Restore a Period Property in Ireland - Articles & Advice

Working With Lime Mortars

Henry O'D. Thompson from The OldBuilders Company is an expert in lime mortar rendering. The OldBuilders Company provide special skills in lime mortar rendering and other conservation and restoration work in one or more areas of your project, or they can handle the entire construction management.

Henry has put together an article for us on two buildings that he has worked on, a martello tower in Co. Dublin and a planter's farmhouse in Co. Tipperary. Henry can be contacted through his website at

1. Martello Tower, Red Rock, Sutton, Co. Dublin, c. 1803

The use of NHL5, eminently hydraulic lime, in conservation is a hotly debated issue, some suggesting that such use is (god forbid) akin to using Portland cement. It should be remembered however that such eminently hydraulic limes were used in harsh conditions in building many of our lighthouses, coastal sea walls, and other exposed structures. Therefore with a heavy heart and a firm grip on logic we accepted the architect's preference for the use of NHL5 on this fine example of a Martello tower.

Located in Dublin Bay, the tower is built on rocks, with its feet in the water, where waves often break on the walls. The structure literally dripped with water. Between the lost original wet dash that covered the rouble stone walls externally, the sloped cut stone roof, and the Portland cement pointed interior it was a sodden disaster.

The building was converted to residential use (a fine bachelor pad if ever I saw one) back in the 1970s. The new owner plans a luxury weekend retreat available for holiday lets.

The logic I referred to earlier is the balance that must be achieved when choosing a lime, that is between capillarity (letting the water in as water) and permeability (letting water out as vapour). The highest permeability is what we strive for because we want as much water to leave the wall as possible. This however comes at a price, capillarity. Not much good if there is more water coming in than going out is there? St Astier have produced some interesting statistics on their limes, and tests have shown some of the highest capillarity vs permeability ratios. This balance is what its all about and why in extreme conditions the use of NHL5 is justified.

The lime used was St Astier NHL 5 with a 5mm down sand and 6mm down aggregate.

2. Planter's Farmhouse, Coolross, Co. Tipperary, c. 1640

When we were asked to plaster this lovely simple two storey farmhouse with external chimney breasts so typical of English planter construction of its time, we planned at first a standard wet dash finish. Further discussion with the Dúchas consultant on the job and an enthusiastic owner made us rethink and we were determined to do something more authentic.

Dúchas showed us some examples of 17th century plastering, including a fine building called Castle Cuffe in Co. Laois. Built in the 1600s it was completed and stood for no more than 20 years when it was burnt down by the former ousted inhabitants and never re-occupied. The remains of the external plaster can therefore be relied upon as an authentic example of the style of the period.

What we saw was faux coin stones fabricated from the plaster, a flat finish with exposed black aggregates/charcoal alongside the wet dash finish of the main walls. This decorative finish extended to the windows as well with a flat finish not just in the reveals but extending 15-20cm out on the facing wall.

It is likely that the windows and coins were blackened against a whitewashed wetdashed wall, quite a dramatic look I would imagine.

The lime used was St Astier NHL 3.5 with a 3mm down sand and 6mm down aggregate.

This article was kindly submitted by Henry O'D. Thompson from The OldBuilders Company. Henry is an expert in lime mortar rendering and can be contacted through his website at