The Retrofit Roofing Game:
The Framing Systems
Posted
09/25/2012
This month, we will discuss the different types of retrofit
framing systems that best suit a project.
Once you have found that ideal opportunity for retrofitting an
existing roof, you will probably already have a pretty good idea
what the new roof's geometry will be. While this was the first
question, will you now know what the best-suited under structure
will be to create the new roof geometry?
For existing flat roofs, there is basically one type of
light-gauge steel framing system with many optional components that
make it adaptable to the multitude of roof structures out there.
For existing sloped roofs, there are several sub-framing assemblies
that have been used, but only a few are code compliant,
structurally correct systems, which I will explain later.
Most of the retrofit manufacturers have framing systems that are
very similar. Why? Because this is not rocket science and you
cannot reinvent the wheel. Their differences are primarily in the
way the framing is braced both transversely (eave to eave) and
longitudinally (parallel to the new metal roof's ridgeline). Some
are easier to erect, but in the end they all make this very
lightweight framing system extremely stable.
Finally, once you begin to compare the new roof's geometry to
the existing roof's framing it becomes clear when the various base
system options come into play. This was discussed last month in our
Fit-for-Use Applications column.
Now let's discuss the correct framing system. Let's say that
your existing flat roof has an open-web steel bar joist support
system. This project will probably be suited for "Post &
Purlin" type framing (see Figure 1), understanding that the base
members may or may not change from intermittent to continuous
members based on the new roof geometry.
However, if your new roof slope is perpendicular to the span
direction of the bar joist and is principally gabled at both ends,
then you may want to look at what we call a "Wide-Bay or Span"
framing system (see Figure 2). Essentially a modified Post &
Purlin system, its value is in reducing the amount of framing to be
erected and needs to attach to the existing roof system. Different
from the Post & Purlin, this system has greater purlin spans
and its vertical supports are located directly over the existing
primary supports, such as beams.
This system is good as long as the purlins are not spanning more
than 20 to 25 feet. The only drawback to this system is in its
anchorage requirements where the required pull-out to be satisfied,
can exceed four or five times that of the Post & Purlin.
As a point of interest, the two different framing systems have
been combined many times for use on the same project. The Wide-Bay
design is usually ideal for existing glulam roof systems due to
their having a structural wood decking in lieu of joists and the
laminated beams are spaced less than 20 feet on-center. It could
also apply to concrete and some heavy timber roofs as well.
If your building is not a flat roof, then you have some options
as well, but be cautious. For decades, the metal construction
industry manufacturers and contractors have been installing new
metal roofs over asphalt shingles and other sloped roof assemblies
including existing metal building roofs (see Figure 3). Today, it
is a little more difficult since most building code authorities
have acquired jurisdiction over roof replacements. Even though some
people may argue that replacing a roof falls under building
maintenance, the permitting bodies don't see it that way. The
problem? You now have to have your new roof engineered to meet the
local code's regulated wind speeds and snow loads. This means a
sub-framing system is engineered to accommodate the ASTM E-1592
wind uplift test results of the new metal roof, as well as
supporting heavier adopted snow loads. It can get tricky, so
consult with a manufacturer that knows this business and can help
you choose the right system.
If you are just entering the retrofit market, there are several
qualified manufacturers that furnish the engineering, framing
components and new metal roofing, which provide a complete package.
The ones I know of are members of the Metal Construction
Association (MCA), which in my opinion establishes them as a
"player" in the retrofit roofing game.
Next month, we will discuss how retrofit systems can help
existing buildings achieve energy independence.
Mark James has more than 40 years experience in
the retrofit and metal construction markets, working for leading
manufacturers and executing great projects. Currently, he is
president of RetroSpec LLC, a consulting company offering direction
and deep knowledge of the retrofit business for manufacturers and
contractors. James can be reached at mark@retrospecllc.com.