The Building Conservation Directory 2021

134 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 1 C AT H E D R A L C O MM U N I C AT I O N S 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. 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 significant 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. TOM BOSENCE is a conservation 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. A cross section through Martello Tower 24 illustrating the arrangement and function of its various internal spaces (Photo: Tom Bosence, by kind permission of Kent Library, Maidstone) 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. (Photo: Tom Bosence)