Retrofitting Gypsum

RETROFITTING / REROOFING POURED GYPSUM DECKS 

The poured gypsum roof deck consists of steel bulb tees, a form board (usually composed of gypsum or fiberglass), wire reinforcing mesh, and the gypsum concrete.  The bulb tees, which are very strong vertically but quite flexible horizontally,  provide the long span strength. The gypsum concrete, which spans tee to tee, encases the tees and prevents them from rotating or deforming horizontally when under load,  thus creating the structural composite strength.  Bulb tees of several shapes and sizes were used, depending on the loading conditions under design as well as the span of the tees.

However, when gypsum decks entered the marketplace, roof decks were not used as the normal method of providing diaphragm shear resistance to the building in many parts of the country.  Steel bracing in the roof structure was the more common method of bracing the walls against wind and seismic loads.  Using the roof deck as a structural diaphragm was a California development and worked its way across the nation in the late 1950’s and early 60’s.  Therefore many poured gypsum roof decks were not designed and installed with diaphragm shear strength in mind; only gravity load and wind uplift loads were considered.  In retrofitting a gypsum roof deck, one should evaluate the steel bracing of the roof structure to ascertain if the roof deck was in fact used as a structural diaphragm.

Poured gypsum decks provide stable structural properties if kept moisture-free.  Damage that can cause loss of the structural composite strength is most likely to come from leakage of water into the roof deck from unattended roof leaks.  Free water will cause the gypsum concrete to decompose and lose structural strength.  Too much water can cause the form boards, which are temporary form members for the gypsum, to soften and sag down below the bulb tees.  In this condition, the gypsum concrete ceases to stabilize the bulb tee and allows it to rotate or bow under loads, thus losing its structural composite capacity.  Repairs are necessary to restore the structural composite capacity and return the gypsum deck to its intended structural capacity.

Where evaluations have shown the structural composite strength of a gypsum roof deck assembly is destroyed or compromised, or where retrofit operations will damage or compromise structural composite strength, remedial steps guided by the performance and engineering requirements of the IBC must be taken.  These measures must be coordinated with removal of the existing roof covering and installation of the new roof covering to keep the building in the dry during operations.

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GYPSUM DECK RESTORATION

Is the roof deck used as a shear diaphragm to resist wind or seismic loads?  If an inspection of the structure by a licensed design professional produces a “no,” then repair methods and techniques are simple. If the answer is “yes,” then things become rather complicated. For a “no” answer, the repair or restoration of the SCA needs to deal only with gravity loads and wind uplift loads.

The Loadmaster dry way involves removing the damaged areas from the bulb tees, cutting the wire mesh leaving a selvage edge, and installing a steel deck section perpendicular to and attached to the bulb tees with screws at intervals sufficient to  brace the bulb tees against rotation under loads. Install optional insulation and mineral board to the steel section at an elevation equal to the height of the surrounding gypsum with an engineered screw attachment pattern to complete the SCA.  Roofing can be installed immediately.  The steel deck section can be flat or corrugated and painted a color to match the underside of the deck.  If needed, an additional layer of insulation and a Duraflex Mineral Board can be installed over the entire roof deck assembly along with the new roof covering.

For a “yes” answer on the diaphragm question, sufficient technical information has not been located by Loadmaster to make recommendations for repair.  One option to consider is to remove the entire gypsum deck, including the bulb tees, and install a new Loadmaster roof deck system engineered to provide all the IBC structural requirements for the building.   A second option is to design and engineer structural steel bracing for the structure to function in place of a structural diaphragm.  Either of these two methods will provide reliable solutions to the problem.

For additional engineering and technical support please contact Loadmaster’s Technical Department at 800-527-4035.

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