Question:

• Why do gypsum ceiling panels of laid-in exposed grid ceiling system often encounter visible sagging after being put in use a short period of time ? 

   

Answer:

• For exposed grid ceiling with laid-in gypsum panels at square pattern 610mm x 610mm or rectangular pattern  610mm x 1220mm, directly suspended from uninsulated metal roof / concrete terrace / tile roof with a slope less than 45 degrees, 9mm / 9.5mm or 12mm / 12.7mm thick gypsum ceiling panels will quickly sag up to 10mm or more, within 6-9 months after dropped into the ceiling grid. It is due to the excessive high temperature at the ceiling plenum (below metal roof / concrete terrace / tile roof and above the gypsum ceiling panels), which is usually at 55 - 60 degree C, totally unsuitable for gypsum wallboards.

   

• For exposed grid ceiling with laid-in gypsum panels at rectangular pattern 610mm x 1220mm directly suspended from well insulated metal roof / concrete terrace / tile roof, 9mm / 9.5mm thick gypsum ceiling panels can sag up to 5mm, sometimes more, within 12 months. Here is the secret: those gypsum ceiling panels are cut from full size gypsum wallboards 9mm / 9.5mm x 1220mm x 2440mm, which are designed and produced for flushed jointed screwed-up concealed grid ceiling, absolutely inappropriate for exposed grid ceiling. Dropped-in exposed grid ceiling systems require special sag-resistant gypsum ceiling panels. It is best to use 12mm / 12.7mm thick Standard core gypsum panels for the installation of exposed grid ceiling with rectangular pattern 610mm x 1220mm if a maximum sag of 2-3mm is required for gypsum ceiling panels. Such a small deflection is hard to detect by normal eyes. In brief, there is no zero deflection, regardless of nature of ceiling panels installed in the exposed grid ceiling.

   

• Another reason of this deflection when 9mm / 9.5mm thick gypsum ceiling panels are used is the incorrect installation method of ceiling grid components, by chance or by intention (to reduce the costs). If Main Tees 3660mm are spaced at 610mm centers, due to having to use more materials, the installed ceiling system will be more costly but its loading capacity is very good. To avoid this high cost system, most ceiling contractors in Vietnam install Main Tees 3660mm with spacing at 1220mm centers. By doing this, they can reduce  the cost and get higher margin, but the loading capacity of the ceiling grid is very poor: Cross Tees 1220mm warp down under the weight of gypsum ceiling panels, and the latter follow this trend.

   

• The fourth reason, but surely not the last one, is the too high moisture level, normally above 90% RH, of the working environment where gypsum ceiling panels are installed. This excessive moist ambience is obviously beyond the limit of gypsum plasterboards.