Joints can generally be split into two types, butt joints and lap joints.  Lap joints have the advantage of being able to accommodate larger movement and being less obtrusive.  However, they are difficult to install and maintain and are hence rarely used.  There is also a triangular fillet joint but this should not be specified as it offers very limited movement capacity.

Lap joint (diagrammatic)	Butt Joint (diagrammatic)

It is good practice to apply a primer before using sealants even if the sealant manufacturer claims that it is not needed.  Experience has shown that the application of a primer is extremely beneficial in reducing loss of adhesion to the concrete.  It also reduces the possibility of staining/migration.

If the outer surface of the panel is ‘rough’ e.g. exposed aggregate/grit blasted, then care must be taken that the actual joint surfaces are detailed and formed with a ‘smooth’ surface.

The ‘design joint width’ (the width of the sealant) may not always be the same as the ‘gap width’ (the clear space between units) as shown.  Depending on the joint profile, the sealant may then come into contact with surfaces which are closer together than the design width of the joint.  This reduces the distance over which the sealant can ‘stretch’ and can cause failure.  In these cases a bond breaker is required in the form of a tape.  This ensures that the sealant only adheres where intended.

On concrete, silicone seals should be nominally10 mm deep, regardless of the width, and not the often quoted 2:1 width: depth which applies to polysulphides.  For joints over 30 mm wide, the depth may be increased to a maximum of 15 mm.  Although not indicated in the diagrams above, it is good practice for the outer edge of sealant to be stopped some 2 – 3 mm from the front face of the joint.  This minimises the risk of primer and sealant being accidentally applied to the ‘seen’ face.  In the case of a recessed joint, this figure can be considerably greater.

Backing strip is usually in the form of closed cell, medium density, polyethylene foam strip or rod.  The surface does not allow adhesion by the sealant.  The size should be 20 – 30% more than the width of the gap into which it is to be compressed.

When positioning joints care is needed to ensure that the sealant gun can positioned so as to get into the joint.  In the example shown, the left-hand detail may be shown by the Architect since it ‘hides’ the joint.  However, it is not possible to get the gun into a suitable alignment, and hence the joint must be as the right-hand detail.