These high-strength, accurately made spacers should be used to ensure the joint widths required for permeable paving. Stone cannot readily be produced with nibs as can concrete paving; these nibs both manage the joint width and provide strength. With stone, this has to be provided with the spacers and tuffgrit as the jointing aggregate. The calculations for SuDS purposes are the same whether using stone or concrete blocks.
The picture shows a selection of the types available. They can provide joints from 5 to 50mm. Some are used at the junctions between units and some in strips. The wide joints can be infilled with soil if a growing medium is required.
Permeable systems with spacers are not advised for the public highway, but some types may be used up to traffic category 5 (pedestrian areas subjected to occasional overrun of commercial vehicles and car parks receiving occasional commercial vehicular traffic). They are suitable for private drives that need to be SuDS compliant.