Thomasons provided project management and structural engineering services for the reinstatement of Wythenshawe Hall, which had incurred significant fire damage in March 2016 following a devastating arson attack.
The practice was appointed by Manchester City Council to act on their behalf in a project which required extensive experience of both restoration of fire damage and working with heritage buildings; an intuitive approach and a delicate touch.
Thomasons focussed on maintaining the building’s structural stability and prevention of further deterioration of the Grade II* listed manor house and to reinstate its external envelope. Restoration of the interior followed. Thomasons brought in Buttress Architects to work alongside them.
Engineering Challenges and Solutions
The fire damage to the core of the property was extensive. It was exacerbated as the hall, which dates back to 1540, is timber framed with infill wattle and daub walls. This allowed the flames to spread quickly and completely gut significant amounts of the roof and first floor. Whilst the necessary heritage design work was being developed it was vitally important to protect the building from the elements, which required careful control of the internal environment specifically targeting against dry rot infestation. A temporary roof was installed following careful removal of the ornate Bell Tower.
Restoration work began in March 2017, with an immediate priority to reinstate the roof, with particular focus placed on the historically and structurally significant North Gable.
Thomasons became involved immediately after the fire and set in place an understanding of the essentials of working with a listed building. These included:
- Retaining the maximum amount of historic timber and minimising the introduction of new elements, replicating the traditional carpentry techniques used during the original construction.
- Reinstatement of the structure as constructed by the original craftsmen.
- Protection of any undamaged parts of the building fabric and structure.
Surveying the damage
Initially, Thomasons conducted a visual investigation to ascertain the extent of structural damage to the timber frame and North Gable, ranging from ‘Structurally Sound’ to ‘Structurally Unsound’. It soon became clear that certain elements would need further, more intensive analysis.
Further inspection comprised a number of stages:
Stage 1. Establish the extent of charring on specific timber sections.
Stage 2. Determine whether sufficient timber cross section remained to transmit required loads.
Stage 3. Use micro drilling to discover if previous insect and fungal attack had, in conjunction with the fire, caused irreparable structural damage.
During this exercise it was established that the historically severely distorted North Gable had been extensively affected by the fire and significant structural stabilising work would need to take place.
The fire had penetrated the mortices and formed cavities around the tenons.
The distortion proved to be a sticking point as various options were considered for the restoration of this section of the building.
Initially, the preferred structural option was to form a substantially new gable utilising any sections of the existing timber that remained in a sufficiently sound condition. The Society for the Protection of Ancient Buildings (SPAB), a statutory consultee for Listed Building Consent, wished the distorted shape of the gable to be retained to maintain this historical aspect of the building, however, this would have put undue stresses into the new traditional timber connections.
The eventual design solution comprised a new oak frame formed in front of the original gable to provide support to the damaged structure in its retained distorted shape. Loads from the new frame were transferred back to the original structure at first floor level.
Following preparations the new oak frame, which replicated the original, was constructed and coach screws installed to retain the fire damaged frame in its distorted shape. The screws were specifically sized to ensure that they penetrated to 25mm away from the internal decorative surface of the wall. This ensured the historical wall paintings were protected whilst restoring the gable’s structural stability.
The apex of the main roof had also been destroyed resulting in the loss of the top of most of the substantial oak roof trusses and all of the common rafters. New timber needed to be introduced but due to existing mortices and notches in the timber a very significant amount of undamaged timber would have needed to be cut away in order to form traditional scarf joints. It was agreed with Historic England and SPAB that saving a maximum amount of historic timber was the priority and steel plates were introduced to transfer loads across a truncated scarf joint in the timber.