Church of St Andrew, Aldborough
The Church of St Andrew, Aldborough is located on Dunsforth Road (the old Roman Road) in the centre of the village which is a Conservation Area and occupies part of the site of Isurium Brigantum, a planned Roman Town. Much of the ground under the village is therefore scheduled and Roman material is incorporated into the church building. This is understood to be the third church to be built on this site and was erected about 1330 by the Dean and Chapter of York Minster to replace a Norman church destroyed by the Scots in 1318.
The interior is plastered and has a fine panelled roof to the nave with guilded 18th Century bosses. The main timbers to the north aisle roof appear to be medieval. Some medieval glass survives in the tracery and heads of the north aisle windows and there is glass by Charles Kempe in the south aisle and by Wailes in the great east window.
The north aisle lead roof was understood to date from the late 1950’s and was suffering from severe underside lead corrosion. The vertical pattern of the corrosion appeared to accord with the medieval oak rafters and trusses below. Even where not blistered, the lead was crunchy underfoot, indicating sulphate crystals below. Pre-conditions of a Grant offer from English Heritage included the undertaking of an extended period of investigation and monitoring by Rowan Technologies of the underside corrosion of lead.
The conclusion at the end of the monitoring period was that the severe corrosion was probably due to the inadequate laps of 160mm to the lead sheets. Rainwater could then have entered below the lead by capillary action. The severity of the underside corrosion and the presence of Massicot suggested that carboxylic acids (probably acetic acid) may also have been involved caused by warm moist air passing over the oak roof timbers below and through gaps in the boards to condensate on the underside of the lead. Discussions then centred around whether the roof construction should be modified to incorporate ventilation. It was eventually agreed that the traditional non-ventilated construction should be maintained and the work be carried out in late spring or early summer with protection against rain, allowing the roof boarding to dry out before recovering and incorporating building paper and the application of chalk emulsion to the underside of the new lead sheet.
The opportunity was also taken to carry out some minor repairs to the nave clerestory wall. A number of stones were found to be crumbling and these were carefully replaced with stone intents set in lime mortar. Other stones were simply dressed back to sound material and mortar ‘fillets’ introduced to assist the shedding of rainwater. Areas of defective pointing were repaired with lime mortar. The tracery stonework to the head of Bay 1 clerestorey window was found to be very fragile with previous repairs detaching from the original stone. Thinned lime putty was injected into open crevasses to hold the fabric together. Old mortar repairs to the head were stabilised and concealed with new pointing and weathering.