Civil Engineering Reference
In-Depth Information
Stumps: materials and installation
Timber floors are required to have clear ventilation space between the ground and the
underside of the bearers. Unless there is a recognised termite barrier on top of the stumps
the minimum clearance is 450 mm. It should be noted that an ant cap is no longer
deemed as a recognised termite barrier in its own right. The combination of a metal ant
cap and Kordon, a product produced by Bayer, would meet this requirement. If you have
an approved termite barrier the minimum distance from the ground to the underside of
the bearer is dramatically reduced. When using strip flooring this minimum clearance is
150 mm, however if using particleboard flooring it is increased to 200 mm. This air space
is to be clear of obstructions to allow uninterrupted airflow between the floor and the
earth. This prevents the moisture of the earth penetrating the timber causing it to remain
damp. If insufficient airflow were provided below the bearers, you would quite quickly
notice a musty smell in the house. This smell is a tell-tale sign that the timber is wet and
most likely rotting.
Stumps are used to support a timber floor. In many instances a house may have a
concrete strip footing around the perimeter of the building, a process that I will cover
later in this chapter, or it may be that stumps support the external walls.
Concrete stumps
The most cost-effective and commonly used stumps are the precast concrete stumps.
These stumps have a steel pin protruding from the centre of the top of the stump. This
pin is fed through a hole drilled in the bearer. Timber stumps are more user-friendly, as
they can be cut to the correct height once in position. It is significantly easier and cheaper
to fit sub-floor bracing to timber stumps. The issue of bracing shall be covered later in
this chapter.
Calculating the length of concrete stumps
Concrete stumps are supplied in lengths with multiples of 100 mm. Only once the stump
holes are dug and cleaned out should you then measure and order the stumps. Let's
assume for a moment that the stump we are measuring for needs to protrude 200 mm
above ground level and that the stump hole is 600 mm deep. At first it would appear that
the stump required would be 800 mm long, but this is not correct. We need to deduct 150
mm from the length of the stump to allow for the required depth of concrete under the
stump: 800 - 150 = 650 mm. Unfortunately, concrete stumps are supplied in multiples of
100 mm. As 150 mm is the minimum of concrete to be under the stump we will have to
purchase a 600 mm stump and increase the concrete under the stump to 200 mm.
Timber stumps
Timber stumps will be cut to the correct height once the stumps are laid. When pre-
cutting the stumps for each hole, add approximately 50 mm to the required length to
allow for cutting. If in doubt add more. It is simple to cut a stump but frustrating to have
to remove it because it is too short.
