Timber vs Steel Decking Frames: Which Is Best?

When you are choosing decking, you naturally focus on what you can see — the boards. Millboard, composite, hardwood, what colour, what finish. But the most important decision is actually hidden underneath. What is the frame going to be made from?

For years timber has been the standard. Lately we are using a lot more galvanised steel, especially for premium jobs, raised decks and anything with curves or levels. Both have their place. A straightforward low garden deck can be absolutely fine on a properly specified treated timber frame. A big raised Millboard terrace, a curved design or a multi-level platform that needs to span a long way — that is where an engineered steel frame starts to make a lot more sense.

There is no single material that is best for everything. It comes down to how high the deck is, how far it needs to span, what loads it has to take, how wet it is underneath, how much build-up you have got, how long you want it to last, access and budget.

Why The Frame Matters So Much

The subframe does all the work. It carries the boards and transfers everything — people, furniture, planters, a hot tub — into the supports and into the ground or building below. On a raised deck that might be foundations or bases, posts, beams or bearers, joists, noggins, bracing, connections and then the boards on top.

The boards are just the skin. If what is underneath is wrong, it will show on top sooner or later. A premium board on a cheap frame is not a premium deck.

What Loads Does It Actually Carry?

It is not just the weight of the boards. A deck can end up carrying people, sofas, big planters, an outdoor kitchen, balustrade, pergola, snow in winter, its own weight and sometimes a hot tub. A filled hot tub with people in it is a serious concentrated load. Same with commercial terraces — much higher loading than a normal garden. That is why "we always use timber" or "steel is stronger" on its own doesn't mean much. The frame has to be designed for how that specific deck will actually be used.

What Do UK Regulations Say About Frame Material?

There is no UK rule that says domestic decking has to be timber or steel. The requirement is that it has to be suitable and safe for what it is being used for. Depending on the job, you might need to look at structural standards, Building Regulations, manufacturer requirements and sometimes engineering calculations.

For timber, the Timber Decking & Cladding Association is clear — use strength-graded structural timber. C16 is considered the minimum for decks above 600mm, and if you are using smaller sections, longer spans or you need more performance, you should be looking at C18 or C24. They also strongly recommend that all exterior softwood substructure — joists, beams, posts — is pressure treated to Use Class 4, even if it is not in direct contact with the ground. A lot of people think only posts in the ground need UC4, but any exterior structural timber that gets repeatedly wet should be UC4.

Timber Decking Frames

Timber is still the most common subframe material in the UK and for good reason. It is readily available, economical, everyone knows how to work with it and it is easy to cut and adjust on site. Low initial cost, easy to fix boards to, flexible for steps and odd shapes, and when it is correctly specified it has good structural capacity.

For a normal residential garden deck, properly specified treated timber is perfectly reasonable. The issue is not timber itself, it is badly specified timber. What grade it is, what treatment, what section size — all of it matters. Depth makes a huge difference to how stiff it feels. Joist spacing and how far those joists have to span between supports matters just as much.

Decay needs moisture. Where we see timber frames fail early, it is almost always where water sits — on top of joists under boards, around beam connections, at posts, at cut ends, where boards meet joists, or where there is no ventilation. It can look dry on top while underneath stays damp for days. That is why treatment, drainage and airflow are so important. And when you cut treated timber, you expose untreated core, so those cut ends need proper end-grain preservative. Joist tape on top of joists helps stop water sitting in screw holes, but it doesn't turn unsuitable timber into suitable timber. It is just one detail in a proper system.

Galvanised Steel Decking Frames

Steel really comes into its own when the deck is raised, heavily loaded, has curves or needs to span further. At Decking Gurus we use galvanised steel frames where the job calls for it, and for raised, curved, multi-level and more demanding projects we measure, cut, fabricate/weld and assemble galvanised steel subframes on site. That way we can build around the real building, not try to make the garden fit a standard rectangle.

You would look at steel where the deck is high, you need long clear spans, you have limited places to put supports, you need a really stiff feel underfoot, you expect heavy loads or you want a fully welded frame. Just saying "it's steel" doesn't tell you if it is strong enough — it depends on width, depth, wall thickness, steel grade, orientation, span, restraint and connections.

And strength isn't the only thing. A member can be strong enough not to fail but still flex so much it feels horrible to walk on. So we look at both — will it safely carry the load, and how much will it actually deflect in normal use. For decking, how it feels is just as important as ultimate strength. Rectangular hollow sections are really efficient when you put the deeper dimension vertical. Square sections are handy for posts. Which you use depends on design.

Mild steel outside needs protection, and for long-term external structures galvanised steel gives you that zinc coating for durability. If you then cut, drill or weld it, you break that coating locally and those areas need treating properly. And welding galvanised steel needs proper precautions — you get zinc oxide fume, so ventilation, preparation and proper PPE are essential.

The other advantage with fabricated steel is you can build everything in — raised platforms, steps, cantilevers where engineered properly, access hatches, curved edges, supports for seating or balustrades. For big raised decks where you want to keep the area below usable, a well-designed steel frame can span further while staying stiff, although span always depends on section, grade, supports and loading.

How Long Does Each Frame Last?

There is no single number for how long timber or steel lasts — it depends on treatment, detailing, moisture, ventilation, workmanship and how it is looked after. A timber frame that sits in trapped water won't last, no matter what it says on the stamp. A steel frame where cut edges and welds are left bare will start to corrode.

A correctly specified and detailed galvanised steel frame that is protected at cuts and welds can give a very long service life. Properly treated structural timber treated to Use Class 4, well ventilated and well detailed, can also give many years of service as part of a complete system. It is the detailing that makes the difference, not just the material name.

What About Recycled-Plastic Systems?

Depending on the job we can also discuss recycled-plastic systems and components. Products like Millboard Plas-Pro are manufactured from 100% recycled plastic and are marketed as a non-rotting alternative to timber in damp locations like roof terraces and low-profile areas.

But plastic shouldn't be treated as structurally the same as timber or steel. Millboard themselves say the spanning capability is around half that of timber and point you to their loading tables. So joist spacing has to be based on the manufacturer's guidance for that exact system, not just copied from timber.

Joist Centres vs Joist Span – People Mix These Up

This is one of the most common points of confusion. Joist centres is how far apart the joists are under the boards. Joist span is how far each joist has to stretch between beams or supports underneath. They are two different things.

You can have joists at the right centres for the board but spanning too far between supports, so the whole thing bounces. Whether that span is okay depends on material, section, loading and supports. The board manufacturer tells you how often the board needs support. A stronger steel joist might let you span further underneath, but the board on top still needs its required centres.

Frames Around Balustrades and Kitchens

Balustrades put a lot of load on the edge of a deck. You can't just screw posts through the boards and hope for the best. The frame underneath needs extra blocking, doubled members, steel plates or proper brackets to take that load. Same with outdoor kitchens, big planters, built-in seating, pergolas and hot tubs. Stone worktops and steel cabinets weigh a lot more than garden furniture. If you are planning those features, the frame needs to be designed for them from the start, not added as an afterthought.

Which Frame Is Best?

For a straightforward garden deck with good ventilation and normal heights, properly specified treated timber can be excellent value. For a large raised deck, long-span platform, heavy loads or a complex curved layout, a galvanised steel frame that we measure, cut, fabricate/weld and assemble on site can give you longer spans, a stiffer feel, fewer supports and much more accurate geometry.

There is no material that guarantees longevity on its own. The best frame is the one designed for that specific project, not simply the most expensive one.

Questions to Ask Your Installer

Before you accept a quote, ask what the frame is actually made from, what grade and treatment if timber, what joist centres and what the joist spans are between supports, how the structure is supported, how drainage and ventilation are handled, how cut ends and welds or damaged coatings are protected, how balustrades, steps and heavy features will be supported and whether the frame meets the decking manufacturer's requirements. For a big raised structure, ask if project-specific calculations are needed.

We design from the structure up — ground or roof first, then drainage and support positions, then the frame, then board layout, steps, balustrades, lighting and finishing details. We install timber and galvanised steel substructures depending on what the job needs, plus recycled-plastic components where appropriate, including Millboard, composite, hardwood and specialist detailing.