Both gasholders were subject to the equivalent of a Level 2/3 historic building record as outlined in Historic England guidelines in their publication Understanding Historic Buildings: A Guide to Good Recording Practice (2016). This included a full photographic, written and drawn (measured survey) record undertaken both prior to (Phase I) and during (Phase II) demolition. The Phase I survey was carried out prior to any contractors on site. Both were subject to the equivalent of a Level 3/4 historic building record as outlined in Historic England guidelines in their publication Understanding Historic Buildings: A Guide to Good Recording Practice (2016). This included a full photographic, written and drawn (measured survey) record. The Phase II survey, undertaken once demolition had begun, was a photographic record only due to the steep slope of the dumpling restricting access to the internal tank. Therefore, the decision was made by SGN to allow only external photography from the side of the gasholder due to health and safety concerns. A general map-regression exercise was also undertaken to determine the general history and development of the site from its origins to the present day. All publicly accessible pre-Ordnance Survey maps and Ordnance Survey maps were viewed. Gasholder No 3 is a 'Type 44' frame guided gasholder with three lifts and 18 wrought iron standards with a nominal capacity of 800,000ft�. Although it is thought this holder was built in the 1940s (perhaps rebuilt after the Second World War), a holder has existed on this position since the late 19th century. Gasholder No 4, which was bombed several times during the Second World War and rebuilt in the late 1940s, is a 'Type 36' frame guided gasholder with three lifts, 30 standards and a nominal capacity of 1,650,000ft�. A second phase of photographic survey was undertaken of the dumpling and crown framing during the demolition of both gasholders. Each concrete dumpling was constructed in a similar way, at a very steep angle, with timber crown framing, although the styles of framing were different. The earlier gasholder, No 3, had two lower tiers and a top tier of vertical timber framing with angle structs offering additional support to the crown. Gasholder No 4 only had one lower tier of vertical uprights to support the crown with the bracing struts angled and socketed into the dumpling itself. Both timber structures, however, had similar jointing with mortice and tenon joints visible. Charred timbers were noted in Gasholder No 4, which suffered several air strikes during the Second World War.
Diana Sproat (Sun,) studied this question.