9. JOINTS

TOPIC: 9. JOINTS

INTRODUCTION: Joints are a very important part in building construction and much of the strength of a building depends on how well joints are made.

This topic teaches students the different types of joints, where they can be applied and how they are made.

OBJECTIVES:

- Students should be able to identify joints, define their characteristics and uses and be able to make some joints independently.

- Students should know that joints are the weakest spot in every timber construction and therefore always choose the proper joint.

METHOD: First we teach this topic in the classroom and students copy down notes and drawings from the blackboard in their trade theory book.

Prepare joints and display them in the classroom.

When possible show students different types of joints on buildings and furniture.

If enough time is available prepare some timber in the workshop and students can make one or two common joints.

NOTE: We prepare a worksheet at the end of this section to reinforce the lessons.

If timber is too short or two timber ends meet in a corner, there has to be joint.

a) Halving joint:

- A halving joint is a joint which forms the junction between two members. Half a thickness is taken out of each member so that when fitted together the surfaces are flush.

- Halved scarf joint: The simplest but also the weakest is the halved scarfed joint. This joint is a lengthening joint, the length of the scarf is about three times the height of the timber. The halved scarf joint is used when both parts are supported in the full length.

Figure

- Corner halving: The corner halving is used on an external corner where two timbers meet, not necessarily at a right angle, but where their surfaces finish flush.

Figure

Figure

- Stopped halving: Stopped halving is used where one piece meets another anywhere along its length and so forms the letter T. Stopped halving is checked out on one piece, only half its width and half its thickness, so that this piece is not weakened as much as when it is checked right through its width.

Figure

- Splayed halving joint: Is used for extending timbers lengthwise and should only be made over another supporting timber. You have to nail nailplates to each side.

Figure

b) Where to use halving joints:

- The halving joint is used for corner plates.

Figure

- For top and partition plates.

Figure

- Splayed halving joint for bearers joists and rafters.

Figure

Figure

c) Housing joint:

- The through housing joint: The through housing joint is mainly used on construction work where the appearance of the joint is not important. It has the whole edge or end of one piece sunk into the side of another piece, which gives a definite positioning of the one to the other.

Figure

- The side housing joint: The side housing joint is where the vertical member is recessed to receive the end of an horizontal member which is usually fixed on edge to the job.

Figure

d) Where to use housing joints: - The housing joint is mainly used on construction work.

Figure

e) Mortise and Tenon joint:

- Open mortise and tenon joint: This is the most common form of connecting members together in framed construction and consists of a parallel sided, thinned projection on one piece (Tenon) fitted into a corresponding slot or cavity in the other piece (Mortise). The two being secured together by glue aided by wedges and/or pins.

Figure

- Haunched mortise and tenon joint: Where a mortise and tenon occurs at the end of a stile it must be reduced in width so that when fitting the door or sash the tenon will not be interfered with when the surplus timber is removed. There should also be sufficient wood left beyond the mortise to withstand the force of the wedges when glueing up. The small stub tenon left when cutting down the tenon is called a "haunch".

Figure

f) Where to use a mortise and tenon joint:

- An open mortise and tenon joint is commonly used on cupboard doors.

Figure

- A haunched mortise and tenon joint is used for external and internal doors.

Figure