CEMP-ET
TI 809-53
01 May 1999
h. Roof Venting. Steep roof configurations are relatively easy to ventilate when an attic space is
present below the deck or by airways created in a cathedral ceiling. Inadequate ventilation may cause
moisture sensitive roof decking to expand, buckle, or degrade. In general, half of the required net free
ventilating area should be placed at the eaves through means of soffit vents, and half at the ridge through
ridge or gable vents to take advantage of the chimney draft. Condensation on the underside of terne
metal can result in corrosion and painting the underside is recommended prior to installation. Zinc metal
must have underside venting to avoid white rust corrosion.
i. Roof Decks. Plywood and Oriented Strand Board (OSB) are commonly used. There have
been problems with fire retarded plywood (degradation due to accumulated heat and moisture). When fire
retarded plywood is necessary the APA recommends AWPA C27 low hydroscopic Interior Type A fire
retardant be used. Lumber and plywood in contact with copper should be non-treated or have a non-
corrosive fire retardant.
j.
Underlayment
(1) Functions. The underlayment can keep the deck dry until the roofing is installed. It also
provides secondary water resistance for unsoldered systems. Sealed underlayments can reduce leaks
from wind driven or ice-dam water which may back up under the panels. The underlayment and rosin
paper forms a cushion for the clips and fasteners.
(2) Icings. At eaves and other locations where ice dams could cause temporary water backup
felts can be sealed together with asphalt mastic, or more commonly, a self-adhesive modified bituminous
sheet meeting ASTM D1970, or a plastic sheet conforming to ASTM D4397. The self-adhesive sheets
have the advantage of forming a watertight seal where roofing fasteners penetrate the sheet. Self-
adhesive sheets should have a melt point above 200F to avoid bitumen melting.
k. Vapor Retarders. If poly film is used as a vapor retarder, a fully compatible tape with equal or
better water vapor permeance must be used. Under most conditions with steep roof system designs, the
vapor retarder will be on the attic floor slowing (but not completely stopping) moisture penetration into the
insulation. Unfortunately, most vapor retarders are not very effective at stopping air exfiltration as little
attempt is made to seal them at edges and penetrations. However, in more severe cases or for high
humidity buildings it is essential to severely limit air exfiltration. If a separate cold-side air barrier is used,
it should have a high vapor permeability.
l. Penetrations. Round pipe penetrations through the metal roofing are handled with soldered
cones or formed metal sleeves. Refer to SMACNA, CDA, Revere Copper and Common Sense, and
NRCA manuals for details. Hot pipes are flashed using isolation details (see NRCA and SMACNA
manuals for details).
m. Historical Roof Restoration. Since by definition steep metal roofings are very visible, efforts
should be made to retain historical accuracy. In many cases lighter metals can give a less expensive
replica of the original metals. Artificial patination is sometimes done to help match the appearance of
adjacent panels but natural weathering will give the best match. See the Copper Development
Association's Copper in Architecture for discussion. Also refer to the SMACNA reprint of its 1929
Standard Practice in Sheet Metal Work.
n.Aesthetics. In addition to the rich color of many of these crafted metals various batten and seam
techniques give the designer a wide number of choices. See CDA, SMACNA, Copper in Common Sense
and NRCA manuals for illustrations.
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