing through this space carries
away moisture vapor that dries off the
back side of the siding. In both instances,
the vent space reduces the chance of
water and moisture vapor being driven
into the wall cavity by wind or sunshine.
Installing any cladding as a vented rain
screen is the best way to make the
cladding last. However, it’s easier to do
with lap sidings than with shingles. Lap
siding, such as cedar clapboard and fibercement planks, easily bridges air channels
between vertical furring strips nailed over
studs. But a vented rain screen with sidewall shingles requires horizontal furring,
which presents some complications.
RAIN-SCREEN OPTIONS
I’ve built cedar-shingle rain-screen sidings
in three ways. The differences mostly
involve the material that forms the vent
space, but this inevitably affects other
details as well.
Vented furring strips. The first time I
built a cedar-shingle vented rain screen, I
laid shingles over a series of back-kerfed
1x3 furring strips (Figure 1). These back
by Mike Guertin
Best-Practice
Wall Shingles
A rain screen offers the ultimate defense against
water intrusion, provided you get the details right
FIGURE 1. The author’s first experiences with venting shingles relied on
back-kerfed 1x3 furring strips. These back kerfs are essential for allowing
water to drain out and air to flow between the horizontal strips, but cutting
the kerfs with a dado blade in a radial arm saw proved too labor intensive.
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kerfs are essential for allowing drainage
and airflow between the horizontal strips,
but they take a considerable amount of
cutting and time: I mounted a dado blade
in a radial arm saw and cut
3
/8- by 3/8inch kerfs a few inches apart along what
seemed like thousands of 12-footers. I
then nailed the furring strips over the
housewrapped wall sheathing, positioning
each strip above every butt line on each
course of shingles. The shingles went up
fine, but I didn’t plan my window, door,
and corner trim details very well. Shingle
butts stood proud of some trim elements,
and I cobbled together less-than-perfect
solutions to mask other problems caused
by the
3
/4-inch furring thickness.
Fortunately, there are now a couple of
commercial products available for creat-
ing vented rain screens with shingles. I’ve
used both nylon spacer mats and plastic
battens over the past few years and found
advantages and disadvantages with each. I
also worked out details for windows,
doors, and trim for these rain-screen systems, as described below.
Spacer mat. Home Slicker by
Benjamin Obdyke is the best-known
spacer mat available. It’s marketed specifically for use with sidewall shingles and
lap siding, though other companies make
similar products for EIFS and masonry
walls that will also work with shingles
(see “Resources,” page 28). Home Slicker
is a corrugated matrix of nylon strands
about
3
/8 inch thick that comes in approximately 40-inch-wide rolls. It gets applied
with staples or cap nails over house-
March/April 2007~CoastalContractor
A spacer mat such as Home
Slicker goes up quickly with no
special layout, creating a
1
/4-inch
ventilation gap behind shingles.
It is important to cut Home
Slicker close to trim such as window casing and corner boards: If
you leave a space wider than
about
3
/4 inch, the unsupported
shingle edge is likely to split.
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wrapped walls, and the matrix compresses
a little when the shingles are installed on
top, leaving an effective
1
/4-inch air
space. The corrugations should be oriented vertically for the best drainage and
airflow, and the edges of the mat should
not be overlapped.
On the plus side, Home Slicker is only
1
/4 inch thick — not nearly the 3/4 inch my
furring strips padded out the shingles. The
butt lines of each shingle course laid over
Home Slicker flush out with
5
/4-inch corner boards applied directly over the housewrap. And the butt lines come close to, but
not past, most flanged window jambs. The
sheets go up quickly with no special layout, but it is important to cut Home
Slicker close to trim such as window casing and corner boards. If you leave a wide
space (
3
/4 inch or more), the unsupported
shingle edge is likely to split.
On the downside, fastening shingles
over Home Slicker takes a deft hand.
The bottom few shingle courses are the
hardest to install. The matrix is spongy, so
hand-driving nails is a challenge, and
pneumatically driven staples or nails easily overdrive even with the air pressure set
low. You’ll end up splitting more shingles
in the first two rows than on the rest of
the wall. Subsequent courses are supported by the shingles beneath, so the
going gets a little easier. There’s a noticeable cushioning of hammer blows when
hand-driving nails into shingles applied
over Home Slicker. The bounce makes it
hard to start nails in the shingles. You
must also use fasteners long enough to
pass through the vent space and penetrate all the way through the sheathing.
Plastic battens. Corrugated plastic
battens offer a good alternative to my furring-strip rain screen. These have hollow
channels that let water and air flow
through them (Figure 2).
The only ones I’ve found marketed
specifically for shingle installation are those
from DCI Products — CedarVent and
RafterVent — but similar products are
available (see “Resources,” page 7).
Standard CedarVent comes in strips 3 feet
long. The four-ply version is
3
/4 inch thick
by 2
3
/4 inches wide. But a two-ply version
that’s just
3
/8 inch thick (my preference)
and a three-ply version that’s
9
/16 inch thick
are also available. While 1
1
/2-inch-wide
strips can be special ordered, I typically just
rip the two-ply version in half (from 2
3
/4
inches down to 13/8 inches) to save material and expose more of the shingle back to
the air. CedarVent is wrapped with a thin
fabric to keep insects out, so it’s great along
the undercourse at the bottom of the wall
and last course at the top. RafterVent can
be used instead of CedarVent in the field of
the wall. It’s essentially CedarVent without
the fabric wrap.
Battens require precise placement, so
they aren’t as fast to install as spacer
mats, but they do provide solid support for
nailing. I lay out a story pole for shingle
course exposure and use it to mark locations for the battens. After I transfer these
layout marks onto window and door trim
and corner boards, I snap chalk lines on
the housewrap between my marks. The
battens get applied above the lines. Since
shingles are nailed about 1 inch above the
butt line of the overlapping course, the
battens are positioned perfectly behind
the nail line. Extra battens are needed
under windowsills and horizontal bandboard trim elements to support the top
edges of the shingles.
Other than selecting longer fasteners,
there’s no special precaution to applying
shingles over battens.
TRIM DETAILS
The devil is always in the details. Corner
boards, woven corners, window and door
trim, band boards, and other trim elements
Best-Practice
Wall Shingles
March/April 2007~CoastalContractor
FIGURE 2. Plastic battens have hollow channels that let water and air flow through them.
Shown here is the two-ply version of CedarVent, which the author rips to 13/8 inches wide
to save material and expose more of the shingle back to air.
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