There are three general categories of metal roof systems used for steep-slope roofing
applications: architectural metal panel, structural metal panel and metal shingle/shingle
panels. Generally, architectural metal panel roof systems are watershedding and
are intended for use on steep slope roofs. Structural metal panel roof systems are
used on low and steep slope roofs. Structural metal panel roof systems can be used
on low slope roofs because of their hydrostatic, or water barrier, characteristics.
Because architectural metal panel roof systems typically are designed to be used
on steep slopes that will shed water rapidly over the metal panels' surface, the
seams typically are not watertight. Many architectural metal roof systems are well
suited for use on roof slopes of 3 inches per foot (14 degrees) or greater. One
exception to the general slope guidelines for architectural metal panel roof systems
is the traditional flat seamed, soldered or welded metal roof system, such as copper.
It may be specified on slopes less than 3 inches per foot (14 degrees). Solid roof
sheathing, or decking, is required for architectural metal panel roof systems, and
NRCA recommends using underlayment.
Example of an architectural metal panel roof system
Most structural metal panel roof systems are designed to resist the passage of water
at laps and other joints, as sealant or anti capillary designs can be used in the
seams. Structural metal panel roof systems possess strength characteristics that
allow them to span supporting members.
Example of a structural metal panel roof system
Metal shingles and shingle panels are available in numerous varieties for use as
steep-slope roof coverings. Most of the metal shingles are press-formed during the
manufacturing process to provide a variety of shapes. These products can take the
shape of individual or multiple asphalt, tile, slate or wood shingle configurations.
There are many categories of metal panels. The term standing seam often is
used as a generic description for a class of metal roof seams. The name standing
seam is derived from the fact that the seams are joined together above the
panel flats. The term also is used to refer to a panel profile that includes
a standing seam: the vertical leg/flat pan and the trapezoidal seam. The trapezoidal
standing seam is more commonly associated with structural metal panels. Other panel
types are batten, flat, bermuda and shingled.
The original batten seam consisted of vertical leg panels placed between
wood batten strips and covered with a cap. Today, many batten seam panels are constructed
entirely of metal. Because they are designed to shed water, batten seam panels primarily
are used in architectural applications.
Example of standing seam profiles
A flat seam is created with individual panels applied in a shingled application.
One panel edge is folded back on top of itself; the other panel is folded under,
and the two panels are hooked together. Again, because flat panels are considered
watershedding, they are more commonly found in architectural installations. Some
flat seam panel systems, such as copper, are commonly soldered, which allows them
to perform on less steep slopes.
Example of a flat seam
Architectural metal panel roof systems and structural metal panel roof systems are
installed over a large variety of substrates. There are two general categories of
substrates: continuous or closely spaced decking that provides solid support for
the metal roof panel, and the other is composed of spaced structural supports (such
as purlins) where the metal panels must span between supports. Most structural metal
panels are used over spaced structural supports without being supported by a solid
Metal shingles and metal shingle panels should be installed over continuous or closely
spaced wood decking, furring strips, or metal or wood purlins using a batten or
counter-batten system. These roof coverings typically are considered to be watershedding
roof systems, so the roof slope should be 3:12 (14 degrees) or greater.
Underlayment (or "felt paper" as it is frequently called) is installed over the
roof deck before the application of a metal roof system. An underlayment performs
two primary functions: it provides temporary weather protection until the metal
roof system is installed, and it provides a secondary weatherproofing barrier if
moisture infiltrates the metal roof panels.
Asphalt saturated, nonperforated organic felts are among the most common underlayments;
they commonly are designated as Type 15 and Type 30 or referred to as No. 15 and
No. 30, which are reflective of a once used pound per square weight designation.
The terms Type I and Type II now are used within the industry in lieu of No. 15
or No. 30, respectively.
For metal panel roof systems, metal shingles and metal shingle panels, NRCA recommends
a minimum of one layer of No. 30 asphalt-saturated felt applied horizontally in
shingle fashion on roof decks having a slope of 4:12 (18 degrees) or more. For roof
decks having slopes of 3:12 (14 degrees) up to 4:12 (18 degrees), a minimum of two
layers of No. 30 asphalt-saturated underlayment should be applied horizontally in
Underlayments typically are not used with structural metal panel roof systems when
intermittent supports are used to carry the roof systems. However, if there is a
continuous or closely spaced roof deck, NRCA recommends an underlayment be installed.
In locations where the average temperature for January is 30Âº F or less, NRCA suggests
installation of an ice-dam protection membrane. An ice-dam protection membrane generally
is a self-adhering polymer-modified bitumen membrane.
An ice dam protection membrane should be applied starting at a roof's eaves and
extending upslope a minimum of 24 inches from the exterior wall line of a building.
For slopes less than 4:12 (18 degrees), a minimum of 36 inches is recommended. See
Figure 1 - Example of ice damming
NRCA also recommends a slip sheet be installed over the underlayment for metal panel
roof systems. A slip sheet is a layer of smooth building paper, such as rosin-sized
or unsaturated building paper. Its purpose is to protect the underlayment from damage,
as the panels can adhere to and tear the underlayment.
Vapor retarders, insulation and ventilation
Condensation should be expected to develop on the undersides of metal roof panels.
Careful consideration should be paid to vapor retarder, insulation and ventilation
issues. Because every building is in some way unique, building owners and designers
may need to consult moisture-control specialists.
NRCA does not make any recommendations about which product or manufacturer to use;
however, NRCA does recommend that metal roof systems meet standards established
by ASTM International.
When purchasing a new roof system, there will be two warranties to consider. First,
there will be the manufacturer's warranty. In general, these warranties cover defects
in the manufacture of the roof covering. In the case of metal roof systems, manufacturers
tend to cover only the metal finishes or coatings. Please read NRCA's consumer advisory bulletin addressing roofing warranties
for more information. Once the project is complete, be sure the contractor provides
you with a certificate for your records.
Second, the roofing contractor will provide you with a warranty covering his workmanship.
Typically, this will cover installation and related issues. The warranty should
contain what items are covered and what will void them. Many contractors offer one
year or two years of coverage; however, there is no industry standard.