Truss repairs

As part of the renovation of St Stephens Chapel, the roof was to be re-slated. When the old slates were removed, the condition of 12no. Truss ends were noted as being poor and required urgent repair. The causes for decay were attributed to several factors; insufficient head lap of bottom run of slates, shallow lead gutters with no upstand to first batten and theft of gutter leadwork; all causing water ingress and damage

 The 12 highlighted trusses were repaired in sequence to not put excessive loads on the existing trusses or thrust to masonry walls. To isolate a truss for repair; first the weight of the truss had to be held using a block and tackle secured to a lifting scaffold. Then, the adjacent purlins had to be cut back to allow the principal rafter to be cut from the bottom chord.

 Once the truss was isolated and in suspension the truss end could be cut off. Due to its section size and span the overall repair length was around 1.2m. A half lap junction with 8no. mechanical fixings was specified by the SE, in addition to this Sikaflex adhesive was to be used between the original truss and new truss end 

 We sourced large section Douglas Fir for the repairs; care was taken to cut the half lap accurately and avoid any gaps larger than 2mm. Once the half lap was cut, we offered the repaired end into position. Ensuring it ran in plane with the existing bottom chord and had a good seating in the masonry pockets at the back of the cornice. At this point we could offer up the principal rafter to the side of the repair to square through the position for the bottom tenon. Using a template, we could accurately mark up and chop out the tenon and housing for the principal rafter. It was paramount that this too was a tight fit.

 Finally, we removed the truss end, glued up and secured the repair, re engaged the rafters tenon and fastened up the 8 bolts. We ratchet strapped the top chord to the bottom whilst the adhesive set to ensure it was clamped down into the housing.