Civil Engineering Reference
In-Depth Information
walls. When a flat ceiling is required, the top of the internal walls must be level with the
top of the external. It warrants mentioning that in this form of construction the internal
walls supporting the ceiling joists are structural walls, so the use of non-structural timber
in this construction would not be permissible.
When constructing a straw bale house where the walls are load bearing, the ceiling
joist will be fixed to the top of the top bale boxing. Prior to fitting the ceiling joists,
ensure that the top bale boxing has been packed up as necessary to provide a straight and
level surface onto which the ceiling joists will be fixed. Refer to Chapter 4 on straw bale
wall construction for details on how to straighten the top bale boxing.
Timber specifications for ceiling joists
Refer to the timber specification schedule on your approved plans for details on the
timber specified for ceiling joists. The most common size and timber for ceiling joists in
Australia is 90 × 35 MGP10 pine. However you will need to refer to the timber
specification schedule included in your building permit to ascertain what is actually
required for your house. It may be that there are different ceiling joists for different parts
of the house. The timber specification sheet will have the material to be used, the
dimensions of the material, its strength and the frequency of its installation. It will
present something like this:
Ceiling joists
90 × 35 MGP10 pine @ 450 crs
If the ceiling joists vary in size for different areas of the house, the timber
specification schedule will give details of the different sizes and hopefully clear details of
their location within the house.
Fixing rafters to ceiling joists
Ceiling joists are to be positioned to enable the bottom of the rafters to be fixed to the
side of the ceiling joist. It is important that the rafters are fixed to the side of the ceiling
joists, as this provides the triangulated strength required for the support of the roof load.
Consequently, the final position of the rafters will have to be marked on the top of the
external timber prior to installing the ceiling joists. At this point it will be necessary to
establish from the timber specification schedule the required spacing of the rafters, and
the position of the first rafter, which is called the 'jack rafter'. This will be covered in the
next chapter, on roof construction.
When a ceiling joist runs parallel to the outside wall onto which the lower end of the
rafter is fitted, the first ceiling joist will be fitted between 450 and 600 mm from the inside
of the external wall. Then trimmers, known as jack joists, are fitted from the outside of
the wall to the ceiling joist (see Figure 7.1).
The process of installing ceiling joists
To begin fitting the ceiling joists, first locate the appropriate size and length material for
each section of the house. Carry the timber around the outside of the house and stand it
on its end against the outside wall of the house so that the other end of the ceiling joist
protrudes above the top of the wall. Position them around the wall approximately
