Termites

Did you know Australia has around 15 species of termite which can damage the timbers in your new house.

Although some species of timber are resistant to termites none are termite-proof. In practice any structure containing wood can be attacked, unless protective measures are taken.

Even if you have got a steel framed house, or double brick, you will still have timber in things like doors and architraves.

photo from Wickipedia

Know Your Enemy

Termites are more like cockroaches than ants.

Subterranean termites do more damage to timber than either damp wood or dry wood termites.

The termites generally remain within a system of tunnels that can extend 50m, from the central nest, to food sources.

Its not unusual for the termites to build their tunnels round any barriers so no matter what termite protection you use you still have to inspect the barriers regularly.

In order to get to their food source of wood, termites can damage materials they cannot digest such as plastics, rubber, metal or mortar.

Protective Measures

In the past certain areas were identified as at risk of termite attack while others were considered termite free. I thick it is much better to consider all properties at risk.

I’m not a fan of regular spraying of chemicals so for me a permanent barrier is a must.

Basically you need a continuous barrier to prevent termites climbing up through the external wall and individual protection around any pipes and conduits that penetrate the slab.

Options for the barrier in the walls, in order of rising cost include:

  • Exposed Concrete This is cheap and effective as it involves leaving the bare concrete of the slab exposed for a minimum of 75mm. Unfortunately not very attractive,  although you could use a concrete paint to match the brick colour.
  • Barrier Containing Insecticide Probably the most common is  Kordon, which is a  combined DPC and termite protection. It is two layers of plastic sandwiching an insecticide impregnated layer. (Expect to pay around $1,500)
  • Termimesh A fine stainless steel mesh. (expect to pay around $2,000)

Last time I built I used Termimesh as I was concerned about appearance, and preferred not to use chemicals.

 

Decisions on your new home? . .  see  Selection/Pre-Start Guide

Only $4

 

Height Above Ground


For the typical modern house with slab on ground base there are two minimum heights above ground level that need to be considered:

  • Next to the building.
  • Distance of 1m away.

The reason for these minimums is to keep water out of the building, including the structure and foundations.

Next To The House

Floor level above external finished surfaces must be a minimum of:

  • 50 mm above impermeable (covered paved or concreted areas) that slope away from the building
  • 100 mm above the finished ground level in low rainfall intensity areas ( 5 minute intensity of less than 125mm/hour for a recurrence interval of 20 years -see: Rainfall Intensity to check your site), or sandy well-drained areas
  • 150 mm in any other case.

Distance of 1m

The external finished surface surrounding the slab must be sloped away from the building, for the first 1m by a minimum  of the following:

  • 25 mm in low rainfall intensity areas for surfaces that are reasonably impermeable (such as concrete or clay paving)
  • 50 mm  in any other case.

Normally in clay soils I would allow another 15-20mm to allow for any ‘Soil Heave’. (See:Building on Clay Soils)

Overall

To meet the Building Code the range of total heights above ground will vary between 75mm and 200mm depending on circumstances. If you want a smaller step between inside and outside (for reasons such as Better Accessible Design) you will need to talk with your House Designer about a detail which meets the intent of the Building Code (Keeps water out of the house).

 

Different Soil Classification Results

When you look at a new house block its always worth asking the Developer’s Agent what the Soil Classification is ……….. Only don’t take what you are told too seriously!

Usually the developers opinion is going to be that the site is going to be better (lower site costs) than your builder.

To understand why you need to consider the differing situations of the Developer and the Builder.

The Developer

  • May have done 20 or 30 tests over the whole development.
  • Is mostly interested in selling blocks
  • Since the tests were taken has had sewers and drains constructed on the site.
  • Probably done some spreading of fill from roadworks construction and site leveling

The Builder

  • Has had 3 tests done on your block.
  • Knows that even with the three tests the information represents less than 0.01% of the soil under your block.
  • Want to be sure the foundations are strong enough.
  • Want to avoid claims, from you, for foundation movement causing structural cracks.

If I got a soil classification from a Developer I would ask the Builder what extra site costs would be required for that soil, and then budget for at least another $5,000.

 

For lots more information why not get the Guide to Buying a Block only $4 at this link: Buying a Block

Drop Edge Beams

If you want to build your new home on a slope you may need to have the additional cost of needing drop edge beams on the low side of the slabs.

These are a sort of retaining wall to hold the fill under the house slab as shown in the drawing below.

Here is a photo of a typical drop edge beam before the brickwork has been started.

Costs (2013) are in the order of $500-$700/sqm of beam, plus the cost of the fill under the slab.

Say. . . $12,000 to $18,000 for a 20m long house with a 1.5m drop beam so a significant extra!

The photo below shows the same wall after the brickwork has been completed.


If you are building on a slope you may also want to know about Retaining Wall Fairness

Thanks to Grumblebum54 for the photos

Building On Rock

Based on the Soil Classification rock is considered one of the best surfaces to build a house foundation.

It comes under the Classification Class A.

That  doesn’t mean its going to be cheap especially for a conventional raft or waffle pod slab.

Any excavation such as leveling the site and excavation for sewerage and drainage trenches is going to be be expensive. This is due to it requiring a heavy excavator and rock breaking equipment.

Keeping Costs Down

If you have a site that has rock close to the surface the most economic base construction is likely to be Piers for Lightweight Construction. (including  Pole Houses and Queenslanders)

If  you want a Masonry House  (such as Brick Veneer or Double Brick) then you need to minimise the amount of excavation by going for suspended floors.
 

Also see Provisional Sums to find out the issues of finding rock during the construction.

 

Building on Fill – Screw Piles

I have previously posted about building on fill using Concrete Piers.

Well one alternative to constructing concrete piers is to use Screw Piles such as the one to the right.

These are made from a thick steel tube with:

  • A cutting edge on the bottom.
  • A  spiral plate close to the bottom.
  • A keyway at the top to lock into the drive tool.

The pile is screwed into the ground using a drive tool mounted on an excavator.

Screw piles will normally be more expensive than concrete piers when only a few shallow piles are needed.

They can be cost effective, and fast to install, when a larger number of piles are required or soil disposal is a major problems.

Other advantages are:

  • There is minimal spoil to be removed from site.
  • You don’t have to worry about rain as much during the installation.
  • They can be easily installed when the base of the pile is below the water table.
  • They can be installed in very restrictive locations.

Edit based on question I received

If you are worried about corrosion normal screw piles are fairly thick steel to stand the driving forces (individual thickness of material may vary)

Typically corrosion for plain steel piles in natural ground can be in the range 0.12 – 0.3 mm in 10 years, although galvanised steel will be less.

See Ground Conditions for more about your new house foundations.

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