Martello Towers

Keeping out the weather

Tom Bosence


View from the top of Martello Tower 24 located in Dymchurch, Kent (Photo: Tom Bosence)

The Martello towers built on the coast in Sussex and Kent were a response to a feared Napoleonic invasion. Later larger versions were built up the east coast of England and then across the British Empire during the 19th century. These small forts were designed to protect vulnerable beaches and strategic positions using their 24 pounder cannons and the gunfire of the stationed soldiers. Their design was inspired by the Torra di Mortella in Corsica, which absorbed an impressive artillery bombardment from two British warships in 1794.

Martello towers present significant conservation challenges owing to their general construction type and their location, which is generally tight to the shoreline. Their external walls are sloped making them more susceptible to saturation from rain than vertical ones would be. Furthermore, their massive copings have no covering, overhang or drip detail, all of which contribute to water ingress. As a result, Martello towers are often cited as having inherent defects in both construction and design which preclude the possibility of them ever being (or ever having been) dry. Certainly, there is some evidence to support this school of thought, and just about the only dry Martello towers are those which have had roofs built on top of them. This article explores the conservation challenges presented by Martello towers and takes a detailed look at the conservation of Tower 24 at Dymchurch undertaken by English Heritage and completed in 2019.

Tower 24

In all, 74 towers were constructed along the south coast of England between 1805 and 1812. Numbered from east to west, the tower at Dymchurch in Kent is Tower 24. It is cared for by English Heritage on behalf of the Department for Digital, Culture, Media and Sport, and its conservation should be seen within the context of English Heritageís asset management plan Sustainable Conservation which was set up to fully survey and record all defects to a structure and to understand their causes.

The required works are then prioritised using targeted assessments of significance and vulnerability. The aim is to bring each building to Ďa steady state we can effectively and confidently maintainí. This makes a clear distinction between that which may be desirable (perhaps to ideal presentation condition) and that which is required to safeguard the building. However, the plan accepts that in the future there may be instances whereby it isnít possible to maintain certain properties in a steady state. An example of this is rising sea levels which are already threatening some of the national estate, such as Hurst Castle where major civil engineering works are currently underway. Erosion is an obvious threat to coastal structures and waves have already undermined Tower 19 at Hythe in Kent.

However, Tower 24 has been aided somewhat in this respect by the sea defences which have been installed at Dymchurch. The effect these defences have on the tower’s setting and significance is perhaps less beneficial, but they are certainly protecting it. Nevertheless, Tower 24 does suffer from significant building defects. Archive research suggests that during the Ministry of Works’ restoration in the 1960s the render, noted at the time as modern roughcast, had failed.

The replacement render we see today is crazed, cracked and certainly causing water ingress. It is cement based and much too hard, as well as not having any proper finish, such as limewash, to help reduce rainwater penetration. In addition, the false ashlar joints are lines of weakness where the render has cracked and it is apparent that moisture is travelling through these fissures. The walls on the seaward side are up to 3m thick, which means that any moisture present may be trapped for a long time, with little chance of escape. While replacement of the render would definitely help with the water ingress (and egress) as well as being desirable aesthetically, it was also one of the most expensive options considered. Ventilation is another concern.

Although the towers were designed with louvred shutters, some, such as Tower 24, were fitted with sash windows during their period as domestic dwellings. Originally two fireplaces would have provided heating which, combined with the effective ventilation supplied by the louvres, would have mitigated a great deal of the moisture we find inherent in the management of the building today. The tower is no longer in constant occupation and when it is, it is not used the way it was designed to be, further aggravating the situation. A further complication is that the towers were actually designed to collect rainwater and to store it for the stationed soldiers.

The different towers have different means of storing and/ or disposing of the water including the Wish Tower in Eastbourne which still retains today its fine subterranean brick-vaulted cistern. The roof of Tower 24 is also presenting problems of water ingress. It appears to have a flat roof but this is in fact a gun platform or terreplein and its primary function was to support the cannon and soldiers during a battle (see cross section). The terreplein falls towards the internal downpipes and logic suggests that some form of waterproof coating would have been required to make the design viable.

Although considerable research has been conducted on these towers, it is surprisingly hard to establish definitive proof of any roof coating. Outline drawings are available in the National Archives as well as in the Kent Library, but none found to date are sufficiently detailed to answer this question. Of course, if evidence for a waterproof coating was found, it might mean a different approach was taken to the conservation of these fascinating monuments. Some anecdotal references have been quoted in texts suggesting that the towers may have once been covered in lead, which does perhaps seem logical as the internal downpipes are formed in this material.


The limewashed exterior of Martello Tower 24 when photographed sometime in the early 20th century (Archive image: Historic England)

However, consultations within Historic England determined that none of the Inspectors of Ancient Monuments ‘have ever found any firm archive evidence for lead coverings to south east martello parapets, or know of any towers that retain any firm evidence for an original (or early) lead parapet covering.’ One interesting observation on Tower 24 raises at least the possibility that lead was used on the roof. The roughly pitched masonry detail evident against the finely tooled ashlar of the Firing Step suggests a coarse modification. The stone would have left the quarry/banker shop with an upstand, and the detail has been removed by someone with much cruder tools than the original mason. The purpose of the upstand could have been for a lead roll to form the back gutter against the parapet.

Of course, the upstand could have been simply to help the water find its way into to the collection system. The 1960s restoration works at Tower 24 removed a coating of bitumen which was recorded at the time as a later addition. It is understood that by 1850, the Royal Engineers intended a general refurbishment of the towers and it is possible that this is when the bitumen or asphalt was applied at Dymchurch. The neatly carved upstand would have proven an impediment to an effective waterproof asphalt layer and was therefore roughly pitched off prior to the work. In the context of the 2019 works, much firmer documentary evidence than this would have been required to even consider any such intervention to a scheduled monument.

One paper which has been helpful in understanding the construction detail of the tower is Substance and Practice, Building Technology and the Royal Engineers in Canada by Elizabeth Vincent. Although Vincent’s research focusses on Canadian redoubts and martellos, it also casts some interesting light on the design approach of the Royal Engineers in the early 19th century. The paper asserts that in general the Royal Engineers were more concerned with ‘value engineering’ than they were with overseeing quality.

Consultation with English Heritage curatorial colleagues concluded that it was most likely that the Royal Engineers assumed or thought that the newly constructed masonry would be sufficient to keep the tower dry. There is evidence to support this thinking in the reports of the Royal Engineers in Ireland who blamed their leaking towers on ‘defective cement’. When formulating plans for the conservation of Tower 24 there were several areas which were obviously in need of attention. Investigations using non-destructive tests such as ground-penetrating radar, microwave meters, thermal imaging and even a simple water test using a hosepipe and sandbags, identified the coping stones as an area for investigation.

As the battered walls of the tower are between 2.5 and 3m thick the coping stones have a significant surface area, much larger in fact than the lower gun platform which is often thought of as a weak point for water ingress. The stone used for the copings is a Yorkshire gritstone and therefore fairly dense and impermeable, but the joints were a concern. Various attempts have been made to repair the coping stones over the years, resulting in some damage to the masonry and the insertion of no fewer than five different layers of fill. These include silicone mastic and linseed-oil putty, finished off with the entirely predictable soft sand/cement mortar.

The deeper original mortar was tested and identified as a non-hydraulic lime. This was matched for the specified re-pointing and mixed in the traditional way of adding the aggregate to the quicklime. It is certainly possible that different mortars were originally used for the bedding and the pointing. However, it was felt that to use the bedding mix as the basis for the pointing was justified, as the existing hard and impermeable mortar was unquestionably causing a problem. It stood to reason that a ‘soft’ and permeable mortar ought to perform better or at least no worse. Importantly it would also do no harm, and if it did not work or failed, it could be easily remedied utilising rope access.

In contrast, overly hard mortars such as the one removed, when paired with a dense stone inevitably crack away from the matrix and allow moisture to be sucked into the structure via capillary action. This effect is generally noted to greater or lesser degrees irrespective of the binder: the harder the set (and the denser the mortar) the greater the likelihood of capillary action. Artificial cements are rightly regarded to be the most harmful, but this is true whether the binder is artificial cement, natural cement, NHL or lime and pozzolan.

moss false ashlar
The cracked surface of Martello Tower 24 showing areas of moss growth in its false ashlar joints (Photo: Tom Bosence)

Although a softer mortar is less likely to cause this effect, it may require more frequent replacement. This trade-off between more frequent maintenance and the building functioning better was considered acceptable. However, scheduled monument consent would not have been granted had the defect and mortar analysis not been undertaken and there had not been a strong justification for proposing the repair. The coping stone repointing was carried out during the fairly mild, albeit wet, winter of 2019 and has already had a 4ignificant effect in terms of preventing water ingress. Another key problem, identified through visual inspection and bung tests was the internal downpipe. This was demonstrably leaking into the structure and was also therefore included in the repair works. An epoxy resin and sock liner (CIPP or cured in place pipe) was installed using air inversion to form a new liner to the relevant section of the downpipe. Importantly, improvements to allow for the maintenance of this internal downpipe were also included, as previous repairs had somewhat obstructed access.

Gentle background heating and ventilation, achieved by keeping the windows ajar, had been previously introduced and the 2019 conservation programme included repairs to the windows and security measures to allow the sashes to be left permanently open without risk. The ventilation is diligently managed by the Friends of Dymchurch Martello Tower, who open the tower on behalf of English Heritage. Another area in which the Friends are playing a key role is the monitoring of moisture content in the building. English Heritage has bought a microwave moisture sensor called the HF Sensor Moist 350B and has set locations for testing.

The Friends take these measurements in the same locations every week at the same time and this repetition helps to reduce the potential inaccuracy of such testing on masonry structures. This monitoring programme is providing a helpful overview of the moisture levels compared to pre-project. As discussed above, it is acknowledged that the condition of the render is far from perfect and is definitely allowing water in. This may in the future be addressed, perhaps first through reinstating limewash. Limewash on cement renders is often described as problematic, but where there is sufficient key it can be surprisingly effective. If the limewash application proved unsuccessful then replacement of the render may be required. However, the results of the monitoring will lead that discussion. If Tower 24 can now be maintained in a steady state with regular maintenance but without further intervention, then the works will have been a success. The money not spent on items such as the render can be spent on one of the many other urgently required projects in the national estate.

Source

The Building Conservation Directory, 2021

Author

Tom Bosence is a onservation certified chartered surveyor and chartered construction manager. He works for English
Heritage as the south region’s Historic Buildings Surveyor, completing quinquennial
surveys and giving professional advice on the care of the scheduled monuments.

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