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West Moat Wall, Tower of London work done by London Stone Conservators West Moat Wall, Tower of London work done by London Stone Conservators

BUILDING CONSERVATION
Tower of London, West Moat Wall 2005 - ongoing

 

West Moat Wall

Insertion of lead drips on embrasures creating a ‘watershed’-like effect to protect the curtain wall. This was an innovation, achieved after extensive negotiations with English Heritage. A successful and ongoing intervention

 

The Moat Wall, also known as the Outer Curtain Wall, is a defensive structure.

A rubble (stone and brick) fabric, it stands at a majestic height of 8 to 15 metres, its thickness varying from between 1.5 to 2.5 metres along its length. Parapet embrasures mount the wallhead. The Casemates*, later structural additions that were built backing into the wall with the top of the parapets forming part of the roof structure, are used nowadays as housing for Yeoman Warder families, and as offices for the Curator of the Tower of London and others. These dwellings were built in the 18th Century, mostly for stores.


Problem
There was serious moisture ingress and dampness in the Casemates*.

Over the last centuries the wall has constantly undergone repairs and maintenance: large areas were consequently pointed and repaired with hard cement mortar rather than the original lime mortars. The top of the parapets were pointed in neat cement and flint, whilst the face of the walls had been largely rendered in a very hard non-flexible cement mortar.

This cement pointing caused corrosion to parts of the wall. Acid rain and other climatic changes had eroded away the soft brick and limestone, leaving the cement pointing to stand proud of the surface of the wall like a honeycomb. These changes allowed a large volume of water to ingress into the wall from the face and the top.

 

Solution
A deeper understanding of the importance of traditional techniques in preserving heritage buildings has brought about the return of lime mortar.

Consequently, after much consultation, we decided to remove all of the failed cement render from the wall and the neat cement from the top of the parapet. It was quickly established that the wall beneath the cement pointing had deteriorated much further than initial assessments had suggested: there were large air-filled cavities behind the first row of stones. Lower down the wall, there were large amounts of loose aggregate (sand), remnant evidence of the lime mortar that had been washed out over the years.

Several samples of original lime mortar were taken from different locations at different heights of the wall and sent for analysis to Rose of Jericho, who recommended a close match to the original by using a mix of natural hydraulic lime (NHL 3.5), stone dust, limestone nodules, pea shingles passing a 5 mm sieve, washed sharp sand and silver sand.

After repointing the walls in natural lime mortar it became clear that further protection was needed for the wall in order to keep the moisture ingress to a minimum.

Therefore we proposed to insert a lead drip, just below the coping stones at the front edge of the parapet embrasures, deep enough to divert the run-off surface water away from the wallhead. The bed joint of the copings could be raked out and the pre-fabricated lead drip inserted into the joint, keeping it in place with compressed lead wedges. Whilst this was a non-invasive, totally reversible alteration, building consent from English Heritage was required, as this addition would be an alteration to the visual intent of the original structure.
A test case was conducted. Initially two embrasures were fitted with the lead drips. The moisture in the wall was monitored for two years. After a successful reduction in moisture on the wall during that period, we obtained planning permission from English Heritage to insert these lead drips around the full length of the curtain wall.

Since that date, we have carried out works to most embrasures of the West Moat Wall, several embrasures of the North Curtain Wall, and embrasures on Brass Mount. The works to the curtain wall continue.

 

*Casemates. Literal translation: houses within walls. They were built in the mid-nineteenth century.

 

Chronology

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