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.
