art of laying modern paving

Changing tastes, renewed interest in the garden and, possibly, climate change, has meant the paved area of a garden has evolved into a popular feature. Thanks to lifestyle magazines and television programmes, domestic clients aspire to use their garden as an extension of the home.

At the same time, global trade has opened endless possibilities to the landscape professional; materials from around the world can be used to striking visual effect in a variety of contexts. 

In most cases, paved areas feature an area of natural or man-made material expertly laid in accordance with good design principles and building regulations, with the potential to last for as long as the customer needs.

Unfortunately, BALI has recently identified isolated cases where modern landscape materials are being laid according to outdated construction methods. This is resulting in areas of hard landscaping which, at best, fail to meet the clients’ expectations, and at worse are unusable.

The cause of the issue is the porous nature of most modern paving materials. Untreated stone, limestone, sandstone, granite and concrete contain tiny pores which allow water, salts and minerals to pass. This means that once laid on a bed of mortar, water, salts and minerals may pass through natural stone either from above or below the surface. 

Modern Paving Patchwork

Correct laying procedure is essential. Mortar acts as a barrier to moisture, meaning water cannot pass as freely through natural paving where the underside of the material is in contact with mortar, as it can when there is no mortar in contact with the underside. A common cause of this is when paving is not laid on a full bed of mortar as specified in BS 7533-4:2006.  

Mortar is, however, a potential source of salts and minerals, which means minerals and salts from mortar in contact with natural paving may be drawn into the material and deposited either on or close to the surface. The movement of minerals and salts into natural stone paving can manifest as different coloured patterns on the top surface of the paving. This may occur within a few days of the paving being laid or over a longer period. This pattern is a stain; it is unsightly and permanent, and in some cases, the only solution is to replace the paving.      

Manufactured materials such as porcelain or ceramics are becoming increasingly popular due to their thin profile, uniform colour and texture, but also their resistance to stains.  These materials are not porous, which means mortar does not readily adhere to the underside. As a result, porcelain slabs laid on mortar alone are likely to become loose within a relatively short period, resulting in rocking, loosening of the grout and eventual cracking of paving units. Instances of slate not adhering are also common. 

The solution to both problems is the use of a specialist coating called a slurry primer or bond bridge. Whilst the use of slurry primers is not new, consideration for their need for paving projects of all types is now good practice.  

Slurry primers or bond bridges are painted on the underside of both natural and manufactured paving materials immediately prior to laying on mortar. They form a strong, impermeable, glue-like layer which performs two functions:

  • Forms a barrier between paving, the mortar and laying course, which helps prevent moisture and cementitious materials leaching into the paving. 
  • Improves adhesion between the mortar and underside of manufactured paving slabs

A range of ready-made primers are available from paving suppliers, needing only water adding to a pre-mixed material. It is also possible to mix cement with SBR to create a similarly effective product. Either way, the product should be painted to the underside of each paving unit immediately before laying. 

The quality of natural paving products has a significant impact on its porosity and long-term durability. Always source materials from a reputable supplier and take their advice when laying materials with which you are unfamiliar.

Thank you to BALI Registered Affiliate London Stone for supporting this article.