Column or Stump Sub Floors

Columns or stump sub floors are a common building practice and are especially useful on sloping sites. The columns can be made of steel tube, pre cast concrete or treated timber these need to be designed by an engineer for correct sizing. Steel columns can fabricated to suit the house sub floor frame and house lay out and then galvanised. However prefabricated galvanised columns are available ready to install and they are adjustable on site. These columns are available from BlueScope steel and can be ordered in through a local hardware store.

Concrete and steel columns

Concrete columns (prefabricated) or stumps are also used however they are not as common as galvanised steel columns. House foundations of old were built of hardwood timber stumps and over time these rot out. Most of these rotten stumps were replaced with prefabricated concrete columns or stumps. However these old rotten timber stumps are easily replaced with prefabricated galvanised steel columns and adjusted on site.

Installation:

Option one:

To install your columns set out your building with string lines making sure that it is correct to the plan and in the exact position on the block. Then mark the centres of the columns by setting string lines square to the building or outside string line that are in correct position.

Step two:

Drill or dig the holes for each of the columns as per the specifications set by the engineer on your approved house or renovation plans.

Step three:

Pour concrete into the holes to the required depth on the plans.(do not use quick set concrete as it is not strong enough).

Step four:

Columns are lowered into the holes and aligned with the string line and then concrete is back filled into the holes. Then the floor bears are installed, however this way requires greater accuracy to get the posts at the right height and position.

Option two:

To install your columns set out the building layout with string lines making sure that is correct to the plan. If you are adding to an existing building make sure you are square with the existing home. Double check your string lines.

Step one:

Drill or dig all of your holes as per the plan mark them by measurement and then with a plumb bob as per the plan.

Step two:

Using props install the entire flooring frame (bearers). This frame should be installed in line with your correct string lines. The props can be acro props from the hire shop or large pieces of timber for low floor heights. When the floor bearer frame is in place and propped at the right height to the plan - double check for square and correct height.

Step three:

Install the columns or post into the correct position as per the plan as if they were holding the floor up. When they are bolted to the floor frame and extending into the holes fill the holes with concrete. Fill each hole with the correct amount of concrete as per the engineers specifications on the plan. When the concrete has property set the props can be removed and the floor should be in the correct position.

Variant:

A slightly different way to this is to fill the holes with concrete in step one and then bolt the columns to the concrete into step three. However an (on site) adjustable post or column should be used like the Blue Scope galvanised post or similar. Also use chemset bolts as per the engineers specifications.