I have been told not to use building sand to bed and joint setts. Is this correct?

According to Part 7 of BS 7533, whether or not sand is used depends on whether the setts are sawn or cropped sided for use in a flexible pavement construction. However, I believe your question stems from the belief by some that natural sands do not work as well as crushed rock fines for laying setts.

The British Standard clearly states that natural sand from a terrestrial source such as a pit, or a marine-dredged source may be used for bedding and jointing sawn sided setts, so long as such sand meets quite specific grading requirements.

This basically means that the sand has to have a variety of different sizes in different proportions that allow it to be compacted, leaving minimal space between the sand grains to reduce the potential for collapse. In this respect you should not be using general building sand unless it complies with the grading requirements.

Sawn setts should be laid with quite narrow joints – down to 2mm – and this requires relatively fine sand. General building sand will probably be too coarse and not contain sufficient fine material. Fine silver sand would be more akin to the type of material used for joint filling.

Cropped setts are typically very rough and inevitably require to be laid with relatively wide joints – 10mm being the nominal width recommended.

If such wide joints were filled with natural sand there would be little to stop the particles from being rapidly removed in use. This is why only crushed rock fines up to 6.3mm in nominal maximum size with a specific grading are allowed, so as to provide good interlock and also adhesion via particle surface friction effects. Once the setts have been compacted, a fine joint topping material is applied to try to fill any gaps remaining in the jointing.

At this point I would like to mention that cement should never be added to either the bedding or jointing aggregates of a flexible pavement design. This is sometimes carried out in a misguided attempt to reduce compaction and movement of the setts. Such action never proves durable as it seems to reduce the adhesion of the jointing with the setts.

The success of flexible sett pavements is ultimately down to maintenance and the regular topping up of joints as they compact and material is lost.

Eventually the joints will fill with dirt and become both fully compacted and hardened, at which point they are able to resist weathering processes, including the effects of street cleaning machines.

So, in answer to your question, I have to state that the use of building sand is not recommended under any circumstances unless you know the grading and have determined that this complies with the British Standard requirements.

Correctly graded bedding and jointing aggregates should be available from those companies who specialise in the supply of natural stone setts at a cost that is not significantly more than that of building sand. The potential consequences of using the wrong sand certainly do not justify the relatively small savings.

BS 7533-7: 2002 Pavements constructed with clay, natural stone, or concrete pavers – Code of practice for the construction of pavements of natural stone setts and cobbles. Published by the British Standards Institution, London.