If you are building a new home in WA one of the important inspections along the construction journey is the inspection of the initial concrete slab and then any subsequent concrete slabs for upper floors.

1      What are We Talking About?

In WA the vast majority of homes continue to be built on concrete slab foundations which remain the preferred method for most builders, given the relatively flat sandy soils which cover most of Western Australia.

Concrete slabs must be installed in accordance with Australian Standard AS 2870 and AS 3600. In Western Australia the most common slab thickness is between 85 mm and 100 mm. This thickness will be stated in the specifications document which forms part of the contractual arrangements between client and builder. Most Perth building sites are flat and sandy so an 85 mm slab will generally be sufficient for a single storey residential construction. If you are building a two-storey home and/or in the hills, engineering requirements usually require a 100 mm slab, with thickening under internal walls. If you have a long slab, perhaps greater than 20 metres in length, any savings from an 85 mm slab will be offset by the requirement for more steel reinforcement. Steel reinforcement increases the tensile strength of concrete and controls the width of shrinkage cracks.

The core issue is that the concrete slab is the very foundation that your new home relies on. It needs to be right! The slab is the first major deliverable by your builder and hence a key milestone. In essence it’s the first chance to see the quality of work that the builder will be delivering to you. From a project management perspective, the question is often raised – what is more important – good beginnings or good endings? There is no right answer, but we can state that starting the construction of your new home with your builder on the right foot can be very valuable in setting the right direction for the rest of the construction.

 2      What is the Process?

The process begins with earthworks preparing the site for construction. This involves clearing and levelling the ground and constructing a sand pad on which the concrete slab is laid. Service penetrations are installed (inward water, gas and electrical and outward grey and black water waste).

The slab is then compacted, termite barrier sprayed or laid, waterproofing membranes laid, reinforcing steel fixed and finally the concrete slab is poured. Once your slab has been laid, it will require approximately 1 week to cure.

After the slab has cured, drains and sewerage are often then connected.

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3      So What are the Issues?

Ideally there should be no issue.

Slab issues that most often arise in disputes between builders and owners relate to the size of cracks that may appear, and the thickness of the concrete slab installed.

Cracking of concrete slabs is a common occurrence, but it is not necessarily a major problem.  Some slab cracking is almost certain because concrete shrinks as it loses moisture, when temperatures change or when there is ground movement.

While slab cracks can look unsightly and affect the application of tiling or other floor finishes, many cracks do not affect the structural integrity of the slab.

Another key issue which clients raise as a potential concern is the level of the slab.  There can be a mismatch in expectations between the tolerances in levels within a slab as defined by the Building Code, Australian Standards, the Guide to standards and tolerances 2015 and the expectations of the floor covering, particularly trades involved in the laying of floor tiles and wooden floor boards. It is well worth ensuring that there is a clear understanding on the extent of floor levels between your builder and floor covering trades before you proceed.

As can be seen below there can be a whole raft of issues associated with a new concrete slab, the majority of which can be relatively easily remedied, if acted upon early enough.  Having an independent set of eyes reviewing the new concrete slab and the overall set up of the construction site for your new home construction can be very valuable.

4      When Do We Inspect the Concrete Pad?

We generally want to inspect the concrete pad (at each level) within 3 days of the slab being laid. It is important that we inspect the slab prior to the brickwork or wall framing commencing.

5       There Can’t be that Many Issues Can There?

Sadly, yes there can.  Here is a small selection of actual concrete slab issues identified in WA over recent months in 2017.

5.1      Slab is not in the correct position on the block

5.2      Wastes not in the correct location

5.3      Minor re-entrant cracks noted

5.4      Drain covers are required to prevent contamination of drains during construction

5.5      Water tap has been run over

5.6      Slab not level, puddles formed in hollows – tolerances to be checked by Builder

 

5.7      Retaining wall not installed as required on the plans

5.8      Drop in slab level, probably due to miscalculation in size of wet areas

There are also gaps where the concrete has not been finished properly, most likely due to a miscalculation of the size of the wet areas. These should be cleaned and filled.

5.9      Not all the slab could be inspected due to soil covering (from Drainage Contractor)

6      What is the Immediate Solution for People Building a New Home?

The simple solution is to have your concrete slab checked by an independent Houspect Building Inspector. Armed with the Construction Plans, a good knowledge of the Building Code of Australia, Australian Standards and many years of experience, your Houspect Building Inspector can review the work undertaken on site so that you can be assured that the concrete slab has been constructed as intended.

7      Can the Concrete Slab Inspection be Done at the Practical Completion or Handover Inspection?

It is almost impossible to undertake a full inspection of the concrete slab at the Practical Completion or Handover Inspection as much of the slab will have been covered by subsequent construction stages, especially if floor coverings have been included as a component of the overall construction package.

Remediation of poor slab construction can take a considerable time and if  inspected at Practical Completion or Handover stages the actual handover of the property to you may be delayed by several months.

8      Summary

• The concrete slabs are a really important structural component of your home.
• There can be issues in the correct construction of the concrete slab and these can generally be readily resolved if identified early enough in the construction journey.
• Independent slab inspections by a Houspect Building Inspector are another set of qualified and experienced eyes reviewing the construction of a key element of your new home.
• Concrete slab inspections should be conducted during construction ideally, as soon as each slab has been laid or prior to the brickwork commencing.

Speak to Houspect today about a concrete slab inspection or our end to end construction inspection packages which can support you through the entire construction of your new home

Ph 9240 8855 Web: www.houspect.com.au/wa Email: enquiry@houspectwa.com.au

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