Joint sizes and spacer pegs
10mm is the most common joint size for specifying and building glass blocks. A 190x190mm block plus a 10mm spacer modulates to 200mm.
10mm is the distance between the blocks, accurately achieved by using spacer pegs. The distance in the centre where the two concave collars are opposite is approximately 18mm. The oval joint is always deeper and this area houses the reinforcement bar.
Spacer pegs serve multiple functions:
When a spacer peg is fitted and the wall is finished, the tabs at the end twist off and can then be grouted over.
Other spacers are available for 80mm-thick blocks (such as 6mm x 6mm and 6mm x 10mm). They can be used to achieve a narrower joint for aesthetic purposes or for smaller openings. These are also ideal for creating curved panels when a tighter radius is required. With the inside vertical joint being narrower the external vertical joint will also be reduced.
A critical golden rule is no glass block mortar joint should exceed 22mm as it is likely the mortar will crack, which could lead to water ingress, leak or freeze.
Spacers are produced to suit 80mm- and 100mm-thick glass blocks. Mortar joints less than 6mm are not recommended because these may prevent curing and affect the stability of the panel. Building freehand is not as accurate as using spacers; weight compression could cause the blocks to slump or joints to be uneven.
(To achieve the slimmest joint achievable, see Easifix...)