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: Sandstone patios we have laid often need to be jet-washed every six months or so as they have become green and slippery. Is this a partiucular problem with sandstone and is there any way to prevent it?

This is a common problem for any type of exterior paving where it does not get sufficient use. Most reports involve sandstone only because it is the most commonly used stone for external paving. Even the most slip resistant stones, such as York stone, suffer from this problem if they are not being used regularly.

Any paving stone that is not sufficiently trafficked collects a veneer of dirt that promotes water retention at the surface and the growth of various organic materials such as algae, mosses and lichens.

When testing such surfaces with the standard TRL pendulum tester the slider tracks across the surface to reveal a very low and often potentially dangerous slip resistance. However, immediately test the surface again and the slip resistance typically improves substantially because the veneer is often easily removed.

This demonstrates how even a thin dirt veneer can create a problem and also how easily that veneer can can be removed by light trafficking.

The problem has become more widespread with the increasing use of sawn rather than traditional, riven (naturally split) surfaces, even though the difference in slip resistance between the two freshly prepared surfaces is typically minimal.

This is because the more obvious texture of riven surfaces provides a means of disrupting the veneer that causes the slip problems.

Sawn surfaces are flat and are wholly reliant on the micro-texture of the grains and voids in the stone to provide their slip resistance.

This provides a clue to resolving part of the problem, in that increasing the visible texture should reduce the potential for slip. This can be achieved by sandblasting the surface. There is now also a water-jet cutting process that can etch the surface of the stone, although this process requires some experience to use successfully.

Removing the problem of surface greening is difficult because wind blown dirt, leaf litter, tree sap and other detritus are always likely to have a presence outside.

Fruit trees are possibly the worst culprits as they also encourage feasting birds with the resulting nutrient laden emissions.

In many locations, greening is a battle that cannot be won without regularly repeated cleaning. Applying biocides or other treatments will generally only treat the current problem and will not remove the build-up of detritus.

In summary, then, if you have or are planning a patio or area of external paving that is unlikely to have a lot of use, make sure the stone has a good texture to at least reduce potential problems with slip.

Otherwise, try to use the area more and enjoy it, and at the very least remove leaf litter at the earliest opportunity after the autumn fall.

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