Why DIY Decks Go Wrong – And How to Make Sure Yours Doesn’t

Decking looks like one of those outdoor projects that’s well within reach of a capable DIYer. The concept is simple enough – posts, bearers, joists, boards – and there’s no shortage of online tutorials making it look straightforward. The reality is that decking is a project where small mistakes in the early stages compound into significant problems down the track, and where the gap between a deck that looks great on day one and one that still looks great in year ten comes down almost entirely to the decisions made during installation. Anyone researching decking in Perth or any hot, dry climate will find that the conditions place particular demands on decking – and that the mistakes which cause problems in milder climates tend to show up faster and more severely when the weather is less forgiving.

This article goes through the most common mistakes first-time deck builders make – not to discourage anyone from attempting the project, but to make sure the pitfalls are visible before you start rather than discovered after the boards are down. Some of these mistakes are easy to avoid once you know about them. Others are more subtle but just as consequential.

Whether you’re planning to build yourself or hiring a contractor, understanding where decking projects typically go wrong makes you a better-informed client and gives the finished deck a much better chance of performing the way it should.

Skipping the Subframe and Getting Straight to the Boards

The decking boards are the visible part of the project, so it’s natural that they get the most attention. But the subframe – the posts, bearers, and joists that everything sits on – is where the deck is really built. A subframe that’s poorly designed, under-engineered, or built from the wrong materials will cause the deck above it to flex, squeak, sag, and deteriorate regardless of how good the boards are.

The most common subframe mistake is spacing the joists too far apart. Joist spacing determines how much the boards flex underfoot – too wide and the deck feels bouncy and unstable, the boards are more prone to cupping and warping over time, and the structure is more vulnerable to movement. The correct spacing depends on the board thickness and species, but as a general rule, closer is almost always better and wider is a compromise that tends to be regretted.

The choice of subframe material matters just as much. Timber used for posts and bearers that will be in contact with or close to the ground needs to be rated for in-ground or ground-contact use. Using standard treated pine where ground-contact rated material is required is one of the most common causes of premature subframe failure – it rots, and once the subframe rots the entire deck is compromised.

Ignoring Drainage and Ventilation Beneath the Deck

Moisture is the enemy of timber in all its forms, and a deck that traps moisture beneath it will deteriorate significantly faster than one that allows air to circulate freely. This is particularly relevant in climates where heavy rain is followed by intense sun – the area under a low deck can become a consistently damp environment that accelerates rot and creates conditions that attract termites.

Ventilation beneath the deck starts with clearance. A deck that sits very close to the ground has limited air movement, which means moisture lingers. Where site conditions allow, maintaining adequate clearance between the ground surface and the underside of the joists makes a meaningful difference to how quickly things dry out after rain.

Drainage also matters. If the ground beneath the deck is graded towards the house or slopes in a way that causes water to pool, that standing water creates ongoing moisture problems. Clearing organic material – soil, leaf litter, debris – from beneath the deck area before construction, and grading the ground correctly to direct water away, is straightforward work that pays dividends over the life of the deck.

Choosing the Wrong Timber for the Conditions

Not all decking timber is created equal, and choosing a species that isn’t suited to the conditions of your specific site is one of the more expensive mistakes you can make. The considerations include hardness and durability, resistance to weathering and UV, natural oil content (which affects how the timber responds to wetting and drying cycles), and how the species behaves in terms of expansion and contraction with seasonal moisture changes.

In hot, sunny climates, a timber with low natural oil content and poor UV resistance will grey out, check, and splinter relatively quickly – particularly if it’s in full sun for most of the day. Hardwood species with higher natural oil content and durability ratings tend to hold up significantly better under these conditions, and the additional cost per lineal metre is almost always recovered in reduced maintenance and longer service life.

Softwood decking boards – treated pine in particular – are often chosen purely on the basis of price. They can work perfectly well in sheltered conditions with regular maintenance, but in exposed, high-UV environments they require more frequent oiling and will typically need replacing sooner than a quality hardwood. The total cost of ownership over ten years often favours the more expensive timber upfront.

Not Leaving Enough Gap Between Boards

Timber moves. It expands when it absorbs moisture and contracts when it dries out, and this cycle happens repeatedly over the life of the deck. If boards are installed too tightly together – or with no gap at all, which does happen – that expansion has nowhere to go. The result is boards that buckle, cup, and push against each other, creating an uneven surface and accelerating the deterioration of both the boards and the fixings.

The correct board gap depends on the timber species, the moisture content of the boards at the time of installation, and the expected climate conditions. As a general guide, a gap of around 5 to 8 millimetres between boards is appropriate for most hardwood decking in outdoor settings, though it’s worth checking the specific recommendations for the species and supplier you’re using.

The gap also serves a practical drainage function – it allows water to drain off the surface rather than pooling. Boards laid with minimal or no gap create a surface that holds water, which accelerates surface degradation and makes the deck slippery when wet.

Using the Wrong Fixings

The screws and fasteners used to fix decking boards are one of those details that people often underestimate, partly because they’re hidden once the deck is finished and partly because the cost difference between appropriate and inappropriate fixings seems trivial relative to the overall project cost. It isn’t trivial – the wrong fixings can fail relatively quickly and cause a range of problems that are disproportionately annoying to fix.

Standard zinc-plated screws are not suitable for outdoor decking. They corrode, and as they corrode they stain the surrounding timber with rust marks and eventually lose their holding strength. Stainless steel or hot-dipped galvanised fixings are the correct choice for outdoor use, and for hardwood decking in coastal or high-humidity environments, stainless steel is strongly preferred.

The length and thread profile of the screws also matter. Too short and they don’t bite sufficiently into the joist; the wrong thread profile and they don’t draw the board down tightly. Pre-drilling hardwood boards before fixing prevents splitting and produces a much cleaner result. These are details that experienced deck builders handle automatically but that catch out first-timers regularly.

Forgetting to Seal End Grain

End grain – the exposed cross-section at the cut ends of timber boards – absorbs moisture significantly faster than the face or edge of the board. In a deck that’s regularly wetted and dried, unsealed end grain is one of the most common entry points for moisture-driven deterioration. The ends of boards check, split, and degrade faster than the rest of the board if they’re left untreated.

Sealing end grain at the time of installation – before or immediately after cutting – is a simple step that significantly extends the life of the boards at their most vulnerable point. End grain sealer is inexpensive and takes minutes to apply. It’s one of those small details that separates a deck built to last from one that starts to show its age within a few years.

Neglecting Maintenance Once the Deck Is Down

Timber decking is not a set-and-forget surface. It needs periodic cleaning and oiling or sealing to maintain its appearance and protect the timber from UV damage, moisture ingress, and surface degradation. The frequency depends on the timber species, the climate, the level of sun and rain exposure, and the finish used – but most timber decks benefit from cleaning and re-oiling at least once a year in harsh conditions, and every two years in more sheltered situations.

The mistake first-time deck owners commonly make is waiting too long between maintenance treatments. Once timber has been allowed to grey out significantly, dry out, and begin to check, restordeing it to good condition requires considerably more work than simply keeping up with regular maintenance would have. A deck that’s well maintained from the start ages gracefully; one that’s neglected tends to look tired within a few years and deteriorates increasingly quickly thereafter.

Built Right From the Start

The common thread running through all of these mistakes is that they’re almost all avoidable with the right knowledge and the right approach from the beginning. Decking isn’t a forgiving project in the way that some outdoor improvements are – the structural and material decisions made during installation are largely locked in once the deck is built, and the cost of correcting them later is always higher than getting them right first time.

If you’re building yourself, take the time to research the specifics for your chosen timber species and your local conditions before you start. If you’re hiring a contractor, these are the questions worth asking – how are you handling the subframe? What fixings are you using? What’s your approach to drainage and ventilation? A contractor who answers these confidently and in detail is demonstrating the kind of care that produces a deck worth having.

Experienced professionals who specialise in decking and similar climates have seen every one of these mistakes – and built enough decks to know exactly how to avoid them. That experience is what the difference between a deck that looks great for a season and one that’s still performing well a decade later is built on.