|Carpentry for Vocational Schools - A Teacher's handbook (GTZ, 252 p.)|
TOPIC: 12. CEILING
INTRODUCTION: This topic teaches students the purpose of ceiling joists, different types of ceiling joists, the way ceiling joists can be joined, hanging of ceiling joists and the correct distance between ceiling joists and how to fit a manhole in the ceiling.
12.1., 12.2. Students must be able to describe what ceiling joists are and their purpose;
12.3. Students must know the only place where ceiling joists can be joined.
12.4. Students must know that ceiling joists can also serve as floor joists for a second floor or as part of a roof construction.
12.5. Students must be able to describe how ceiling joists are hung correctly so that the ceiling construction is level and strong enough to carry the weight of the plywood.
12.6. Students should be able to define what a manhole is and its purpose.
12.7. Students should be able to describe a stairway and its purpose.
METHOD: First we have all the lessons in class where students take notes from the blackboard and copy down drawings or, if prepared, glue photocopies in their trade theory book.
Where possible demonstrate this topic on a building project. If not, again use the model house where students fix the prepared ceiling joists in the scale 1:10.
NOTE: At the end of this topic prepare a worksheet with questions about ceiling joists and hand it out for students to complete. Collect it later on for correction and assessment.
The pieces of timber placed on top of the wallframe are called the ceiling joist.
a) Strengthening the house construction:
- If there are no ceiling joists, the weight of the roof will tend to spread the walls.
- Through the ceiling joists exist a triangle construction which is immovable This means the pressure of the roof is straight downwards.
Ceiling joists are fixed across the narrow way of the house. Every second or third ceiling joist has to be placed near a rafter and connected to that rafter, so the ceiling joists act as ties to the bases of the rafters.
b) carrying the ceiling material: - Plywood, masonite etc. is nailed onto the ceiling joists. Depending on the ceiling material being used, the distance between each ceiling joist will vary from 500 mm - 700 mm. When plywood is used, the distance from the centre of one ceiling joist to the centre of the next ceiling joist is 610 mm.
Ceiling joists can only be joined on wallframes. The butt joint is used in this case. You have to nail both pieces.
a) Roof truss: - A truss consists of one bottomcord, two topcords, one king post and few diagonal truss members. The topcords are acting like rafters and the bottomcord acts like a ceiling joist.
b) Bottomcord: - When roof trusses are used, the bottom-cord acts like a ceiling joist. In such cases the roof trusses have to be fixed, depending on the ceiling material, at the correct distance (Plywood: centre to centre 610 mm or 1220 mm). Where roof trusses are set 1220 mm apart, a separate construction has to be made to support the plywood in the centre between the two trusses and at the cross joints of the plywood.
c) Two storey house: - In buildings with two stories, the ceiling joists of the ground floor are the floor joists of the first storey.
When ceiling joists have to cover longer distances, it may be necessary to give them extra support. This is done with hangers. Hangers should be placed from one side right through to the other without any joints. The joining between ceiling joists and hangers is done with a fillet. Hangers are placed above joists at intervals of 1500 mm to 2000 mm.
The manhole is used as an entrance into the attic. In many cases the attic is used as storage space although it is usual for the electrical wiring and water pipes to run across the attic floor for connection to the individual rooms underneath. This makes them easily accessible for work and repair at any time.
When a stairway is required from the ground floor to the first floor we have to cut out one or more ceiling joists to allow space for the stair cases. To hold the remaining ceiling joists in the correct places, we use "trimmers". Trimmers are placed from the first ceiling joist on either side of the space for the stairway. To join trimmers to the ceiling joists, we use a halving joint.