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Why do gypsum ceiling and drywall partition flushed jointed with... Print E-mail

Question:

 

  • Why do screwed-up concealed grid gypsum ceiling and drywall partition systems flushed jointed with perforated paper tape and plaster based jointing compound have joints with far superior anti-cracking property than similar systems treated with self-adhesive fiberglass mesh tape and calcium carbonate based putty compound ?

     

 

 

 

Answer:

 

  • A first glance at perforated paper tape and self-adhesive fiberglass mesh tape tells us that fiberglass mesh tape is pretty strong and difficult to destroy, whereas paper tape can be torn off manually. However, the issue is when using these tapes to reinforce gypsum plasterboard joints, the continuous and cross fiber structure of perforated paper tape helps the joints consisting of a bedding coat of jointing compound, following by paper tape, and three successive coats of jointing compound on top achieving an excellent anti-cracking property; while due to its mesh pattern, fiberglass mesh tape is easily to deform (glassfibers woven into mesh are expanded and contracted, mesh in square pattern are distorted), inducing a movement of dried coats of jointing compound on top of the tape, hence resulting in a joint crack.

     

  • It should be noted that above facts are only valid for perforated paper tapes and self-adhesive fiberglass mesh tapes produced in USA to meet strictly the American standard ASTM C475. Most renowned gypsum wallboard manufacturers in the world have intensively tested and demonstrated that wallboard joints reinforced with perforated paper tapes possessing an anti-cracking strength about 50% better than joints treated with fiberglass mesh tapes on the basis of using the same jointing compound.

     

  • We should also recognize the importance of jointing compound which in turn, is the second component of the joint system, apart from reinforced tape being the first component. Plaster (CaSO4.1/2H2O) based jointing compounds in powder form, after thoroughly mixed with clean water and coated on top of paper tape / fiberglass mesh tape, will react with water and gradually turn back to gypsum (CaSO4.2H2O) when fully dried. Thanks to this chemical reaction, dry coats of plaster based jointing compound have excellent anti-cracking strength. Wheareas putty compound used by most non-professional workers in Vietnam for joint treating gypsum drywall and ceiling are finely ground calcium carbonate (CaCO3) particles, inert to water, only bond together due to polymer additives, have a very poor performance of anti-cracking when fully dry. Calcium carbonate based putty compounds are preferred by non-professionals since they are soft and very easy for sanding when fully dried.

     

  • In all cases, gypsum wallboard joints have the best anti-cracking strength when treated with perforated paper tapes and plaster based jointing compounds. In some special cases (building structures are extremely sturdy, buildings are not exposed to vibration during use, the surrounding ambience does not have high humidity, temperature between day and night does not change much, etc.), wallboard joints can be treated with self-adhesive fiberglass mesh tapes and plaster based jointing compounds. Never try to reinforce wallboard joints with self-adhesive fiberglass mesh tapes and calcium carbonate based putty compounds because apart from a very poor anti-cracking strength, high alkali level putty compounds might destroy glassfibers that form meshes of the fiberglass tapes.

 

  • Summary:
    *  Flush jointed with Perforated Paper Tape & Plaster based Compound: board joints are excellent crack resistant
    *  Flush jointed with Fiberglass Mesh Tape & Plaster based Compound: board joints are fair crack resistant
    *  Flush jointed with Perforated Paper Tape & Calcite based Compound: board joints are fair crack resistant
    *  Flush jointed with Fiberglass Mesh Tape & Calcite based Compound: board joints are poor crack resistant  

 

 
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