Building Basics – Understanding Subfloors

When you’re buying a house, pretty much everything (the entire structure!) rests on making sure the foundations are up to scratch. That’s why your house inspection checklist really needs to start at the ground floor – or in this case, the subfloor.

The subfloor is the bottom most structure of a floor which is made up of piles, bearers and joists, all designed to lift your timber off the ground. (These are not to be confused with concrete or raft foundations – which we’ll cover in a separate article.)

Piles are the stilts, legs, or supports that come out of the ground and hold the bearers up. These are usually made from timber, concrete or a mixture of both (usually in this case called jack studs). The subfloor bearers are the main beams that run along the structure, while the joists are what the flooring materials are fixed to. In these sorts of floors you’ll generally have a subfloor space that you can get underneath.












Pile                   Jack Stud                      Bearer                  Joist


Plenty of New Zealand homes have these suspended timber floors – and many suffer from some common issues that aren’t visible from the street or the garden. So, let’s crawl under and check off what these issues might be, and what you should be on the lookout for.

First – is the subfloor space accessible?

Being able to access the space under the subfloor is key to finding out what’s really going on. Many are ‘walk in’, and big enough for people to store household goods, suitcases and outdoor sports gear under. Others are tight and claustrophobic. You ideally need at least 400mm clearance – or you’ll be crawling on your belly.

It’s very important to keep the access way clear, so you, a property inspector, or tradie can easily get in and under, to check out any issues. Sometimes hatches and doors get built over, as in the case of one property we looked at where the deck had been constructed over it. The owner had to spend the day taking his deck apart so we could access the subfloor – not advisable if you have an urgent issue such as a plumbing leak.


Once we are in, there are plenty of things to look for:


Dampness, leaks and soggy subfloors are one of the most common problems in suspended timber floors and can lead to poorer health for residents. Dampness issues can also have a ‘flow-on’ effect and lead to other problems like rot and corrosion, vermin and/or borer infestation, poor insulation performance and even instability if piles (holding the floor up) become unstable. During our inspection we also look for ground built up around the cladding and the subfloor which can lead to moisture, rotting timber and mould.


Ventilation and moisture barrier

Ventilation plays a big role in allowing airflow through the subfloor area and assists in keeping timbers dry. It’s important that vents built into the subfloor are not blocked off and that the ground level around them doesn’t allow moisture into the subfloor. Also that they are not damaged as that will make a path for pests. It’s now part of the Healthy Homes rules that you need to have a moisture barrier on the ground if subfloor areas are accessible. Things like a polythene sheet will help stop any damp rising and make life a lot easier and stop you getting wet or dirty.










Vent blocked off by planter


Insulation plays a crucial part in keeping the interior of a home warm and dry. There are so many different types of insulation and they have changed over the years. The older foil insulation we commonly see is not rated anymore and can be extremely dangerous if it meets live electricity. Other insulation types might be polystyrene blocks, or a fiberglass or polyester. If insulation is poor, or non-existent, it can add significant cost to your house expenditure, and buyers need to be aware of it.


Subfloor Connections

Bearers sit on piles or jack studs and can move over time – some older homes may even be resting on tree stumps! And in plenty of older houses the bearers were never attached to the piles – resting not fastened – not ideal in an earthquake. It is important to check for any gaps between the piles and bearers to ensure the piles are doing the work they should. Also check to ensure no piles appear to be missing as sometimes they can be removed for various reasons and either not replaced correctly or at all.









Gaps under piles


Age of the building

If a subfloor is built from original native timber or untreated timber it can show signs of cracking due to age, or it could be a sign of dry rot inside the joists and bearers, Conversely in wet areas, such as under bathrooms, showers and laundries, leaks from above can cause issues to flooring materials and joists.


Newer builds

These days, to be up to current code, you need the correct fixings, bracings and connections, called ‘subfloor bracing’. Simple bracing has to be bolted to the timbers using square washers. In the past, a lot of it was just nailed rather than bolted, or no bracing was installed at all. If a property is re-piled it will need to be brought up to these current codes.

subfloor bracing









New compliant bracing on left, older uncompliant bracing on right

While most foundations constructed now are concrete (more on this next time), you do still see the odd timber one in something like a replica villa, or perhaps an area that is susceptible to flooding, such as a floodplain where you need to lift it off the ground.


Quick checklist for older timber subfloors:

  • Access/clearance – identify the access point and the size of the crawl space
  • If there is moisture, know where it’s coming from.
  • Check whether timbers are sound or if are they showing signs of rot.
  • Quality of the insulation, if any.
  • Loose piles – where there is a gap between the piles and the bearers.
  • Missing piles – does there seem like an odd gap or is there a pile not included in a row?
  • The cladding around your foundation – it could be asbestos if it is a fibre cement board. Is the ground level built up around it and is this causing any visible moisture damage?
  • Where the wires and pipes are, and if they are sagging, and/or need replacement or clipping up.
  • If there are signs of borer. These are generally small holes in the timber and/or piles of dust. This is common especially in old native timber and untreated timber – and we see a lot of it in Auckland.

wood and wire









Borer dust and borer holes present


  • Evidence of vermin such as mice, rats, possums, or other pests.

Bottom line – there a lot to look for!

As we said earlier, literally everything rests on getting the foundations right, so don’t put off an inspection because you can’t ‘see’ any issues and you think ‘she’ll be right’. Get the subfloor sorted and have peace of mind.

While there are risks with any type of property you’re buying, it’s important to understand the building basics to ensure you’re able to make an informed choice about one of the biggest purchases of your life. Book a Property Inspector to do your home inspection report before making any commitments.

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