Details of roof truss manufacture
After timber for the truss has been cut to size, the truss is assembled and pressed. There are broadly two methods of doing this:
The jig method uses a set of independently moveable jigs (approximately 800 to 900 mm high) onto which the timber members are placed and supported at the joints. The members are then clamped in place with nail plates positioned across the joints on each side of the wood. Once the truss has been configured correctly a press is used to compress the nail plates into the wood, securing the joint.
The work bed method consists of a large flat bed and a travelling press head. The bed heights range from 590 to 840 mm high. The timber members are laid out on the bed and clamped/secured in position. A nail plate is positioned either side of each joint. Once assembled, a gantry-mounted hydraulic press moves down the length of the work bed and exerts pressure onto the nail plates one at a time, fixing the timber members into position.
In both cases once the nail plates have been pressed, the truss is released and lifted off the jigs/bed by teams of operatives, then carried to the stack. Trusses tend to be stacked with their peaks upwards (peak-up stacking). The stacks are banded and then typically removed by side loader.
Trusses can vary a great deal in size and shape. There are several configurations, for example: attic, duo-pitched, single pitched, and laminated (or plied). Trusses can measure from a few metres wide and high, to 20 m long and 5 m high.
Most fabricated trusses weigh below 75 kg, typically in the region of 50 to 70 kg. Some single trusses can weigh as much as 150 kg and if plied together with another up to about 200kg. Attic trusses are particularly heavy as they have large timber members to make up for the lack of triangulation timbers that would interfere with the habitation area.
The Roller Press method is similar to the work bed method but this time the pressing is achieved by a roller located at one end of the bed, as shown below. Wooden blocks are first secured to the bed to allow the correct positioning of the truss components. The bottom metal plates are then put in place prior to the timber sections. Finally, the top plates are positioned and secured by hitting them with a mallet to hold the truss together until final pressing. Once all joints are secured the truss is prised out of the blocks and fed into the roller. The roller pinches the timber and draws it through, pressing the plates fully into the timber. The truss is then handled from the out-feed side of the roller over to the stack.
The height of the truss is limited to the width of the roller. This will limit the type and size of truss that can be made this way.
Images courtesy of Crendon Timber