Civil Engineering Reference
In-Depth Information
Installation of floor joists
Check the bearer positions
Before starting work on the floor joists, check that the bearers are sitting flat on the same
plane. It is sometimes necessary to plane timber off bearers that are too high, or
alternatively fit packers where the bearers are too low.
Floor joists are mounted straight on top of the bearers. I have known of people fitting
light wire netting between the floor joists and the bearers. Bulk insulation is then laid on
top of this netting just prior to installing the flooring. While I agree that this method will
work I have some reservations. It is important to have the floor joists fixed securely to the
bearer to avoid movement between the joint that will generate noise in the floor. To fix
the floor joists over wire netting is likely to increase the possibility of movement in this
junction as the wire could hold the two surfaces apart initially. After some time the wire
netting would bed into the timber and thereby create space between the timbers
generating noise.
When building a straw bale house on stumps it is much easier to lay a sheet flooring
prior to erecting the walls. Strip flooring would suffer greatly from the amount of water
that will get on it during the rendering process. If the bulk insulation was inserted prior
to the floor it is still likely to get wet from water that leaks through the flooring. There are
other alternative methods of insulating the floor that are covered in Chapter 13.
Details of floor joist material
The materials to be used for floor joists will be specified on the approved plans, as will the
spacings of the floor joists. As a general rule the floor joists are fitted at 450 mm centres.
On a recent inspection of a house being built by an owner-builder I discovered that the
floor joists had been fitted at 460 mm centres. The owner explained that by just stretching
the floor joists out that little bit he saved a whole floor joist.
When the specifications stipulate that the joists are to be fitted at 450 centres, to fit
them at 460 is breaking the law. The building surveyor would not only have the power to
force you to rectify it, but would be doing the morally correct thing. This particular job
had progressed well beyond the floor joist stage. The floor had been fitted and most of
the walls built and in place. Consequently it was unrealistic to remove and refit the floor
joists, so the only remaining solution would be to fit a second floor joist between each of
the existing floor joists so that the span between the floor joists was legal. To save one
floor joist can end up being a very costly and time-consuming saving.
Legal variations from the specifications.
The only way to vary from the materials specified on the approved plan with the building
permit is to have an engineer provide calculations for the alteration. In most situations it
is possible to use a range of timber types and sizes or even steel to perform a particular
task. A good consultant will often be able to advise you on variations that will not require
engineering calculations but it is wise to check before you change any thing.
If the material schedule on the approved plans specifies that the timber used for the
floor joists is 'continuous span', the floor joist has to be supported in a minimum of three
