Philosophy of Repair
The approach to the proposed work is detailed below:
- The aim of our proposals is to stabilise the stonework deterioration through the replacement of stonework which is in a very poor condition or which has been damaged to a degree where weatherings & other mouldings are not effectively shedding water or are channelling water in a way which is promoting further deterioration. What became evident during our investigation is that virtually all the mouldings play a weathering role as well as a decorative role. If the mouldings are not shedding water they are directing water to a moulding which will shed away from the face of the tower. Where seemingly decorative mouldings have failed we have noted increased water or wind erosion to other elements which can be directly related to the moulding failure. Where this is the case we would strongly recommend the reinstatement of these mouldings and have specified works accordingly.
- We are trying to maintain as ‘light touch’ as possible in the specification of these works, although we have tried to make allowances for the fact that due to the timetable of HLF works it is likely that works will not start till a year after our initial inspection. To do this we have tried as far as possible to specify for the worst case to ensure that we will not be ’embarrassed’ when works start in earnest next April or May. Hopefully once the scaffolding is erected we will be able to reduce the scope of works slightly while having a budget to carry out the worst case repairs.
- In the course of these works ‘Defrassing’ is defined as the dry brush vertical surface of walling and wall tops with a stiff bristle brush without damaging the stonework. The intention is to remove general detritus, dirt, webs and loose vegetation rather than scrubbing the stonework with the brush. The process helps expose the extent of problems before final decisions on repairs are made and will form the chief cleaning method employed on the tower. We will ensure that the appointed contractor is aware of the presence of the remnants of limewash and in these areas defrassing is to be avoided
- The other form of cleaning which will be employed is intended to reduce the thickness of the localized areas of heavy sulphation crust, which can both promote and conceal the conversion of limestone to gypsum leading to significant erosion. The intention is not to fully remove the crust but to reduce its thickness to a level which will reduce the likelihood of damage progressing. The main method of addressing this will be by poulticing which from experience can leave patchy results even when carried out by skilled operatives. The intention is to gently use the Jos system to even out the results and graduate or ‘feather’ the edges to avoid stark results. Good results will depend on the skill of the operatives we are quizzing the tenderers on who they propose to carry out the work to ensure they have adequate experience. In areas where historic limewash is present this form of cleaning will not be used also care will be taken to avoid run off from higher up the building affecting these areas.
- Maurice Rogers has carried out an analysis of the stone used on the building. The results of his analysis have been incorporated in the specification and have been supplied to the tenderers. This has revealed that the two types of limestone used on the tower are similar in colour texture and property to the following: The light cream stone (the Syreford type stone) is used for all stones with a weathering function, moulding etc. (i.e. all stones on the East & West Elevations, All stonework of the upper moulded panels & parapet of the North & South Elevations, & All weatherings, mouldings & quoins of the buttresses). The more golden stone (The Oathill type) is used for the unmoulded lower stages of the north & south elevations and the sides of the buttresses.
- The vast majority of the stonework repairs and replacements are to the former stone type largely due to the exposed locations in which this type of stone is used.
- The repair of stonework is specified on a location specific basis with the aim of ensuring it is at least fifty years till the next major repair programme is required. A pallet of techniques ranging from shelter coating and insitu mortar repair to the complete replacement of stones will be used. As previously stated, it is hoped that our specification errs on the side of caution and can be scaled back once a detailed inspection from a working scaffold can be carried out.
- Insitu mortar repairs are mainly specified in areas not subject to weathering by water flow and where the nature of the damage means that the mortar will achieve a firm key from both the stone substrate and any stainless steel armature required. Insitu mortar repairs will also be used to fill features such as solution pockets, which while not strictly speaking being damage form areas where erosion and decay can originate. In these cases the repairs are being used to fill solution pockets in the stonework rather than carry out a formal repair. We believe that in these cases the pockets will provide sufficient key to ensure their long term survival. Their purpose will be to slow erosion to help prolong the life of the affected stone.
- Indent repairs will be carried out in a manner respecting existing mortar joints rather than attempting a ‘dental’ approach. We believe that the results of such an approach will both have a longer life and will be aesthetically less disruptive than an alternative approach. The intention is to ‘respect’ the original stones. We will avoid harsh junctions between new and old mouldings. The main intention of the reinstatement of moulding is to re-balance the weathering and water flows down the face of the building.
- The replication of lost mouldings is simplified on the tower by the design of the tower. In general each moulding is repeated at least four times on the building and is again repeated a further four times in mirrored form. In many other places the repetition is far in excess of this meaning that in each case where a missing or eroded moulding is to be replace there is at least one location where the moulding exists in close to its original detail. In all cases we ask for templates to be made and shop drawings to be submitted for approval.
- The existing mortar will be fully analysed to guide the formulation of repair mortars to ensure that it matches the colour and texture or the existing while ensuring that the mortar is weaker than the existing and replacement stone. Our investigations showed relatively little variation in the mortar used across the tower, probably due to the limited extent and infrequency of repair carried out in the last three hundred years
You can see how our philosophy of repair has been applied in practice during the Delivery Phase.