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458 Barry’s Advanced Construction of Buildings
Mastic sealant
and backing strip
Mastic
sealant
Single skin
GRC panel
Mastic sealant and
backing strip
Figure 7.38 Mastic sealant joint.
A mastic sealant joint should be no wider than 15 mm as the sealant in a wider joint
might sag and no longer seal the joint. The sealants most used are two part or one part
polysulphide, one part polyurethane, epoxy modified polyurethane and low-modulus silicones.
Which is used depends on the skill of the operative and the ease of access and
application. One part sealants are easier to use but less effective than two part sealants.
Silicones tend to be the most difficult to use and the most effective. In time, sealants may
oxidise and harden and require renewal after a number of years.
Gasket joint
A gasket joint is used for larger GRC panels where accuracy of manufacture and fixing can
provide a joint of uniform width. The elasticity of the gasket is capable of accommodating
the moisture movements of the panel while maintaining a weather seal. A gasket joint may
be used for vertical and horizontal joints or for vertical joints alone with overlapping
rebated horizontal joints, as illustrated in Figure 7.39.
For gasket jointing, the edges of the panels are rebated to provide space for insertion of
the gaskets. A backing strip and mastic seal is formed on the back face of the joint to exclude
wind. The strip of neoprene gasket is rammed into the joint to make a weathertight seal.
Where both vertical and horizontal joints are gasket sealed, preformed cross-over gaskets
are heat welded to the ends of the four straight lengths at junctions of vertical and horizontal
joints and then rammed into position. Being set in position some distance behind
the GRC panel faces, the gaskets are protected against the scouring effect of wind and rain
and also the hardening of the material due to oxidisation caused by direct sunlight. It is a
reasonable expectation that these gasket joints will be effective during the life of most
buildings. The advantage of the overlapping, rebated horizontal joint illustrated in Figure