Lime Repointing of a Red Sandstone Barn

“Private residence, Kirkby Thore, Cumbria”

Lime mortars protect stones from erosion, but they do so sacrificially – at the expense of themselves. This is particularly true on softer, porous stones, like the red sandstone on this barn in the Eden Valley. Here the previous pointing mortars (probably dating from the early 1900s), were deteriorating badly and no longer protecting the building. This project involved repointing with a hot-mixed fat lime mortar.

red sandstone barn conversion near Kirkby Thore, Cumbria, after repointing in hot-mixed lime

↣ Mixed rubble and red sandstone barn conversion after repointing in hot-mixed lime mortar.

Protecting red sandstone with lime

Part of the reason the red sandstones of the Eden Valley, and other areas, are so prone to erosion is its porosity. It readily absorbs water in copious quantities. Far more water will cycle through it in its lifetime than almost any other stone type. And the evaporation of all that water causes salts to precipitate and crystallise, and it is those crystals that generally cause erosion

rubble and sandstone barn conversion before repointing in lime

↣ Barn prior to repointing, having been converted to residential use some 20 years previously.

Lime mortars draw water (and salts) away from the stone. To do so effectively, it needs to envelop the stone as much as possible. As the pointing mortars erode over time, the protection given to the stone reduces and the stone itself begins to erode. The erosion of the stone in turn accelerates the deterioration of the mortar.

Such was the situation here.

eroding stone and mortars in old sandstone and rubble barn wall

↣ Eroded stone and mortars, along with missing stones and previous cement repairs; the wall before repointing

It’s important to consider carefully the style of pointing applied to sandstone buildings. Attempting to retain the exposed, weathered appearance of stone buildings inevitably means that the repointing mortars must be recessed slightly. Doing so might preserve the character of the building, but the exposed stone will continue to erode – although, hopefully, more slowly.

The most sustainable solution is to reinstate the original finish – which, in the case of many non-domestic buildings, was flush pointing and limewash. If exposed stonework is desirable, the mortars should be pure lime (rather than a bagged hydraulic lime).

Hot-mixed lime pointing

To start with, the crumbling mortars, flaking stone and other cement additions were removed to provide a good depth for repointing. Because of the friable nature of the eroding stones, getting good adhesion with new mortars is often difficult – so pointing at depth provides a good grounding for the mortars.

lime pointed barn repairs, Eden Valley, Cumbria

↣ Repointed wall having had cement and crumbling mortars removed and various stones reinstated.

The mortars used were lime rich (at 1 quicklime to 2.5 aggregate). In addition, the aggregates included new crushed limestone dust (from the same source as the quicklime) and reused mortars which were taken from the wall. Together with the addition of crush brick, this should make a very breathable mortar.

(The addition of limestone – whether in the form of crushed stone or reused mortar – has interesting properties, because it provides a supply of calcium bicarbonate. Calcium bicarbonate forms in the presence of carbonic acid (rainwater, say) and reacts with calcium hydroxide (the slaked lime that is used to make mortar) to creating calcium carbonate. Consequently, the addition of limestone may help promote early setting at depth, rather than just the exposed surface – which is the normal process for lime mortars.)

hot-mixed lime pointing helping to reduce stone erosion

↣ Lime pointing mortars slightly recessed to retain the exposed stonework, whilst still helping to reduce erosion.

Historical snippets

Limewashing was once ubiquitous across the UK – not only for the reasons outlined above, but because it’s an efficient means of construction. On this building there were clear signs it was previously limewashed, but then later repointed without. Exactly when that happened isn’t clear.

evidence of earlier or original limewashing of barn in Cumbria

↣ An earlier limewash extending behind a subsequent repointing (left); limewash covering an earlier (perhaps original) mortar (right).

There were also signs of even earlier earth mortars. The building itself wasn’t built with earth mortars, so these may be fragments clinging to the stones from a previous building – which would be a tantalising glimpse into the past! The survival of such fragments confirms the durability clay as a construction material.

evidence of earth mortars in a Cumbrian barn

↣ Fragments of earlier, earth-based mortars – a ubiquitous building material in pre-Victorian Britain.