Hot-mixed Lime Construction

“Private residence, near Appleby, Cumbria”

The brief for this project was for a retaining wall which sat well against the adjoining property, using materials reclaimed from an unwanted nearby wall. I retrieved as much limestone rubble as possible (which was the predominant material used in the converted barn), and constructed it using hot-mixed lime mortar. The construction method would be immediately recognisable to any craftsman working two or three hundred years ago. There is every reason to expect that the structure will stand for a similar length of time.

lime mortared retaining wall constructed for a barn conversion near Appleby-in-Westmorland, Cumbria

↣ Retaining wall for new garden path (pathway built by customer), designed to blend nicely with the limestone rubble of their barn conversion.

Lime Mortared Solid Wall Construction

Countless historic structures have been damaged in modern times by builders looking at the original mortars and saying they are too soft and weak. They should have looked at overall structure – rather than its individual components – and said: “can I build anything better than that?” My answer would always be ‘no, but I can certainly hope to replicate it’.

Solid wall construction differs very little from dry stone techniques, but with the additional support of a durable, if not strong, lime or earth mortar. Hot-mixed lime mortars (made by slaking quicklime with sand) seem to be in their element here. The heat (caused by a chemical reaction between quicklime and water) encourages the mortars to stiffen quickly. This holds the stones firmly during construction. Excess water is also driven off, whilst retaining enough to ensure a steady rate of carbonation.

solid wall construction with limestone rubble and hot-mixed lime mortar

↣ Solid wall construction techniques like this differ not at all from those used to build countless pre-20th century buildings.

The most common mistake (in all forms of stone walling) is to use too much mortar between the stones. Vernacular buildings generally use rubble stonework, which requires good stone contact to provide strength. (A small number stone types call for more generous mortar beds, but these are less common.) The amount of exposed mortar at the face might vary considerably – depending how much the stone is worked – but the beds will rarely be more than 3/4″ thick over an extended area. Over-mortaring will encourage excessive movement over time.

↣ Click here to find out more about lime mortar and historic construction methods.

hot-mixed lime wall construction methods

↣ Despite consuming considerable quantities of mortar, the wall is still overwhelmingly stone. Striped bare, most vernacular buildings would look like this.

Hot-mixed lime pointing

hot-mixed lime pointing of limestone rubble wall

↣ Applying hot-mixed pointing to the newly constructed wall.

Historically masons would have ‘pointed’ as they went. Few structures were left bare, so they’d more specifically “dub out”, or harl or limewash as they went. There are good reasons for doing so. Modern tastes call for exposed stonework, which is more intricate – and less durable! – so I often point up afterwards. The pointing mortars included crushed limestone from Shap and crushed brick from York Handmade Brick Company. Crushed brick and limestone seem to work particularly well with hot-mixed lime mortars and were widely used historically.

lime mortar pointing scraped back after initial firming up

↣ Freshly applied lime pointing, scraped back after initial firming to aid carbonation.