ASHRAE Journal - January 2015 - 41

COLUMN BUILDING SCIENCES

deal 203for uplift ("How Not to Build
Roofs," ASHRAE Journal, March 2008).
Adapting a good standard commercial compact roof design gives you
Figure 2. Simple, elegant, robust. So
what can go wrong? A whole bunch
of things. First of all, you really, really,
really need an air barrier on your
roof deck. In Figure 2 with traditional
board sheathing the air barrier
should be a fully adhered membrane.
Sheet polyethylene does not work as
it rips, tears, punctures easily and
otherwise sucks as an air barrier. If
you use sheet goods such as plywood
or OSB as your roof deck, you have
two options. First, go with a fully
adhered membrane as you would
with traditional board sheathing.
Second, select a sheet goods system
that allows you to tape the joints of
the sheets to create the air barrier.
Taped OSB works if you use the right
tape with the right OSB ("Stuck on
You," ASHRAE Journal, February 2013).
What else do you need to know
about? Well, if you are in a high
snow load zone, you have to deal
with ice dams and the best way to do
that is to construct a vented "overroof" over the top of the unvented
"under-roof" (Figure 3). See "Dam
Ice Dam," in ASHRAE Journal, June
2010. The new twist to these roofs is
more roofing membrane options.
The roofing membrane on the
top-side of the unvented "underroof" has traditionally been a vapor
permeable roofing felt, but now
with new materials available, it can
be a fully adhered "vapor permeable" membrane. Yes, you read that
right...there are vapor permeable
fully adhered membranes available. Alternatively, you can go with a
taped OSB sheathing system that is
semi-vapor permeable.

FIGURE 3 Control Ice Damming. Construct a vented "over-roof" over the top of the unvented "under-roof." The new

twist to these roofs is more roofing membrane options. The roofing membrane on the top-side of the unvented
"under-roof" has traditionally been a vapor permeable roofing felt but now with new materials available it can be
a fully adhered "vapor permeable" membrane. Yes, you read that right. There are vapor permeable fully adhered
membranes available. Alternatively, you can go with a taped OSB sheathing system that is semi-vapor permeable.
Roof Sheathing
Roofing Paper
Shingles

Minimum R-50 Rigid Insulation in Two or More
Layers with Horizontal and Vertical Joints Staggered
Roof Sheathing
Roofing Membrane
Vented Space

Air Barrier Membrane
Wood Decking
Timber Rafter or Exposed Joist

Are we done yet with the "adding
all the insulation to the top of the
roof deck" approach? Nope. The
layers of insulation are "thick" and
they affect the aesthetics of the roof
edges. We dealt with this before
when we looked at retrofit overroofing ("Don't Do Ugly Things,"
ASHRAE Journal, August 2012). Figure 4
illustrates how you could extend the
overhang to "hide" the thickness of
the insulation-if the thickness thing
irritates you.
Anything else? Yes. Amazingly
enough you have to attach the upper
layer of roof sheathing (plywood
or OSB) through 8 to 10 in. (203 to
254 mm) of rigid insulation. You
need long screws. How many and
how far apart? The good news is
that this is less complicated than
installing continuous insulation on
walls. Huh? Yes. Less gravity, better
friction. Uplift is the problem, not
slippage and bending movement
of the fasteners. The folks who do
commercial flat roofs have this uplift

FIGURE 4 Thick Roof. The layers of insulation affect
the aesthetics of the roof edges. You can extend the
overhang to "hide" the thickness of the insulation-if
the thickness thing irritates you.

New Roof Deck
New Crown
Molding
New Fascia
New Soffit
Dentil Molding
Frieze Board
Siding

thing dialed in. The same fastening requirements for uplift for flat
roofs will work for these roofs while
simultaneously handling the slippage issue. Except where it snows
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of
Building Science Corporation in Westford, Mass. Visit
www.buildingscience.com.

JAN UARY 2015

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