A beautiful deck starts with what you don’t see. In this guide, we walk you through the three main types of substructures we use at Timbermasters — treated timber, aluminium, and composite. Each one has a specific role depending on your design, layout, and long-term expectations.
Why Substructure Matters More Than You Think
The substructure is what keeps your deck solid, levelled, and safe – no matter how much wind, rain, or weekend wear it gets. And in Ireland, where moisture is pretty much a permanent feature, this hidden part of the build matters more than most people realise.
A lot of builders use “treated timber” from standard suppliers, but here’s the issue: many of those timbers are only rated for above-ground use. That might be fine in a dry climate – but not here. In shaded gardens or areas with poor drainage, we’ve seen rot begin in just a couple of years. It usually starts in the joints and contact points, where water gets trapped or splashes back up from the soil. Left unchecked, it eats away at the structural integrity until what looked solid on day one becomes soft, unstable, and expensive to repair.
Moisture also means movement – timber swells and contracts with weather changes. Over time, that shifting can lead to warping, squeaking, popped screws, or even joist failure. And once that starts, the surface above doesn’t stand a chance.
In Ireland’s wet climate, substructure failure is one of the top causes of premature decking repairs – and it’s usually avoidable with the right materials and planning.
Bonus issue? Rodents. We’ve come across more than a few nests tucked away under decks – not always because of the structure itself, but because no one thought about airflow or access. A properly planned subframe can discourage that – and we build with that in mind, too.
1. Pressure-Treated Timber Substructure
Let’s clear something up: not all “treated timber” is ground-rated. A lot of builders use timber meant only for dry, above-ground use. And in Ireland? That’s asking for trouble. Close to the soil, in shady or damp spots, those timbers can start breaking down from the inside in just a few seasons – especially around joints where moisture collects.
That’s why we do things differently. At Timbermasters, we use construction-grade timber that’s been properly pressure-treated at source – and then we treat it a second time ourselves, in-house. We developed our own sealing process, inspired by marine-grade timber systems, designed to resist prolonged exposure to moisture. It’s the kind of barrier you’d expect on a boat, not just a backyard deck – and it makes a massive difference in long-term durability.
Whether your deck sits low to the ground, hugs a shaded garden, or faces a rainy west-facing wall, this double-treated timber holds its shape, keeps its strength, and delivers a moisture-resistant substructure that’s ready for real Irish conditions. It’s not just “protected” – it’s prepared.
Proper installation is just as important. These frames must sit on pads or footings – never directly on soil – and need plenty of airflow beneath to avoid rot and movement. In wet or poorly ventilated spaces, we often build in extra clearance or drainage detailing to keep the subframe healthy long-term.
Sure, timber will always need occasional checks – it’s a natural material, after all – but if it’s treated, supported, and ventilated the way we do it, it holds up beautifully even in Ireland’s wettest corners. And that’s exactly why we back our decks with a full Timbermasters warranty – because when it’s built right from the ground up, it lasts.
2. Composite (WPC) Substructure
Composite joists (WPC – wood plastic composite decking) sit right between timber and aluminium — both in performance and price. Think of them as a smart middle-ground: more stable and weatherproof than wood, but not quite as rigid or load-ready as aluminium.
WPC subframes are made from a blend of recycled wood fibres and plastic resin, extruded into structural profiles. They’re completely resistant to rot, insects, and splintering, and they require zero chemical treatment or sealing — a big plus in damp gardens or low-maintenance builds.
Where they shine: composite joists are ideal for decks built over compacted gravel or slab bases, especially where height is limited but airflow still exists. We typically recommend them for low-to-mid-height decks with consistent load distribution — like ground-level platforms, garden seating areas, or composite-clad terraces. Their slight flexibility makes them easier to work with on gentle slopes, but they’re not suited for raised structures or spans over 40–50 cm without extra support.
It’s important to note that WPC joists need closer spacing than timber or aluminium — usually at 300mm centres (sometimes less, depending on the deck board used). And while they won’t absorb water, they do expand and contract slightly with temperature, so fixings and gaps need to account for that.
If you’re going for a full composite decking build — boards, frame, and finish — this gives a seamless visual result and keeps everything colour-coordinated. Plus, it’s one of the most eco-conscious options available, made from recycled materials and fully recyclable at the end of its life.
In short: for low-profile, low-hassle decks that still need to hold their shape in Irish weather, WPC is a very solid choice — as long as it’s used in the right context, and installed properly.
3. Aluminium Substructure
Aluminium might not be the first thing that comes to mind when you think of decking – but once you understand what it offers, it’s easy to see why we love it.
First off, it’s incredibly durable. Properly installed, an aluminium substructure delivers long-term, worry-free performance. It won’t rot, warp, or absorb moisture, and it’s completely resistant to insects and decay. For homeowners looking for a “fit and forget” solution – especially in wetter climates – aluminium offers peace of mind that lasts decades.
It’s also surprisingly versatile. While many people assume it’s only for raised or commercial decks, we often use it in two very different scenarios:
- For very low-clearance decks – especially those under 10 cm – where airflow is limited, and timber wouldn’t survive. Aluminium is perfect here: it’s slim, stable, and unaffected by ground moisture.
- For higher decks, we typically rely on our Timbermasters double-treated timber – specially engineered for strength and weather resistance – but the two approaches share the same principle: choosing the right material for the right structural height and environment.
That said, there’s one important detail: Aluminium needs a fully stable, level base. Unlike timber, which can tolerate some unevenness, aluminium frames require a solid sub-base (typically concrete or compacted aggregate) across the entire footprint. That means the prep work matters – and it has to be done right from the start.
Used correctly, aluminium gives you a clean, rigid, moisture-proof substructure that performs year after year, rain or shine. It’s not the cheapest option up front, but it pays off in longevity and peace of mind. It’s the kind of detail that makes a deck feel engineered – not just built.
Quick Comparison Table
Not sure which substructure suits your project best? Here’s a quick side-by-side look at the three main options we work with — how they handle moisture, what they cost, and where they shine. It’s not about “best overall” — it’s about the right match for your garden, layout, and long-term expectations.
| Substructure Type | Timbermasters Double-Treated Timber | Composite | Aluminium |
| Moisture Resistance | Good (if well ventilated) | Very good | Excellent |
| Long-Term Durability | 10–15 years | 10–20 years | 20+ years |
| Weight | Medium | Light | Very light |
| Maintenance | Occasional checks, minor upkeep | Very low – no sealing or staining | None – no rot, warping, or decay |
| Cost | Budget-friendly | Mid-range | Premium |
| Best For | Ground-level decks, adaptable layouts, traditional garden builds | Fully low-maintenance gardens, matching composite decking, eco projects | Raised decks, very low clearance builds, porcelain or modern systems |
Still not sure? That’s normal. Most clients aren’t 100% certain at the start — and that’s exactly where we come in. We’ll look at your site, your layout, your long-term goals, and help you pick the right structure from the ground up. Because a deck that lasts always starts with a smart plan underneath.
How We Choose the Right Structure for Each Project
At Timbermasters, we don’t believe in one-size-fits-all solutions — especially when it comes to what holds your decking together. Every garden, every site, and every client is different. That’s why our substructure planning always starts with the context, not the catalogue.
We look at the full picture:
- Site conditions — including the slope of the land, how exposed it is to wind and rain, and how well the soil drains. These factors determine how the base should be built and what materials will last.
- Deck surface — whether you’re going with timber, composite decking , or porcelain, each material has specific weight, flexibility, and ventilation needs. The substructure must be matched accordingly.
- Maintenance and lifespan expectations — some clients want a zero-maintenance solution that lasts 20+ years. Others are happy with occasional checks and light upkeep. That shapes the decision.
- Clearance and structural load — is it a low-level garden deck or a raised terrace with built-in planters and furniture? Height and weight impact everything from footing depth to frame spacing.
We often plan the substructure alongside other key features — like drainage solutions, lighting systems, and even future proofing for access panels or service zones. For us, a deck isn’t just a surface. It’s part of how you live outside — and it should be built with the same thought as anything inside your home.
Side Story: How a Simple Grass Job Turned into a Full Decking Project
Not long ago, we visited a client who wanted artificial grass installed for his dog — mainly to escape the endless cycle of mud, paw prints, and patchy lawn problems (you can read more on that in our guide about artificial grass for dogs).
While chatting during the site visit, he mentioned he’d also been thinking about adding a deck — but had one big concern. “I’d love to do it,” he said, “but I’ve heard the mice in this area love hiding under them.”
And he wasn’t wrong — we’d actually worked on a few homes in that same estate where rodent nesting had become an issue under poorly built decks. So, we told him: no problem, we’ve dealt with this before — and here’s how we handle it:
• We use our Timbermasters double-treated timber, which is not only rot-resistant, but naturally discourages nesting. Rodents don’t like the scent or texture of it — especially after our in-house marine-inspired sealing process.
• We design the subframe layout to avoid warm, undisturbed corners, making it harder for mice to settle in.
• And when the risk is higher, like in this area, we go one step further and install a galvanized rodent barrier mesh under the structure. It keeps airflow healthy while blocking entry points completely.
That client ended up getting both the grass and the deck — and hasn’t had a single unwelcome visitor since. The garden stays dry, clean, and functional year-round — and most importantly, the dog absolutely loves it. Happy owner, happy dog, no more muddy paws… and no mice.
FAQs – Substructures for Decking in Irish Gardens
Q: What’s the most durable substructure option?
A: Aluminium, no contest. It doesn’t rot, warp, or absorb moisture — and with the right prep underneath, it can last 20+ years without needing a thing.
Q: Is treated timber a bad choice?
A: Only if it’s the wrong type. Many builders still use store-bought timber that’s not rated for ground contact — and that’s when rot sets in fast. Our Timbermasters double-treated timber is a whole different story: pressure-treated at source, then sealed in-house using a method inspired by marine timber protection. With good airflow and occasional checks, it holds strong for up to 15 years, even in wet gardens — and it’s far more budget-friendly than aluminium.
Q: Can composite joists be used for all deck types?
A: They’re great for low or mid-height decks on solid bases — especially when everything’s composite. But for raised structures or heavy loads, we don’t recommend them unless heavily reinforced.
Q: What substructure needs the least maintenance?
A: Aluminium again — no sealing, no upkeep. Composite decking is also low-maintenance. Timber needs periodic checks, yes — but if it’s treated properly like ours, it’s reliable, cost-effective, and long-lasting.
Q: Can you mix different substructures in one build?
A: Absolutely. We often combine aluminium in low-clearance areas with our double-treated timber elsewhere. It’s all about matching the material to the space — and getting the best balance of price, performance, and durability.
Final Thoughts
No matter how beautiful the surface, the deck only lasts as long as its base.
Whether you want a budget-friendly garden platform or a precision-engineered terrace, the right substructure ensures your deck stays strong and safe for the long run.
Let’s chat about your project – even if it’s just an early idea or a sketch on paper.
