Woodworking: Lengthening Joints
Choose the correct joint for the job
Timber often has to be lengthened in big projects such as garages, carports or floor renovation. Smaller work, too, such as furniture repairs and general odd-jobbing, may need an occasional length ‘graft’.
This is the simplest of the lengthening joints and suitable for lightweight structures. Cut the laps to half the thickness of the timber, and make sure that both ‘boulders butt against the end of the joining pieces exactly, otherwise the joint will be badly weakened.
Glue andthe joint, with the staggered to avoid splits along the grain.
Paint it orit to protect the joint against the weather in all outside jobs.
The joint is mainly resistant to a load acting on its edges, so arrange the work so that the timber is edge-on to any weight or pressure.
A variant of the half-lap, with the lap cut in the thickness instead of the width, can be used where the joint itself is supported by a joist or wall. Its chief use is to keep the
timber in line where a centre nailing line has to be straight for fixing sheet materials such asor flaxboard.
Cut a splay along the grain to resist any tendency for the joint to be pulled apart. Make the length of the joint equal to the width of the timber.
Drive a cut nail diagonally through the lower lap into the joist or supporting wall plate — timber running along the top of the wall. Butt on the joining length and secure it with another nail driven through diagonally from the top.
This is cut on the splay and is mainly used in cabinet-making and joinery.
It is usually only glued, but the splayed faces need to be cut and planed with great accuracy. Properly made, it is as strong as the timber it joins.
Ideally make the length of the scarf eight times the width of the timber. Screws can be used for extra strength. Cramp well whilesets.
This is used mainly in furniture repairs or wherever appearances are important.
Cut the V with a fine-tooth saw. Then cut and plane the joining piece to an exact fit. Glue and cramp into position.
With careful wood selection and colour matching, it is often difficult to see where the repair has been made.
Coach bolts used with timber connectors make strong face-to-face joints for roof trusses.
Insert the connectors — metal washers with toothed edges — on the bolts between the joining faces. Tighten the nuts and the connectors bite into the wood, increasing the shearing strength of the joint. Use washers under the nuts.
Using joining plates
A ‘sandwich’ construction using joining plates gives great strength to end-to-end joints. Cut the joining plates four times longer than the width of the timber. They should be the same width as the timber and half its thickness.
Glue all surfaces, and stagger the screws or. Coach-bolt together.
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