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  A Quarterly Publication by Boral Plasterboard Spring Edition 2000 Targetingthe Issues Boral  for   Builders F  o  l l o  w  i  n  g  S  p  e  c   i   f   i c   a   t  i   o    n   s M a  n  u  f  a  c   t  u   r  e   r s G  u a r  a  n  t  e  e  s  &  W  a  r   r  a   n   t   i  e   s  W  o  r  k  m  a  n  s   h    i   p  H  i  d  d  e  n  C  o  s   t  2 About This Edition Welcome to the third edition of Boral Plasterboards’ ‘Boral forBuilders’ Magazine.Once again we focus on thereal issues facing tradespeoplein the plaster industry.In particular this issue, we target back-blocking, effects of lighting on finished plasterboard walls & ceilings,Expansion and Control Jointsand warranty problems associated with using ‘mix andmatch’ products. And as withevery issue, the Bureau of Statistics have provided uswith the ‘housing and residential building approvals’for the ‘June Quarter 2000’.The Editor.Boral for Builders MagazineBoral PlasterboardMarketing ServicesDepartment676 Lorimer Street Port Melbourne Vic 3207 Contents  The Back-blocking Story ...2,3Getting the Right Angle4,5on LightingExpansion or Control Joints 6 & 7Building activity statistics 8, 9 The Mix & Match Trap10, 11 TecASSIST 12 Some years have passednow since the SouthAustralian experience of peaking, joint cracking and joint distortion that affectedmany homes. In the meet-ingsthat followed, isolatingthe exact cause proved onceagain to be elusive due to the many variables in thebuilding process. On the positive side however, the jointsponsored investigation conducted atMonash University that followed andthe inclusion of back-blocking intoAustralian standards and plasterboardmanufacturers specification must surelybe recognised as one of the most significant contributions toward bothmaintenance reduction and the overallpresentation of plasterboard seen in theindustry for a long time.In some quarters, slow recognition of the benefits has been governed somewhat by the belief that anythingthat impacts on the final selling pricewill reduce competitiveness in the market place or by resistance at timesfrom trades people claiming they havenever experienced ceiling problems.Are they serious??It is of interest to find that decisions regarding a few dollars for extras thatcan add significantly to someone’sprestige home receive little consideration (particularly if they can’t be seen).My experience has shown regular comments from frustrated owners whowould have gladly paid any extra costsfor back-blocking if only they had beengiven a choice rather than be faced withthe dust and disruption that follows any rectification program carried out overtheir new carpet and furnishings.As plasterboard manufacturers, wehave witnessed and are happy to report a noticeable decrease in costly maintenance issues relating to ceilingproblems as a result of the back-blockingsystem.Many plasterers and builders now implement back-blocking of ceiling joints as part of their normal practiceand jobs are priced accordingly.Unfortunately some confusion stillexists in the industry in relation to: a)  The exact requirements of thestandard. b) Whether it is a manufacturers specification or not. c) Is back-blocking required with13mm ceilings? d) Is back-blocking required when battens are used? The Back-blocking Story... Inverted peaking due to the ageingprocess.  3 A. Extract from Australian/New Zealand Standard AS/NZS 2589.1:19977.7.2.4 Recessed joints in ceilings – All recessed joints on a ceiling shall be back-blocked in any areacontaining three or more recessed joints.7.7.3 Back – blocking procedure. Ceiling recessed edge joints – the procedure shall be as follows:a) Cut back-blocks at least 200mm wide and long enough to fit loosely between the framing members.b) Apply plaster based setting type adhesive/cement to back-blocks over the full face of the back-block. A notched spreader to give 6mm x 6mm beads at approximately 20mm centres at right angle to the joint would be satisfactory. B. Back-blocking does form part of manufacturers specifications and is recommended by leading industry authorities. C.  The standard makes no differentiation between board types or thickness; the reference is for all ceilings to be back-blocked. D. Battens either metal or timber can be no … Substitute for back-blockingdue to the separate functions they are expected to perform. (Battened ceilings must be back-blocked.) The requirements for back-blocking make no differentiation between the variousapplications or building systems, so it should be noted that any warranties orguarantees from your plasterboard manufacturer will in all probability revolvearound compliance to these recommendations, should problems such as joint distortion, peaking joints or cracking occur.Failure to comply may render your plasterer liable for any associated cost relatingto maintenance and/or rectification caused by suspected jointing defects.Our views on the merits of back-blocking remain unchanged despite a trend inthe industry to exclude anything that remains unseen or contributes nothing tothe overall short-term appearance. Particularly if there is an added cost similarto other systems such as sisalation, paint sealers, ventilation etc. Have you everwondered why a large ceiling that looked perfectly flat at handover was almostan embarrassment hen you are called back some time later. At this point wemay have blamed settlement, movement etc. and over time this explanation forthe deterioration has been accepted as a foregone conclusion.We now know that ceilings that have been back-blocked correctly will perform satisfactorily for the life of the building and eliminate joint distortion, crackingpeaking joints etc. Cheap insurance for any slight additional cost. Plasterboard peaking has probably been part of the housing industry for a long time…however modern design techniques particularly the switch from relatively small rooms toopen-plan living and the use of lights fitted flush with the ceiling tend to accentuate the peaking says Shane McCartin, HIA National Technical Director. Although the sheets are flat when they are laid, any growth or ‘creep’ by the plasterboard orits supporting structure may result in distress of the plasterboard joints, resulting in ‘reversepeaking’ or cracking if the joint is pulled apart or peaking if it is forced together. The distressat the joint may be small but is highlighted as shadow by the flush-fitted lights. The HIA together with Victoria’s Building Control Commission, plasterboard manufacturersand the timber industry are funding a $30,000 research project aimed at eliminating the problem of ceiling joint cracks and peaking commonly known as ‘plasterboard peaking’. The results of the research will hopefully enable the industry and the HIA to work together to determine a solution to this problem. The Facts are: Rafter or joistBack-blockTemporary battensBoral PlasterboardsheetTemporary nailingCornice adhesivePacking strip Back-blocks in placeBoral Plasterboard ceiling Ceiling joist or bottom chordof roof truss Wall Back-block, from pieces ofBoral Plasterboardsheet 200mm wideminimum, glued withBoral PlasterboardCornice AdhesiveRoof trussBoral Plasterboardceiling Cross section through back-block.View of back-blocking in ceiling from below.View of back-blocking in ceiling from above.  4 Much has now been said about the effectsof lighting on finished plaster surfaces.Despite our best efforts with education, literature and illustration, there are still manyhurdles to overcome particularly whenattempting to demonstrate to a less thanhappy owner (one who has just viewed forthe first time the ceilings of his/her new home at night) the circumstance behind thisunwanted phenomenon occurring in theirresidence. When considering the type of finishrequired from plasterboard, it isimportant to understand how theoverall appearance is to be affectedby glancing light. Getting the Right Angle on Lighting We are aware that the common requirement is for a flat blemish-free surface. However, despite every-one’s best intentions that is not always possible toachieve. As industry professionals, we are faced with a constant challenge to give a better understandingto designers and builders so that they may indicate to the end user the standard that can reasonably be expected from the choice of materials and design within their nominated budget.Unfortunately there is still a gap between owner’s expectations of plasterboard finishes and what is realistically attainable, particularly when consideration is given to the materials, substrates and systems that are available. Plasterboard ceiling under glancing light conditions.
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