In this regular column, Consultant Barry Hunt answers reader enquiries. If you have a question, email it to: nss@qmj.co.uk marked for Barry’s attention.

Q: During the past cold winter a stone patio I laid using 20mm thick slabs cracked and the pavers lifted, even though I built it to the British Standard. Can you explain?

I suggest first checking that the patio actually was built in accordance with the version of the British Standard current at the time of construction. Standards go through revisions and guidance does change. BS 7533, which covers the design of paved areas, is no exception. Parts 4 and 12 have both been updated within the past few years.

Assuming the patio installation does conform, the question raises an issue of increasing concern regarding the use of thinner slabs for paving.

The British Standard covering the quality of the stone to be used for paving is BS EN 1341, which includes a table of loading requirements for different uses. For garden patios and other similar low traffic and supposedly low loading requirements no actual strength requirement for the stone is stated. Therefore, any thickness stone can be used.

For other uses, the strength of the stone is used to calculate the minimum thickness required and this results in stones of only 20mm thickness now commonly and legitimately being used for paving using procedures originally intended for stones of traditionally 70mm thick or more.

It is strange that the requirement for laying 20mm stones for internal floors, is far more onerous than for laying 20mm stones outside, even though inside it does not have to contend with frost, rain, direct sunlight and other weathering issues.

Outside, thin stone must not be laid on top of loose or flexible bedding materials as hollows will inevitably be present or will soon form. Pavers will be at risk of moving, especially if adversely loaded by having lawnmowers driven over them, for example, or large planters put on part of them. Poor compaction at corners can cause pavers to lift or simply break.

Thin stone laid internally typically requires a concrete base or reinforced screed, and something similarly strong and resistant to movement, shrinkage cracking and other issues should be expected of external installations employing thin stone. It is my personal belief that stones less than 50mm thick should not be installed using traditional paving methods and 70mm might be a better minimum for peace of mind. The main difference between internal and external installations using thinner stone should be that outside a bedding mortar proven to be frost resistant is used.

There is no doubt that the British Standards, including the part of BS 5385 that deals with external tiling, need to be altered and unified. Definitions need to be created to differentiate between tiles and slabs used externally and pavers.

Furthermore, many stones thought to be durable in the UK climate have suffered damage in this year’s cold and snowy winter and a re-think is now needed. I advocate the setting of minimum performance requirements for stones to ensure that more durable materials are used for thinner stone installations. Performance for thicker paving units is less of an issue but a guide to a minimum acceptable quality would still be useful.

Unfortunately, the problems you have experienced may have been unavoidable and were just waiting to be shown up by the return to a ‘traditional’ British winter. In the meantime, it is difficult to offer good advice other than to think carefully about re-bedding the stone and to ensure that drainage is rapid.

References

BS EN 1341:2000. Specification for slabs of natural stone for external paving.

BS 5385-5:2009. Wall and floor tiling. Design and installation of terazzo, natural stone and agglomerated stone tile and slab flooring. Code of practice.

BS 7533-4:2006. Pavements constructed with clay, natural stone or concrete pavers. Code of practice for the construction of pavements of precast concrete flags or natural stone slabs.

BS 7533-12:2006. Guide for the structural design of trafficked pavements constructed of concrete paving flags and natural stone slabs.