When carrying out our surveys we sometimes come across timbers that have been lengthened. In this short article we will examine the various methods of joining timbers to achieve longer spans and the applications to which they are most suited.
01 Lapped Joint
Image 01 shows a lapped joint. Lap joints are the simplest of the lengthening joints and are most commonly used for lightweight purposes. The laps of the joint are cut to half the thickness of the timber and both shoulders butt up against the end of the joining piece. The joints are usually glued and screwed together, and it is common for the screws to be staggered in order to avoid splits along the grain of the timber. When this joint is used on external projects it is important to ensure that the internal joints are treated to protect against decay. This type of joint is only really suitable for loads which act on its edges, therefore it is important to arrange the work so that the timber is edge onto any weight or pressure to prevent failure.
02 Splayed Lap joint
The joint in image 02 is known as a splayed lap joint. It is similar to the lap joint described above but with the lap being cut into the thickness of the timbers rather than the width. It can be used where the joint is supplementary supported by a joist or wall. It is not intended for use in carrying loads without this additional support. The joint only really acts to keep the two joining timbers in a straight line so that coverings such as chipboard can be secured above. The lower adjoining piece is secured to the supporting member below with a nail driven diagonally through the side of the joining timber into the supporting member. The second joining length is butted on top of the lower joining links and a nail is driven through diagonally from the top.
03 Scarf Joint
Image 03 shows a scarf joint where the joins are cut on a splay. Glue and screws can be used for extra strength however scarf joints are not suitable for use in heavy load bearing joinery such as roof structures unless they have supporting members directly beneath the join.
04 Spliced Joint
A spliced joint is shown in image 04. Spliced joints, unless situated above a supporting member such as a timber truss or load bearing wall, are again uncommon in roof voids as they have limited load-bearing capacity.
05 Bolted Joint
Bolted joints such as that shown in image 05 are generally secured with coach bolts and incorporate timber connectors to make a strong face-to-face joint. They are most commonly found in roof trusses and purlins. As the nuts are tightened, the connectors bite into the wood increasing the shearing strength of the joint. Washers are used between the bolt and the nut.
06 Joining Plates
The final joint shown in image 06 is a joining plate joint. Joining plates ‘sandwich’ the two timber pieces. This method of joining gives good strength for end-to-end joints. The timber joining plates should ideally be four times longer than the width of the timber; they should also be the same width and at least half of the timbers thickness.
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