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will water pool under my home



It’s something that only a few people consider, but when building a home with a suspended steel floor – which has many advantages – the space underneath the home needs to be carefully planned so that water doesn’t pool under the home. This is important for a few reasons, but in answering the question, will water pool under my home?, we need to analyse the bigger picture. Of course, this can happen to a home that’s built on a concrete slab, but the implications and applications are different, and not what we’re talking about in this blog.


Before we do talk about this though, let’s clarify why it’s important that the ground is kept dry. By the way, when we say dry, we don’t mean crumbly dirt that blows into dust, but we just mean soil that isn’t continually saturated with moisture or puddles of water ponding around the place. So here’s why it’s important –


A. A wet sub-floor area can negatively affect the longevity of the materials that your home is built from. This could be anything from steel to timber. It’s important that these materials aren’t in a state of constant immersion in water. This doesn’t necessarily mean ‘under water’ but it can also mean ‘bathed in sweat’.

B. Moisture in the ground can place additional stresses on the structure of the home by causing the ground to move and thus causing the home to move more than it’s been designed to handle. This could eventually cause things like cracking in the plasterboard, doors not shutting properly and sealant coming away from tiles.

C. Moisture rising up (or rising damp as it’s sometimes called) can contribute to discomfort inside the home. This typically only occurs in extreme cases, but if the moisture is extensive enough, it can affect the interior of the home and the occupants are then influenced by it.


So onto the discussion around whether water will pool under your home and how to prevent it. There are 3 aspects to this, and we’ll discuss them in their order of importance.


1. The most important is the slope or gradient of the site, and especially the footprint of where the home is going. Because this is so important, the Building Code of Australia (now called the National Construction Code or NCC) is really clear about this – it’s under a clause called Surface Water Drainage. Without boring you with too much detail, it specifies that there must be a fall of at least 10mm for every 3mtrs of house. This doesn’t seem much, but in reality, water doesn’t need much of a fall to run, and for a house that’s 10 mtrs wide, that’s 34mm fall. Of course, there’s other things they spell out such as filling undulations underneath the home so that it’s a smooth (not level, but smooth) surface.


The reason for all of this is fairly obvious – it’s so water doesn’t stay there, it keeps moving on and away from underneath the home. We see an instinctive thought among many people that the area under a home should be flat. Under a slab, yes, but under a suspended floor, no. And then we also hear the comment that the surface must be an impervious surface so that the water runs over the top of it and doesn’t sink into it.


There’s some truth in this, but how impervious is the question. Just plain dirt with weeds is clearly no good; it has to be at least a skin of crushed rock that’s had at some form of compaction. And then the lower (or gentler) the slope is, the more important this is. If the slope is quite steep, then the impervious quality of the ground isn’t as important. The whole goal is to make sure that if water gets in there that it doesn’t stay in there.


2. The next thing is the location of the home, and although we’re looking at this as a separate aspect, it’s really part and parcel of the slope or gradient of the site. And by location, we don’t mean whether you’re building in Trentham or Teesdale, it’s the location of the home vs the area around it. Let’s use an example. Say your home is going to be in a hilly area with some beautiful views. So far so good. But then you realise that your home is going to be half way up the hill, so this means that there’s a lot of ground above the home. The key then is to look at this higher ground – is it a like a track for water flowing down the hill from other areas? Or is it just a small isolated crest that won’t pose any problem?


We’ve seen situations where a home has been placed in the way of water flowing down from higher ground, and the thing isn’t immediately obvious because the water’s flowing ‘through’ the soil, not necessarily just an overflow on the surface. So in wet weather, the ground underneath the home slowly becomes waterlogged because of what’s going on maybe 200 or even 500 metres away. So the answer here is to make sure there’s some form of diverting that water before it gets underneath the home. Because even though you might have a gradient under the home, prevention is better than cure, and the dryer you can keep the underfloor, the better.


So it could be a simple as making sure the ground is profiled such that the water runs away and around the home, or it could involve putting some drainage pipes in to pick up the water and literally send it in a different direction away from the home.


3. And then there’s the soil type. This may or may not play a significant part, but if the soil you’re building in is clay, it can have a negative impact. You might think that clay, being fairly thick, would stop the water coming in. But the very nature of clay is that it reacts to moisture, and when that happens it moves – sometimes a lot!


Typically clay is layered deep over massive rock, and when water is continually flowing into / under / around this clay, it starts to move and heave, and the ground at the top can move and create areas for water to pond, or create creek beds that channel water in the wrong direction, both on the surface, and deep down inside.


Of course, you might be wondering about the strength of the home in moveable clay – see our other blog on that. But once again, the key here is to be aware of it and look at the overall site situation to ensure that any potential problems are minimised.


It really all boils down to one thing – being aware of the overall situation on the site and planning accordingly. And this is what a good builder should be doing. Once levels are taken, plans are drawn and the ins and outs of the site are taken into consideration, then a plan of attack is formed that could involve simply doing absolutely nothing to major excavation and drainage system. There’s no one size fits all for this issue, except that it’s important that the area under the home is kept dry.


Just a note of caution here – although wet sub-floors are not good, it doesn’t mean that every home has this problem and that this problem is major. 99.9% of the homes and sites will be fine – it’s just important that this aspect is looked at so that all is well in the long-run. It also doesn’t mean that slabs are better, there’s no right or wrong way to build – there are just as many if not more homes on slabs with serious moisture problems because the home is then sitting right on top of the problem.


As always, feel free to ask for more information!
Disclaimer. This blog is our opinion only. The information provided in our blogs is accurate and true to the best of our knowledge, but there may be omissions, errors or mistakes. The information presented in our blogs is for informational purposes only and we are not professionals, so the content we provide shouldn’t be taken as legal advice. We strongly recommend consulting with a professional before taking any sort of action. We reserve the right to change how we manage our blog and we may change the focus or content at any time.

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