There is only one category of metal roof systems used in low-slope applications
structural metal panel. Structural metal panel roof systems can be used for
low slope roofs because of their hydrostatic, or water barrier, characteristics.
It is important to note structural metal panel roof systems can be used for steep
slope roof assemblies, too.
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
Structural metal panel roof systems are installed over a large variety of substrates.
There are two general categories of substrates: one is 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 roof deck.
Underlayment (or "felt paper" as it is frequently called) is installed over the
roof deck before the application of a metal panel roof system. An underlayment performs
two primary functions: it provides temporary weather protection until the metal
panel roof is installed, and it provides a secondary weatherproofing barrier if
moisture infiltrates the metal roof panels.
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.
Asphalt saturated, nonperforated organic felts are among the most common underlayments
used for metal roof systems; 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.
If an underlayment is to be installed, 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 shingle fashion.
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 panel roofs, 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.