Why stonemasons dislike precast ‘stone’

I was recently asked to repair a window in Kingston that had been made with precast stone. It was a real challenge and that was what prompted me to write this post.

What is precast stone?

So what is precast stone? Essentially, precast stone is sand and cement coloured and then poured into moulds to create different shapes and left to dry. Masons refer to it as concrete, and most stonemasons hate the stuff. Manufacturers sometimes call it natural precast stone or reconstituted stone but don’t be fooled; there’s nothing natural about precast stone. It doesn’t quite look like any indigenous stone and just doesn’t look right.

In my opinion the main reason stonemasons object is to the use of the word ‘stone’. Precast stone is not a naturally occurring stable and hard wearing mineral that has formed naturally in the geology of the earth over millions of years, but a composite of different materials made in a factory, containing little or no stone dust at all!

What is wrong with precast stone?

Precast stone products cannot be trimmed or ‘dressed’ and in my experience many of even the better quality ones are bowed, thus compromising the finish, especially where complicated architectural mouldings are required. I’ve also found that the arrises (edges) of precast stone to be extremely fragile and even the slightest brush of a glove can ruin the edge.

Look at the photo below – the arrises have chipped after only a year. This would never happen if real stone were used. Another thing to note is the pointing. Precast stone products are usually used by non-masons who leave 10mm joints and use the pointing mix the precast manufacturer has supplied. Masons use hydraulic lime, stone dust and sand to get a near identical colour and hue, working to 3 to 4mm joints to give that perfect finish.

Here the vertical mitre on the middle/left corner is fracturing and the base has chipped.

If you’re a home owner or church, why real stone is probably the right choice for you

There’s obviously a place for “precast stone” in the building trade – not every project needs the best finish and cost is often a factor. But for longevity, serviceability and appearance, I would always recommend real stone. It may cost more but real stone achieves a better result and will last longer and wear better.

In addition, should you be the custodian or owner of a listed or historical building which is in part or wholly comprised of stone, the same stone (or a closest match) should be used for restoration or new build additions, as the following cautionary tale will tell.

This article appeared in Natural Stone Specialist magazine in March 2014:

“Reconstituted stone is not natural stone. While it may seek to replicate and reflect natural stone, its different appearance and behaviour becomes increasingly obvious with time, when the natural processes of weathering reveal the inferiority of the artificial product.” – Inspector.

An independent inspector has upheld a decision by Bath Council planners to refuse retrospective planning permission to use reconstituted stone rather than natural Bath limestone for four houses in the city.

Planning permission was granted for the houses in Gibbs Mews, in the Bath Conservation Area that is part of the World Heritage Site, in February 2009, on condition they were built using “natural Bath Stone”.

They were not. What the council has described as “reconstituted faced Bath stone blocks” were used instead.

The developer, Thameside Property Company Ltd, maintained that the reconstituded stone fulfilled the condition of the planning permission to use ‘natural stone’.

The Council considered this assertion “perverse”. The officer responsible reported when the Council considered a retrospective planning application to change the planning approval in April last year, that: “Natural stone would suggest in its natural state, and reconstituted stone is a product of a manufacturing process, and could never be described as ‘sawn’, as it is on the relevant drawings. If the definition of ‘natural stone’ is extended to encompass reconstituted stone, it would appear extremely difficult for a condition to be set which requires ‘real’ Bath stone to be used.”

The restrospective planning application was rejected. The developer appealed against the decision. On 8 January this year the appeal was heard by Inspector Jennifer Armstrong. It included a site visit. On 22 January the appeal was dismissed. 

Among the reasons given is: “Reconstituted stone is not natural stone.”

Ms Armstrong reported: “While it may seek to replicate and reflect natural stone, its different appearance and behaviour becomes increasingly obvious with time, when the natural processes of weathering reveal the inferiority of the artificial product. 

“This can be seen in a number of 20^th century buildings in the city and, as the Council stated at the hearing, such examples illustrate why natural stone is routinely required for new development in the Conservation Area. 

“And while I have considered the applications before me on their own merits, any acceptance for the use of artificial stone could be cited as a precedent for development elsewhere.”

The Council (its full name is Bath & North East Somerset Council) says it will now write to the applicant to ascertain how it intends to comply with the original planning permission.

No copyright infringement is intended by reproduction of this article from Natural Stone Specialist magazine. All other opinions/musings on ‘precast stone’ are those of a stonemason of 20 years’.