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: What treatments would you recommend for external paving?
The problems with leaf litter and other products that may be expected to drop on to patios were highlighted in this column in November. My advice was that it is best to remove leaf litter as the leaves fall and wash the patio down but otherwise leave the stone surface to mature.
However, some people still want their stone patios to remain looking pristine. This is an unrealistic expectation but one that needs to be addressed because such a wish is relatively easy for someone to propose to fulfil with their latest wonder product.
Using such products can be an expensive waste of time and in the worst cases result in premature failure.
The major problem with any product applied to stone used externally is that the application creates a barrier not just to whatever falls from above, but also to moisture that will inevitably be present below the stone.
This can retard the drying rate and result in higher moisture retention in the structure below – and not just the stone. In maritime countries that experience regular frosts the stone can easily be damaged by ice formation.
Treatments also are rarely able to cope with joints, which will open up and allow moisture through the barrier.
Manufacturers may claim that their products allow the flow of moisture vapour, but this is only if water vapour is actually being formed. The process might potentially be very slow relative to the input of moisture from below.
Furthermore, if freezing occurs from the outer surface this will retard vapour movement and create a barrier to moisture below not yet frozen.
Frost attack can and will occur, no matter how good a treatment is, even presuming it has been applied correctly.
The long-term effects on slip resistance should always be queried. Initially good results are typically promoted, but I have yet to see long-term results that would surely be promoted if the products worked in this respect?
When confronted with any marketing data quoting test results and trying to present a scientific spin, try to find out who undertook the tests, whether they are reputable and whether a proper long-term study has been carried out.
Look for test results on a range of stone types and query where there appear to be holes in the data, which there often seem to be.
Returning to slip resistance, when this is checked it is often found that any initial increase in slip resistance quickly disappears and might, in fact, end up being reduced.
Also, sealed stone surfaces that do not allow surface water to soak in may develop a film of ice quicker than untreated stone.
The only time I believe treatments applied to natural stone surfaces to be walked upon might be of benefit is when the stone is a granite or similar crystalline rock.
Such stones are typically too strong and exhibit extremely low absorption rates for them to be significantly affected by frost attack and other weathering factors. Therefore the minimal changes to the stone properties brought about by treatments are extremely unlikely to have a substantially negative impact.
In this instance, staining might be reduced and the maintenance of the original appearance prolonged.
It is difficult to predict all the potentially negative issues associated with treatments and thus be able to ask all the right questions in order to make an informed decision about whether their use will really be of benefit.
Unless the purveyors of these products can back up their claims with proper research relevant to the situation you are faced with, I would simply avoid them.
Stone sold for paving should be fit for purpose without treatments. If anything goes wrong you should be able to claim compensation. If you treat a stone and it fails, obtaining redress may be more difficult because it will not necessarily be clear whether it was the stone the failed or the treatment that caused the failure. Both the stone and treatment suppliers are likely to blame each other.