The Building Conservation Directory 2021

BUI LDING CONTRACTORS 2 53 C AT H E D R A L C O MM U N I C AT I O N S 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 CONSENTS Structures which are listed may require listed building consent for works including temporary fixings to the building. The control of works affecting scheduled monuments are more stringent and it is likely that any work to the monument requiring scaffolding will require scheduled monument consent. In both cases there is likely to be a preference against drilled anchor ties, particularly so with scheduled monuments where there is a greater emphasis on the control of works. ARCHAEOLOGICAL AND HISTORIC SITE CONSIDERATIONS To make the installation of scaffolding as easy as possible, the delivery vehicles need to be parked as close as possible to the site to minimise carrying. On historic sites, access may be limited due to narrow passages, archaeological remains or soft ground, particularly if installation or removal is proposed in the winter. For short distances it may be possible to unload using a lorry fitted with a small crane, but consideration would have to be given to the location of its outrigger legs. For longer distances and where slopes are steep, scaffold walkways can provide safe access for the workforce, allow the transport of materials, and reduce damage to the ground from footfall. As part of the pre-construction information, existing drawings and archaeological investigations in the area where scaffolding is proposed should be supplied. This ideally should identify shallow buried structures incapable of carrying load, buried voids or projecting uneven ruinous walls. The scaffolding can then be designed to avoid areas of weakness, or back- propping may be used to reduce the bearing. If the latter, the conservation team should undertake a risk assessment to determine the consequences of insufficient bearing capacity. Where there is a risk to archaeology or buried structures with inadequate load bearing capacity, the principal contractor will need the advice of a structural engineer in determining an acceptable maximum allowable bearing capacity. There is limited guidance on this subject and engineering judgement must be used, based on the type and scale of likely archaeology, depth and relative compressibility of the surrounding ground. Scaffolding may need to be supported from historic fabric. Where this is understood in the design phase, it should be included in the pre-construction information showing where load can be carried, together with information on allowable loads or back-propping requirements. TEMPORARY FOUNDATIONS Where scaffold legs bear on historic fabric, the contractor will need to ensure the scaffold does not cause damage. This is achieved by adequate load spreading and protection to the historic fabric. If proposed scaffold legs are initially poorly positioned, there may need to be several iterations of design to resolve this. Where appropriate, foundations will need to be designed to resist uplift or the additional weight from any ballast (kentledge). The options for this are: • excavation to provide substantial (heavy) foundations to resist uplift directly (which may require archaeological supervision) • spreading of the additional weight of kentledge at ground level • installing temporary piles (which risk unknown archaeological damage). Where excavation is required for foundations on a scheduled monument, consent will be required. If concrete foundations are proposed on a historic site, the design should consider how they will be removed without harming the historic fabric following completion of the project. If a TG20-compliant scaffold is proposed and the temporary foundation design indicates significant load, spreading of load will be required, either to avoid archaeology or reduce the applied bearing pressure. It may be beneficial to instruct the scaffold designer to undertake a site specific analysis of the scaffold, which may reduce the scaffold leg load and therefore the amount of load spreading required. Where the scaffolding is likely to remain in place for a number of years traditional timber sole plates are unlikely to have sufficient durability. In these circumstances, concrete foundations may need to be considered. TIE TYPES The damage caused by ties is a key issue when scaffolding is used adjacent to historic fabric. They are needed where the height of the scaffold is less than four times its width, so the need to tie the scaffold to the building may be avoided by keeping the height down, or by using rakers (inclined poles) to brace the verticals against the ground, increasing the width at ground level. Putlog scaffolds, where one end of each horizontal pole is supported by the wall of the building, may be used where a wall is being built, but they are generally not used against existing fabric. Types of scaffolding ties include: Return ties These are installed around the corner of the building, restrained one bay beyond the return. Through ties or lip ties These generally pass through window openings to prevent outward movement of the scaffold. Provision should be made in the pre-construction information for the removal and reinstatement of either the window or the glazing by a suitably qualified person. These types of ties may present a security issue and if the building is to remain occupied, the openings may affect the internal environment of the building. Reveal ties These are equivalent to light duty ties. TG20:13 permits the use of reveal ties which rely on friction, but recommends that only 50 per cent of ties to a façade should be reveal ties. Due to the need to frequently check reveal ties throughout the life of the scaffold, these are not appropriate where scaffolding is likely to remain in situ over an extended period. Drilled anchor ties There may be instances, for example an occupied building at the rear of a pavement, where there is insufficient width to reduce the height to width ratio to achieve a free-standing scaffold and where the other types of ties are unable to provide sufficient coverage for the scaffolding. In such instances a compromise may be required and drilled anchor ties which cause permanent damage may need to be considered. The design should be developed carefully to minimise the number of anchor ties and to locate them in areas of less significant material. If anchor ties must be fixed into historic fabric, it would be helpful if the scaffold designer could provide a statement explaining why they cannot be avoided. Lateral stabilisation achieved with a through-tie (left) and a lip tie (right)