When you’re building a house you start from the ground up, with the foundations, so this week we’re taking a closer look at one of the most important elements of your building.

A proper foundation does more than just hold a house up. It keeps out moisture, insulates against cold weather and resists movement of the earth around it. One more thing: It should last forever, according to Tracey McKenzie of Builderscrack.

The most popular choice of foundation in New Zealand is the traditional concrete slab. Reinforced slab-on-ground concrete floors that combined the floor and the foundation became popular during the late 1970s as they were considered to be cheaper (on flattish sites), termite, borer and vermin-proof, fire-resistant, draught-proof, and did not deteriorate.

They could distribute loads to the ground over the whole slab, thus creating a lower load per square metre than a floor supported on piles, making them particularly suitable where the soil bearing capacity of the ground was lower. 

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With the concrete slab, the crew puts up wooden forms to serve as a template for the foundation and digs the holes and trenches. Footings (structures where the house interfaces with the earth that supports it) are installed. The plumber will install all the pre-plumbing, such as the sewer pipe or hydronic underfloor heating, and the electrician will install electrical conduits, such as wires to an island bench in the kitchen, as these need to be embedded in the slab. An inspection is carried out before the concrete is poured. Slab foundations may be the least expensive to construct but they’re the most expensive to repair.

If the slab is a ‘raft’ slab (such as MaxRaft or RibRaft - more on this below), no footings are required as the slab ‘floats’ on the surface. Specialised subcontractors will usually come in and complete these slabs. 

Once concrete is poured it will need time to cure (a day at least). During this period, there will be no activity on the construction site.

Word to the wise, though, for those building within a tight budget. Ensure whomever has the responsibility of ordering the concrete has a good grasp of measuring what’s needed. Misjudgements in the size or depth of the space means over-ordering of concrete is commonplace. And while it’s good to err on the side of caution perhaps, excess concrete is a waste that the house owner usually pays for if you don’t have a fixed-price building contract.

With the drive for more energy efficient homes the thermal properties and performance of a building are becoming more and more of a consideration. The foundations of your home will have an impact on the energy used in heating and cooling your home. 

Twenty years ago, Firth Industries Ltd created Ribraft, New Zealand’s first pod flooring system.

Firth RibRaft® system has now become one of the country's most popular solutions for residential and light commercial flooring. The system uses polystyrene pods, steel reinforcing rods, plastic spacers and RaftMix™ concrete. Each of the components fit simply together, dramatically reducing labour time and costs.

This innovative method of concrete floor construction sits 'on' the ground not 'in' the ground, making it seismically strong.

More recently the MAXRaft® products have also come on the market, including MAXSlab (which is based around the three polystyrene components, all of which are installed on a damp proof membrane, and suitable for flat sites), MAXSpan (which delivers the benefits of MAXRaft® to the upper levels of your home), and MAXRaft (which is particularly suited to less stable ground).

MAXRaft®, a fully insulated foundation system, is a reinforced concrete slab-on-the-ground contained entirely within a thermal envelope of specifically designed MAXRaft® insulation. The slab encases the perimeter and puts a continuous layer of insulation between the ground and concrete. It’s also engineered to be seismically stronger in earthquake conditions compared to traditional concrete foundations.

PHOTO: Wanaka App