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Perimeter expansion joints 

Glass blocks will expand and contract by 0.25mm per 25OC  temperature change. Allowing for expansion and contraction is one of the most critical aspects of any glass block construction, no matter what the size of panel. Soft expansion joints must be incorporated into the perimeter between the substrate opening and block, being caulked with a white silicone (or fire-stop mastic). This will visually look similar a standard mortar joint.

For the head and jambs of an opening, 10mm-thick foam is used. This is a white expansion fibre made from polyurethane pellets so when a drill bit pierces it, the residue material will not snag or spin out.  

The horizontal expansion joint between the first row of glass blocks and the base of the opening is formed using high-density bitumen or neoprene material. This supports the weight of the panel and copes with the expansion by absorbing the compression. It also creates a barrier to prevent the mortar adhering to the aperture. After construction, this joint should be cleared of any residue mortar and caulked with Rods & Mortar expansion sealant. Bridging the joint would restrict flexibility and movement and negate the expansion fibre. 

If the aperture height has been calculated incorrectly and around 10mm needs to be gained, two coats of bitumen emulsion can be applied as the barrier between the bottom course mortar joint and base of opening.

Reinforcement rods are carried through the expansion material by piercing a hole in the foam or bitumen. 

Creating a large glass block panel may require being split into sections where it exceeds 6m in either direction. Slip joints, structural steel or wind posts should be incorporated. Expansion fibre is also required between the glass blocks and intermediate support.

 In curved walling as well as perimeter expansion, where the curves change plain a vertical soft joint should be incorporated. This rule also applies when using corner blocks, this joint is usually placed between one and three joints away from the corner glass block.

It is extremely important not to bridge the expansion joints with mortar, plaster or render. All soft joints should be caulked over using Rods & Mortar expansion joint sealer (or fire stop mastic). If the joint is bridged or expansion restricted in any way, it can cause glass blocks or joints to crack.

If the external facade of a building is rendered, it is advisable to incorporate framing to render up to.

The perimeter expansion joint must separate the framing and glass blocks. The framing can be U-channel, box section or even timber.