There are three main materials I\u2019d consider when choosing a decking substructure:<\/p>\n
They all have their place.<\/p>\n
The mistake is assuming that one material is automatically the best for every deck.<\/p>\n
The right choice depends on where the deck is being built, how high it is, the expected lifespan, the type of decking being installed and, of course, the budget.<\/strong><\/p>\n So let\u2019s look at each option properly.<\/p>\n Timber is still the most common material used for decking subframes, and when it is specified and installed correctly, it can be an excellent option.<\/p>\n But here\u2019s the important question:<\/p>\n Is all treated timber the same?<\/strong><\/p>\n Absolutely not.<\/p>\n Two pieces of timber can look almost identical but have very different levels of protection and structural performance.<\/p>\n One of the biggest things people overlook is the Use Class (UC)<\/strong> of the timber.<\/p>\n UC2 is intended primarily for internal applications where the timber is protected from the weather but may occasionally experience moisture<\/strong>.<\/p>\n UC4 is designed for much more demanding outdoor applications where the timber is exposed to significant moisture and biological hazards, including situations involving ground contact.<\/p>\n That\u2019s a pretty important difference when you\u2019re building a deck.<\/p>\n And yet, the treatment specification is something many homeowners would never think to ask about.<\/p>\n For an external decking subframe, I personally prefer UC4 treated structural timber<\/strong>, rather than simply accepting whatever treated timber happens to be available.<\/p>\n For my own builds, my go-to timber is:<\/p>\n UC4 treated C24 structural timber, normally 6\u201d x 2\u201d.<\/strong><\/p>\n C24 is a higher structural strength class than C16, giving you a stronger structural timber option.<\/p>\n A 6\u201d x 2\u201d section also gives you a substantial joist compared with lighter timber sections.<\/p>\n There are situations where a 4\u201d x 2\u201d section may be appropriate, particularly where height is restricted, but I would generally prefer the larger section where the design allows it.<\/p>\n This is another area that gets overlooked.<\/p>\n When you cut treated timber, the freshly exposed end isn\u2019t protected in exactly the same way as the factory-treated timber surrounding it.<\/p>\n That\u2019s why I treat cut ends with a suitable end-grain preservative<\/strong>.<\/p>\n It\u2019s a small job, but it\u2019s another layer of protection against moisture getting into the timber through the cut end.<\/p>\n And if you\u2019re spending money on good-quality treated structural timber, it makes little sense to ignore the exposed ends.<\/p>\n A strong timber frame is only as good as the connections holding it together.<\/p>\n I wouldn\u2019t recommend buying a box of generic wood screws and assuming they\u2019ll do the job.<\/p>\n A decking subframe needs to support the decking, people, furniture and the loads transferred through the structure.<\/p>\n That\u2019s why I use structurally suitable external fixings designed for the job.<\/p>\n For example, I personally use Carpenter Mate Pro structural screws, including longer fixings for joining joists and ring beams and shorter fixings for components such as noggins.<\/p>\n The exact fixing specification should always be appropriate for the particular structural connection.<\/p>\n Another relatively simple upgrade I recommend is joist protection tape<\/strong>.<\/p>\n The tops of your joists are one of the areas most exposed to trapped moisture.<\/p>\n Water can sit around the decking boards, debris can accumulate and moisture can find its way around fixing points.<\/p>\n A good-quality decking tape creates a barrier over the top of the joist and around the fixing penetrations.<\/p>\n It isn\u2019t going to magically make a poor-quality frame last forever, but it\u2019s another sensible precaution when you\u2019re trying to maximise the life of the structure.<\/p>\n This is where I start looking at alternatives to timber.<\/p>\n For many of my lower-level decks, I prefer recycled plastic posts<\/strong>.<\/p>\n Why?<\/p>\n Because I\u2019ve seen what can happen to timber posts over time.<\/p>\n A timber post sitting in or very close to the ground is exposed to some of the harshest conditions a timber structure can experience.<\/p>\n If you\u2019re building a deck that you expect to last for decades, I don\u2019t want the weakest part of the structure hidden underneath the ground.<\/p>\n However, recycled plastic posts aren\u2019t suitable for every situation.<\/p>\n There are limitations to how high they can be used, particularly because of movement and structural requirements.<\/p>\n Once you\u2019re getting into significantly raised structures, I\u2019d start looking at alternatives such as appropriately designed timber posts with suitable protection, steel or aluminium systems, concrete solutions, ground screws or other engineered support systems.<\/p>\n The correct choice depends on the structure and the loads involved.<\/p>\n This is another really important point.<\/p>\n Every decking system has its own installation requirements.<\/strong><\/p>\n Joist spacing, support spacing, fixing methods and allowable spans can vary between products.<\/p>\n Don\u2019t design the subframe first and then simply assume every decking board can go on top of it.<\/p>\n The board manufacturer\u2019s installation instructions should form part of the design.<\/p>\n And on raised decks, you may also need to consider a beam-and-joist system<\/strong> rather than simply running joists between individual supports.<\/p>\n The beams allow the loads to be distributed through the structure and can make a much more efficient raised-deck design.<\/p>\n There are certain situations where I would seriously consider another material.<\/p>\n Below-ground applications<\/strong><\/p>\n This is where I become much less enthusiastic about relying on timber.<\/p>\n Even well-treated timber is still timber.<\/p>\n It needs the right conditions and detailing to remain durable.<\/p>\n If I can remove timber from a high-moisture or below-ground environment altogether, that\u2019s something I\u2019d seriously consider.<\/p>\n Rooftop decks and balconies<\/strong><\/p>\n Weight can become a major consideration on rooftops and balconies.<\/p>\n This is where aluminium can become particularly attractive because of its combination of low weight, dimensional stability and durability<\/strong>.<\/p>\n Recycled plastic is a completely different proposition.<\/p>\n Products such as Millboard\u2019s Plas-Pro<\/strong> are designed for situations where resistance to moisture and decay is extremely important.<\/p>\n And that\u2019s the big advantage:<\/p>\n It doesn\u2019t rot like timber.<\/strong><\/p>\n For certain below-ground applications, that\u2019s a huge benefit.<\/p>\n But there\u2019s another side to recycled plastic that isn\u2019t always talked about.<\/p>\n It\u2019s difficult to work with.<\/strong><\/p>\n In my experience, constructing a perfectly straight, level and accurately dimensioned frame from recycled plastic can take considerably longer than constructing the same frame from timber.<\/p>\n The material can be difficult to manipulate, and fixing it requires more thought.<\/p>\n You may need to pre-drill components, countersink fixings and deal with the fact that screws don\u2019t behave in recycled plastic in quite the same way they do in solid timber.<\/p>\n Some recycled plastic sections can also be less uniform and more difficult to get perfectly straight.<\/p>\n And because the material can move with temperature, you also need to take thermal movement into consideration.<\/p>\n All of this adds up.<\/p>\n No.<\/p>\n Far from it.<\/p>\n It has a very specific purpose, and when that purpose suits the project, it can be an excellent material.<\/p>\n But I wouldn\u2019t automatically choose it for an entire deck simply because it won\u2019t rot.<\/p>\n For many projects, I\u2019d rather put the money towards the third option.<\/p>\n This is where things get interesting.<\/p>\n For me, aluminium is the future of decking substructures.<\/strong><\/p>\n It\u2019s lightweight, extremely durable and doesn\u2019t suffer from the same rot problems as timber.<\/p>\n You don\u2019t have to worry about treating cut ends.<\/p>\n You don\u2019t have to worry about bowed timber joists.<\/p>\n You don\u2019t need to cover every joist with protection tape.<\/p>\n And you don\u2019t have the same problems with screws relying on timber to hold their threads.<\/p>\n It also makes an extremely clean and accurate frame.<\/p>\n Cost.<\/p>\n There\u2019s no point pretending aluminium is cheap.<\/p>\n The material cost can be significantly higher than timber.<\/p>\n But there\u2019s another side to the calculation:<\/p>\n Installation time.<\/strong><\/p>\n A system that is faster and easier to install can reduce labour time.<\/p>\n So while aluminium costs more to buy, the overall difference in project cost isn\u2019t necessarily as dramatic as the material price alone suggests.<\/p>\n And you\u2019re also investing in a substructure that should significantly outlast a conventional timber frame.<\/p>\n One aluminium option is Millboard DuoSpan<\/strong>, which was designed specifically around the Millboard decking system.<\/p>\n It\u2019s a very neat system and can make installation considerably easier.<\/p>\n There is one limitation, though.<\/p>\n When using certain Millboard fixing methods, you still need a suitable material within the aluminium profile for the decking fixings to engage with.<\/p>\n So it isn\u2019t simply a case of replacing every component of the system with aluminium.<\/p>\n Another system worth looking at is the aluminium framing system from Outdure<\/strong>.<\/p>\n One of the things I like about this type of system is the flexibility of the components.<\/p>\n Longer joist lengths can allow larger areas to be constructed with fewer joins, while different profile sizes can be used depending on whether you\u2019re building a low-level deck or a more substantial raised structure.<\/p>\n The fixing system also makes assembly much more straightforward compared with recycled plastic.<\/p>\n And because the connections are designed as part of the system, you\u2019re not relying on improvised fixing methods.<\/p>\n That matters.<\/p>\n A subframe isn\u2019t just a collection of pieces of material.<\/p>\n It\u2019s a structural system.<\/p>\n Here\u2019s my honest answer.<\/p>\n Best for:<\/strong> My preference:<\/strong> Advantages:<\/strong> Disadvantages:<\/strong> Best for:<\/strong> Advantages:<\/strong> Disadvantages:<\/strong> Best for:<\/strong> Advantages:<\/strong> Disadvantages:<\/strong> This is where I think the answer becomes more interesting.<\/p>\n If you\u2019re building a normal above-ground deck and budget is important<\/strong>, I\u2019d still choose a properly designed UC4 C24 timber subframe<\/strong>.<\/p>\n You don\u2019t need to spend a fortune on aluminium just because it\u2019s technically the premium material.<\/p>\n A well-designed timber structure can provide many years of service.<\/p>\n But if you\u2019re building a raised deck, difficult site, rooftop deck or a project where longevity is the priority<\/strong>, I\u2019d seriously consider aluminium.<\/p>\n And if you\u2019re building something where parts of the structure are going below ground<\/strong>, recycled plastic or another engineered support solution can make much more sense than burying timber and hoping for the best.<\/p>\n The real answer isn\u2019t \u201cwhich material is best?\u201d<\/strong><\/p>\n It\u2019s:<\/p>\n Which material is best for YOUR deck?<\/strong><\/p>\n Because spending more money doesn\u2019t automatically make a deck better.<\/p>\n The best subframe is the one that is correctly designed for the environment, correctly sized for the loads, compatible with the decking system and installed properly.<\/strong><\/p>\n You can put a \u00a310,000 decking system on top of a badly designed \u00a31,000 frame and still end up with a problem.<\/p>\n Conversely, a properly designed timber frame can be an excellent solution when timber is the appropriate material.<\/p>\n And if you\u2019re prepared to invest more, an engineered aluminium system can potentially take the substructure to another level.<\/p>\n That\u2019s why I don\u2019t believe in simply selling someone the most expensive option.<\/p>\n I believe in choosing the right subframe for the job.<\/strong><\/p>\n","protected":false},"excerpt":{"rendered":" There are three main materials I\u2019d consider when choosing a decking substructure: Timber Recycled plastic Aluminium They all have their place. The mistake is assuming that one material is automatically the best for every deck. The right choice depends on where the deck is being built, how high it is, the expected lifespan, the type […]<\/p>\n","protected":false},"author":1,"featured_media":915,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[31],"tags":[],"class_list":["post-1758","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-insights"],"acf":[],"yoast_head":"\n1. Timber \u2014 the traditional choice<\/h2>\n
UC2 vs UC4 \u2014 what\u2019s the difference?<\/h3>\n
So what should you be looking for?<\/h3>\n
What happens when you cut treated timber?<\/h3>\n
The fixings matter too<\/h3>\n
Don\u2019t forget the top of the joists<\/h3>\n
What about the posts?<\/h3>\n
Don\u2019t forget the deck board manufacturer\u2019s requirements<\/h3>\n
When wouldn\u2019t I choose timber?<\/h3>\n
2. Recycled plastic<\/h2>\n
So is recycled plastic bad?<\/h3>\n
3. Aluminium \u2014 the premium option<\/h2>\n
The downside?<\/h3>\n
Millboard DuoSpan<\/h3>\n
Outdure aluminium framing<\/h3>\n
So which one would I choose?<\/h2>\n
Timber<\/h3>\n
\nMost above-ground decks where budget matters.<\/p>\n
\nUC4 treated C24 structural timber, correctly sized and installed.<\/p>\n
\n\u2714 Cost effective
\n\u2714 Easy to work with
\n\u2714 Widely available
\n\u2714 Easy to modify
\n\u2714 Excellent for conventional decking<\/p>\n
\n\u2718 Can rot
\n\u2718 Can move and warp
\n\u2718 Requires good detailing and maintenance
\n\u2718 Quality varies enormously<\/p>\nRecycled plastic<\/h3>\n
\nSpecific high-moisture and below-ground applications.<\/p>\n
\n\u2714 Won\u2019t rot like timber
\n\u2714 Excellent resistance to moisture
\n\u2714 Useful for posts and difficult environments<\/p>\n
\n\u2718 More expensive
\n\u2718 Slower to install
\n\u2718 More difficult to work with
\n\u2718 Thermal movement needs consideration
\n\u2718 Fixing can be more complicated<\/p>\nAluminium<\/h3>\n
\nPremium projects, raised decks, difficult environments, rooftop applications and anyone prioritising long-term durability.<\/p>\n
\n\u2714 Lightweight
\n\u2714 Doesn\u2019t rot
\n\u2714 Excellent dimensional stability
\n\u2714 No treatment required
\n\u2714 Fast and accurate installation
\n\u2714 Excellent long-term solution<\/p>\n
\n\u2718 Higher material cost
\n\u2718 Requires a compatible system and fixing method
\n\u2718 Not every decking board system is designed to work directly with it<\/p>\nSo what\u2019s the best value?<\/h2>\n