DCI Products RafterVent - Manual (Page 6)

6
spacer mats, I install blocking as a base
for the band boards, just like the corner
boards (Figure 5, page 5).
Regardless of the material used to cre-
ate the vent space, you still need to install
drip cap flashing over horizontal trim. But
don’t run the drip cap all the way to the
wall sheathing over the band boards.
Doing so will break the continuity of the
airflow. Instead, treat the cap flashing like
a Z flashing, as shown in Figure 5. Its
main function is to redirect water that
enters at the siding/trim joint back out
and protect the top edge of the band
board. Run the “wall” leg of the flashing
over the face of the rain screen so air can
freely flow from intake to exhaust.
Windows and doors. Over windows
and doors, the flashing practice is differ-
ent. Run cap flashings all the way to the
wall sheathing and integrate with the
housewrap (Figure 6). Any water draining
in the vent space will drain out over the
drip cap. Be sure to leave a
3 / 8 -inch air
space between the bottom of the shingles
and the cap flashing for air circulation.
And remember to provide insect screens
on the rain-screen material.
Keeping the vent space thickness down
to 1 / 4 to 3 / 8 inch doesn’t pose much of a
problem. However, if thicker drainage
mats or battens are used ( 3 / 4 inch, for
instance), the windows and doors will
need to be padded out. The simplest fix is
to mount spacer blocks around the rough
openings that equal the thickness of what-
ever spacer material you’re using. Some
March/April 2007
~
CoastalContractor
Window Flange on Nailers Window Flange on Sheathing
Self-adhesive
flashing
Housewrap
Plastic battens
Metal Z flashing
Metal Z
flashing
Plastic battens
Sill flashing
Self-adhesive
flashing
Housewrap
Housewrap
under nailer Housewrap
under sill
flashing
Plywood or
OSB nailer
Plywood or
OSB nailer
Sill flashing
BARK SIDE: FACE IN OR FACE OUT?
While most red cedar shingles are milled vertical grain, white cedar shingles are
usually cut flat grained. Many installers like to face the shingles “bark-side out”
hoping that the shingles will be less likely to curl at the outside edges and stay
flat on the wall. Checking every shingle’s growth rings is an extra time-consuming
step, though, and I’ve given up on the practice. Although I don’t have a study to
back up my position, I’ve noticed that I end up with many fewer curled shingles
since I began applying them over vented rain screens. I speculate that shingle
curling has more to do with the concentration of moisture inside a shingle than
with the ring orientation. Shingles will tend to curl toward the “dry side.” When
shingles are applied directly over a sheathed wall, the sun will drive moisture
toward the back (cooler) surface. Shingles applied over a rain-screen space will
be able to dry more readily, reducing the excess moisture built up on the back
surface and thereby reducing curling. — M.G.
FIGURE 6. The best way to handle
the exterior trim is to mount the
window flange on furring strips
(far left). The alternative is to fur
out for the window trim after the
window is installed (left), which
works with a 1 / 4 -inch to 3 / 8 -inch
spacer material.