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Our 'estate'... stone garden walls

This article is one of a number offering expert but general advice about maintenance and repair of property in our area.

This, the first article, on garden walls, has kindly been contributed by
Stephen Morris
, architect, who lives and practices in Redland.


Preface...

This is not intended as a substitute for professional advice, but may help in getting that advice and choosing a mason who has the necessary understanding and skill to deal with our largely Victorian properties. We also point out that planning consent may be needed for anything other than repair.

This can be checked out at the Planning Office, tel 922 3097.

RCAS Committee believe that the way work to garden walls is executed is very important to our visual amenity –
this is shown by the regular awarding of Gold Stars for particularly good examples.

However, stone walls are generally very simple objects, so the right materials and details are really significant. This means that inappropriate details can stand out badly – for example copings formed with large stones set on their end in cement – an ugly version of the top of a Cotswold dry stone wall and wholly out of place.

We hope members will let us know if articles of this type are useful.

Roger Mortimer, RCAS Planning Group

Also see...

Trees  Common Property Issues No.3
Ironwork  Common Property Issues No.4


Garden Walls...

Click on headings on list of contents or scroll down the page


Introduction

Redland and Cotham are blessed with many fine garden walls built of rubble stone and those fronting onto streets contribute greatly to the character of our area.  Now, after more than a century, many are in a sorry state of disrepair. Some have been patched up over the years, for better or worse, while others look distinctly unstable.

Matters to consider are the general appearance of the wall, the type of stone used,
how the wall was built, the type of coping on top and the mortar that holds it all together.



General Appearance  

Typically, front garden walls finish at a pier. Almost all of these would have been of ashlar, neatly worked and finely jointed Bath stone, with a pier cap.  Sadly, many have been lost when a wider opening is made for an off-street parking space, which spoils the rhythm of the street frontage.

Almost as bad as nothing at all, are poor imitations built of rubble stone or brick. Undoubtedly, a fine stone pier
is the best option, though they do not come cheap.
 
Many front walls were also finished with iron railings. Sadly, most of these were removed in World War II.



Stone 

Many different types of locally quarried stone were used to build our garden walls, including
Pennant sandstone, Clifton Down limestone and Brandon Hill gritstone

Indeed, more than one type of stone can often be found in the same wall. It is best to save and reuse existing stone unless this is badly fractured as weathered material always fits in better than new. 

Appropriate salvaged stone can usually be sourced by a good mason.


Construction 

Rubble walls were traditionally built with a facing of stones on either side, selected for their shape and with a reasonably fair face, with occasional ‘bonders’ - stones that spanned the whole width of the wall.  The core of the wall was packed with pieces of broken stone, pebbles and mortar. 

They were built without much in the way of a foundation, often straight off the clay, after the topsoil had been removed.

In an attempt to save cost, new garden walls are sometimes built of concrete blockwork on one side (usually rendered), with a facing of rubble on the other.  However, this type of wall will need a concrete strip foundation and involve much more in the way of excavation than constructing a traditional wall.


Copings 

Copings of different kinds were used, though most usually they were of Oolitic Limestone

(the best known of which is Bath Stone) or Pennant, often of substantial thickness and set  flush with the face of the stonework. It is possible to find acceptable modern copings made from reconstituted stone (concrete with a smooth facing) and these are cheaper than natural stone.

However, the standard ones found in most builders merchants and garden centres are invariably much too thin and always look hopelessly wrong, particularly when the edges are projected out from the surface of the wall beneath.

Far better is to save up and use natural stone!  Sometimes old ones can be found in the yards of demolition merchants, as can supplies of reclaimed rubble stone. Some existing stone  walls are capped with hard blue or red bricks, usually bull nosed, laid on edge. This can provide a robust and tidy solution, better than some modern alternatives to real stone.


Mortar

Traditional mortar was based on the use of lime, rather than cement, mixed with sand and/or stone dust (actually a mix of crushed stone, graded from grit down to a fine dust).  Such mortar takes a long time to set and, because it remains slightly flexible, allows settlement to take place over the years, without cracking.

The mortar between the stones in the wall is a sacrificial element, which does need redoing from time to time, perhaps every 50 years or so. 

Mixed to be weaker than the stones themselves, a lime-based mortar is able to ‘breathe’, allowing moisture to evaporate naturally from the wall. Making the mortar too strong with cement prevents moisture from evaporating easily and causes the wall to degrade rapidly in frosty conditions


Pointing

The surface layer of the mortar, the pointing, is crucial to the final appearance of the wall.  The sand used may be different from the general mortar or a pigment may be added, and these measures can enable the tone and colour of the pointing to be close to that of the stone, a desirable aim.

Usually it is best, if the pointing is flush or maybe ‘bagged back’ slightly - certainly not spread over
the surface of the stones.
 
Sadly, many examples of bad pointing can be found. The worst type is so-called ‘ribbon’ pointing, made of strong cement mortar left proud of the wall and cut off in straight lines. This catches rainwater, prevents the wall from drying out and looks absolutely horrible.

A good recent example of lime mortar pointing can be seen on the walls of Avenue House,
on Cotham Park North, facing the side of Colston’s Primary School.



Further Reading

‘Bristol – heritage in stone
by Eileen Stonebridge
£2.40
 
A well illustrated handy introduction to the many building stones used in central Bristol.


RCAS March 2004