The church is on a small hill at the base of a valley and is exposed to strong wind gusts. Due to a lack of weather protection, the repair options were limited.
The scaffold design limited access to some repair locations and could not serve as temporary support; therefore, all repairs had to be planned accordingly.
The Crown of Thorns is a Grade-A-listed sculpture created off-site in 1964, replacing a previous masonry structure. It is made of laminated redwood, painted and then clad in aluminium.
Timber decay on leg two was examined, revealing no significant defects, and a new laminated timber section was subsequently fitted.
The four timber legs are hollow and formed from 110mm-thick redwood laminations that connect to the cruciform. The connection to the church is made via steel brackets and linked steel rods, which allow for the necessary movement. Four outer timber members are redwood laminated into solid triangles. They connect to the timber legs via a tenon joint and pass through the outer timber arm above.
The outer timber arms (4rn) are crafted from laminated redwood, seamlessly integrated with the cruciform, creating a striking visual and structural element. Rising above is the pinnacle column, composed of two robust sections of solid laminated timber that gracefully connect to the cruciform.
The Architect in charge of the repair scheme was Gareth Jones of Pollock Hammond.
Structural engineer Andrew Turtle from Blyth & Blyth monitored the investigation and repair works.
Godon Matheson of Matheson Plumbing was the main contractor.
Timber assessment by Nigel Foster CSRT with >30 years in timber repairs.
The wood-rot analysis revealed that the southwest area of the structure, exposed to wind and rain, is experiencing significant timber decay.
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Given the structure’s size, I used a Tramex moisture meter in deep timber scan mode to generate a moisture profile, and the damp locations were marked using coloured tape. Red (24->30%), Yellow (20-23%), and White (≤19%) and 20% moisture content is quite normal for an external structure. This helped me focus on repair work and understand the types and patterns of timber decay.
The initial investigation involved removing approximately 70% of the damaged timber from the structure, in accordance with the established assessment plan. Subsequently, a thorough visual inspection of the remaining wood was conducted to determine the scope of repairs or replacements required. This phase was protracted because scaffolding had to be completed before access to the sculpture’s upper levels was permitted.
Timber decay occurs in laminated sections because the lamination adhesive traps the moisture. Water accumulated in the substantial timber at the base of the legs. Reverse interstitial condensation, as solar heating caused water vapour to gradually seep through the paint. A comprehensive understanding of these factors is crucial for developing innovative solutions and enhancing the structure’s long-term durability. Penetrating damp caused wood rot due to failures in cladding joints and screw fixings. Proactively addressing current cladding design issues will prevent future damage and maintain optimal structural integrity.
The inside of the legs was visually inspected from access points formed at the base. However, the entire length was not visible because the OSB board formed bulkheads. They had drilled holes for ventilation. Leg one had soft rot at the building contact point due to penetrating dampness, but this did not compromise the connection section, which was treated with a Boron Preservative.
The main types of wood-rotting fungi seen:
Soft rot fungi are actively present.
White rot fungi primarily target the upper surfaces.
Brown rot fungi are found in the timber legs and the principal timber of the outer timber.
Drying of the timbers
Due to the exposed site conditions and scaffolding limitations, drying the timbers before cladding was not feasible. We protected certain areas to promote drying and facilitate repairs.
Matheson Plumbing carefully covered all the timber with breathable building paper. Subsequently, they applied a copper-based cladding system. Notably, the joints in this system are specifically designed to prevent rainwater penetration. Moreover, the plumbers planned to install high and low vents on all four legs for ventilation.
The timber structure will absorb the sun’s heat, creating an internal water vapour pressure. As a result, water vapour will gradually escape through the new cladding joints, causing the timber structure to dry over time.
Just a tiny section of the timber repairs carried out
Gareth Jones, Architect, RIAS, IHBC, Director, Pollock Hammond Ltd. The repairs were unusual and very challenging. It was not possible to fully assess the extent of decay in the timber structure before scaffolding. We worked closely with ADPC and the Structural Engineers to survey the structure as the cladding was removed. ADPC provided significant input into the specification for the repairs and developed several strategies for addressing decayed timber based on the extent and depth of decay, the location of the rotten timber, and structural requirements. The work was carried out efficiently, economically and to a very high standard.
