Lime Mortared Sandstone Wall

“St John’s Church, Murton & Hilton, near Appleby-in-Westmorland”

St John’s Church, Murton & Hilton, was built in the mid-nineteenth Century to avoid the need for parishioners to make to the 3 or 4 mile walk to the church in Appleby – at factor which, so the story goes, was leading many local residents to attend one of the nearer non-CofE chapels. The church and its surrounding walls are a fine example of the Victorian fashion for exposed stonework.

The boundary wall is in remarkably good condition after more the 150 years, but was suffering localized distortion – largely due to pressure from a now felled tree. Cumbria Stone Walling was asked to repair these sections.

red sandstone squared rubble wall in Cumbria repaired with lime mortar

↣ Restored section of red sandstone wall at St John’s Church, Murton & Hilton, Cumbria.

Structural Integrity

The worst section of wall was leaning at close to 30 degrees, whilst retaining made ground and carrying the load of coping stones weighing as much as 150kg each. It was still standing, not because any of the individual components or materials was particularly strong – in fact, quite the opposite. It was still standing because the structure as a whole worked to absorb external stresses.

leaning retaining wall built in red sandstone and lime mortar - Murton, near Appleby, Cumbria.

↣ The tightly jointed stonework with hand-dress internal faces and soft lime mortar allowed this wall to remain standing.

Although no analysis was done on the surviving mortars, visual examination showed the material was likely very lime-rich with well graded course sand as the principle aggregate. The abundance of very fine material (lime and, possibly, stone dust) was most likely intended to make a very workable mortar. This is pretty much essential when using finely jointed soft stonework (like red sandstone).

retaining wall built in red sandstone and lime mortar - dismantling prior to rebuilding using lime mortar

↣ Despite appearing to be little more than a crumbled mess, the original lime mortar was still doing its job well. Props were added to allow safe removal of the heavy coping stones.

It was testament to the materials used that virtually every single stone – right down to the smallest of pieces. Red sandstone has a reputation for weathering badly, and is liable to degrade rapidly if used with hard hydraulic limes and cements. With a suitable non-hydraulic lime, and with adequate routine maintenance, however, it can last almost indefinitely.

Reconstruction

The deformed sections of wall were dismantled sufficiently to create a stable structure, but without removing any more of the original than strictly necessary. In the case of the section affected by the tree stump, this meant repairing the full height, in order to extract the tap root and several lateral roots (which were obscuring the original line of the wall.)

retaining wall dismantled and tree roots removed prior to rebuilding

↣ The width of the wall was only 14”, but its strength derived from good stonework, soft mortars and top stones which have prevented weather penetrating the wall.

The wall was rebuilt as faithfully to the original as possible – although gradations in the range of lichens up the wall (themselves a valuable feature) made exact replication impossible.

Mortars were made on site, hot-mixed in order to benefit from the initial fluidity of freshly made lime mortar. The mortar consisted of quicklime and limestone dust – both from Shap – along with red sand and sharp grit sand, plus a small amount of crushed brick. The crushed brick was added to stiffen up the mortars in anticipation of the approaching winter.

Crushed brick – which acts as a mild pozzolan – appears to have been widely used, certainly by the nineteenth century.

like-for-like conservation repair to red sandstone retaining wall

↣ Reconstructed wall, built with like-for-like lime mortars and hot-mixed on-site, being brought up to through level.

The original stonework would be classed today as random-coursed squared rubble walling. Each stone was hand worked on five sides – work that was probably done at the quarry, then transported to site in its finished state. The roughness of the unseen internal faces are critical to the overall strength of the wall – acting to resist movement by generating shear between the stone and the mortar.

flush squared through stones in red sandstone retaining wall

↣ A single, continuous band of through-stones were originally incorporated mid-way up the wall, and reinstated in the same manner.

Much of the modern stone facing applied to new buildings in this region is similarly described as random squared rubble. In reality, its internal faces are sawn perfectly flat and bear little resemblance to the organic form of true squared rubble walling. Despite this it is still routinely accepted and required by planning authorities.

flush squared through stones in red sandstone retaining wall

↣ The key to the strength of this wall: hand-dressed internal faces prevent individual stones moving independently, whilst the properties of lime mortar have preserved the stones in immaculate condition – despite being a very soft stone-type.

Finished

Rebuilt sandstone retaining wall complete with lime pointing

↣ Reinstated lime pointing – flush with the stone surface – helps define the stonework, whilst protecting the faces from erosion.

The completed wall sections had their heavy tops reset in position and the stonework pointed flush to protect the stones from erosion. Whilst much of the existing stone is still in good condition, the original mortars have been lost (and would have, ideally, been repointed many years ago). As a consequence, some of the facing stones have begun to erode quite badly.

Repointing arrests this erosion both by managing moisture and by sacrificially weathering in preference to the stonework.

Whilst repointing can change the appearance, it is absolutely vital if we are to pass such structures on to future generations.

Rebuilt sandstone retaining wall complete with lime pointing

↣ Repointing necessarily changes the appearance of the wall, but is essential for its long-term protection. The recessed appearance of the existing stonework is due to weathering and some of the stones have begun to suffer as a result.

The process of repointing was not entirely without incident – being hit by several days of severe gales and driving rain, followed by prolonged frosts. The exposed mortars responded by blistering and forming a hard crust at the surface. This will be kept in place over winter before being brushed off in spring.

The underlying mortars will set up over the coming months and should age gracefully, whilst providing many decades of good service.