The Retrofit Roofing Game:
Fit-for-Use Applications
Mark James ,
Posted
08/30/2012
This month, we will discuss the different types of conventional
roof construction and how each of these influence the type of
retrofit framing system that best suits the project.
Believe it or not, conventional roof construction hasn't changed
much over several decades. It utilizes a structural grid of primary
supporting members such as girders and a secondary joist. There are
exceptions to this, but the basics stand true for the majority of
buildings.
Existing roof construction typically includes wood, steel or
concrete framing. The following are the most common systems that
have been retrofitted over the years:
• The most common roof system to retrofit is openweb steel bar
joists with a decking material that will range from: rigid
insulation over ribbed metal, lightweight concrete over metal deck,
acoustic decking such as Tectum, and even wood on much older
buildings. Most of these will support the positive loads imposed by
the new retrofit system, but they should still be analyzed by a
qualified engineer. Acoustic and wood decking require a thorough
inspection to determine the amount of deterioration from moisture
infiltration. You will find these roof systems in warehousing,
schools, commercial/retail and industrial facilities. Depending on
the type of new roof geometry (hips, valleys, gable, etc.), a
typical zee-shaped purlin and cee-shaped post system is the best
choice for the retrofit framing.
This retrofit system is best known as "Post & Purlin." Base
member type will depend on the geometry and the orientation of the
joist spans relative to the new roof slope direction.
• A roof system of dimensional lumber with wood decking can be
challenging because of its inability to receive concentrated
positive loads from the retrofit system, plus its exposure to
moisture deterioration. You will find these in offices, lodging and
light commercial buildings. The retrofit framing for these will
include components similar to the steel-framed roof, except the
base member will be continuous and vertical members should be
spaced no more than two-times the spacing of the existing
joists.
For example, if the joists are spaced at 16 inches, then the
vertical retrofit member would be spaced at 32 inches maximum.
• Heavy timber and glulams will usually require a special
framing system due to the distance between the beams. These beams
commonly are spaced from 10 to 15 feet on center. Also, such a
roof's structural wood decking is not intended to receive
concentrated loads. Therefore, the retrofit framing must distribute
its roof loads back to the existing beams. The new framing will
consist of vertical framing directly above the existing beams with
deeper zee-shaped purlins because of the inherent spans.
• Steel or wood trusses found in flat and barrelshaped roofs are
framed very similarly to heavy timber and glulam systems. They are
common in school gymnasiums and other facilities that require
clear-span framing without intermediate supporting columns.
• Concrete roofs typically are retrofitted similarly to bar
joist systems. Concrete roofs can include structural,
poured-in-place concrete decks; factory-formedconcrete planking
over steel or concrete beams; pre-cast concrete tee construction or
even intermittent tees with acoustic decking. Concrete systems are
found in schools, warehousing, lodging and high-rise buildings. The
retrofit system for structural concrete decking often will be
limited by the amount of point load it can impose on the concrete
deck. It is always best to locate your vertical supporting members
directly above the existing beams instead of on the deck
itself.
•The best way to enter the retrofit roofing market is to start
with pre-engineered metal buildings. These roof replacements
usually do not include removal of the existing metal roof and they
utilize the simplest type of retrofit sub-purlin members similar to
those manufactured by Roof Hugger, Lutz, Fla.
If you are considering retrofit as an alternative to your
current construction skills, you will surely be exposed to very
different roof framing systems. Some will challenge the design of a
conventional retrofit framing system, and a few will not be suited
for retrofit. The retrofit roof game is always filled with
surprises because in renovation and remodeling you never know what
is behind that wall. Unfortunately, it is never that long-lost box
of money the original building owner left behind. Next month, we
will discuss the retrofit systems themselves and what programs are
available to help you.
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.