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: Was someone joking when they said you must be wary of mushrooms growing near paving stones?

It does sound like a joke and it is something you should not take too seriously, but it has been reported that some mushrooms have caused problems with paving.

The first report I have found discussing the phenomenon was by the British mycologist Mordecai Cubitt Cooke in 1862. He mentions a Dr Carpenter, who describes an instance occurring in the town of Basingstoke.

Not many months after paving stones were laid they became uneven and, upon lifting them, large toadstools were found growing beneath them.

One of the stones weighed 36kg, although the force to lift the stone would have been even greater when you consider the interlock resistance that would have been afforded by the jointing mortar.

A similar tale is also told of a hearthstone that only settled once the bedding had been replaced to a depth of 150mm to remove the invading fungus.

Arthur Henry Reginald Buller determined that even quite small mushrooms could exhibit Herculean strength. The turgor pressure of the rather fragile looking 5mm diameter stem of Coprinus sterquilinus is capable of lifting a weight of 250g.

Turgor pressure, or turgidity, is created by the growth of vertically oriented cells that fill with water by osmosis creating a miniature hydraulic ram that enables mushrooms to push their way up through even the most compacted earth.

Fortunately the problem remains little known today as most paving stones are laid on beds of mortar rather than compacted earth or sand.

It is quite possible that the problem affects garden paving but remains either undetected or unreported.

The most likely culprits of such problems could be some of the strongly growing ink-cap mushrooms (Coprinus) or Agarics (Amanita).

Agaricus bitorquis is better known as the \'Sidewalk Mushroom\' in the US because of its often observed ability to punch through asphalt.

The best way to get rid of potentially damaging mushrooms is to dig out the soil, because the mushrooms you see are only the \'fruiting\' bodies of the main growth, known as the mycelium, that lives otherwise unseen in the earth.

However, the problem is something you should neither lose too much sleep over nor be likely to encounter if the paving is properly built.

References: 1) Cubitt Cooke, M. (1862). A Plain and Easy Account of British Fungi, Robert Hardwicke, London. 2) Reginald Buller, AH (1909) Researchers on Fungi, New Phytologist, Vol 8, No 10, pp 391-392

Barry Hunt is a Chartered Geologist and Chartered Surveyor who has spent 20 years investigating issues relating to natural stone and other construction materials. He now runs IBIS, an independent geomaterials consultancy undertaking commissions worldwide to provide consultancy, inspection and testing advice. Tel: 020 8518 8646