In a traditional post-and-beam structure, you can sometimes plane down protruding spots or stiffen the twist. However, in a raised pedestal system, where each joist rests directly on an adjustable head, this kind of movement causes immediate chaos. The pedestal does not move, but the wood lifts out of its seat, creating a hollow springing sound, uneven board lines, and irritating squeaking.
Choosing the material for joists is one of the most important construction decisions you will make for a raised deck terrace. Here is how aluminum and wood really compare when paired with adjustable pedestals.
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The role of the joist in a pedestal system
On raised deck terraces at ground level or of low profile, adjustable pedestals replace poured concrete footings and vertical timber posts. Instead of burying timber in the ground or screwing it to wall plates, a grid of pedestals is laid directly on a concrete slab, compacted gravel base, or roof waterproofing.
The joist rests directly in the seats of the pedestal heads. It has three specific tasks:
- To span the clear gap between pedestal heads without excessive deflection.
- To carry the deck boards and transfer live loads (furniture, foot traffic, snow) down to the support pads.
- To remain flat and stable through years of seasonal temperature fluctuations and moisture cycles.
Because the pedestals are individual point supports, the joist acts as the structural backbone keeping the entire assembly vertical. When designing our pedestal heads, we create joist holders to securely cradle structural rails. However, if the joist warps, twists, or rots, the millimetre-precise levelling you set in your pedestal system is lost.
Aluminium joists: precision, span and low maintenance
Aluminium substructures were once reserved for commercial rooftop terraces and urban plazas. In recent years, they have become the standard choice for residential raised deck terraces, especially on waterproof membranes or around swimming pools.
Clear advantages
- Perfectly straight, every time: Extruded aluminium does not bend, twist, bow or sag. Once you set your line of pedestals with a laser level and drop in the aluminium rail, your surface is flat on day one and stays flat ten years later.
- Exceptional rigidity and spanning capability: Aluminium profiles are engineered with internal ribs and closed-section geometry. A 50 mm (2 inch) or 80 mm (3 inch) deep aluminium joist can often span significantly greater distances between pedestals than a timber joist of comparable depth. As a pedestal manufacturer, we regularly review site layout drawings where switching to a rigid aluminium profile allows contractors to increase pedestal spacing from 400 mm to 800 mm or more. This reduces the number of pedestals required across the entire project area by 20% to 30%.
- Lightweight handling and height adjustment: Hauling wet, pressure-treated 2x8 beams up three flights of scaffolding onto a roof terrace is brutal work. Aluminium profiles weigh a fraction of what wet pressure-treated timber does, making installations on roofs and elevated balconies far safer and faster. The lightweight, rigid profile also makes fine adjustment of slope correction and pedestal height much easier, because you are not fighting the enormous weight of wet timber while turning threads.
- Resistance to moisture and rot: Water pooling in the joist pocket will eventually destroy timber. Aluminium does not absorb moisture, support mould growth or suffer insect damage. In coastal environments or over damp substrates with poor airflow, it is practically eternal.
Disadvantages
- Higher material cost: Aluminum profiles cost significantly more per linear meter than standard pressure-treated pine.
- Tools and fasteners: Aluminum cannot be cut with a standard wood blade; you need a carbide blade for non-ferrous metals. Fasteners must also be carefully selected: driving standard carbon steel screws into aluminum causes galvanic corrosion, so compatible stainless steel screws or dedicated groove-profile fasteners must be used.
Timber joists: economy, familiarity and natural durability
Pressure-treated timber remains the most commonly used deck substructure in the world, and not without reason. It is familiar, readily available at any local timber yard and forgiving to work with.
Clear advantages
- Budget-friendly: If you are building a simple terrace where initial cash flow is your main constraint, treated softwood (such as pressure-treated pine or Douglas fir) keeps material costs low.
- Every carpenter knows it: No specialized blades, no proprietary clamp connectors. Every construction carpenter knows how to make cuts, trim, reinforce, and screw wooden joists without reaching for a technical system manual.
- Solid screw holding: Wood provides a dense, solid seat for standard composite and wood decking screws. You don't have to worry about the threads of self-drilling screws stripping out in thin-walled metal sections.
Disadvantages
- Moisture-related movement: Wood constantly absorbs and releases moisture. Even kiln-dried and pressure-treated wood will move with the changing seasons. When a timber joist bows upward on top of the pedestal, it rocks. When it bows downward, it loses contact with the pedestal head.
- Shorter spans, more pedestals: A standard timber joist 47x100 mm (2x4) or 47x150 mm (2x6) has greater flexibility over distance than a structural aluminum box beam. To eliminate the trampoline effect underfoot, pedestals must be spaced closer together. When calculating project costs, remember that savings on cheaper wood are often absorbed by the need to purchase additional pedestals to support shorter spans.
- Rot at the pedestal interface: Water readily accumulates where the bottom surface of a timber joist rests in the pedestal cradle. Although we form drainage channels in our pedestal cradles specifically to limit retained water, standard untreated wood contact points will over time remain damp long enough to accelerate fungal rot exactly at the support point, unless protected with joist tape.
Direct comparison
Dimensional stability: Aluminum joists maintain zero deformation, twisting, and bulging over time. Treated wood joists remain susceptible to bending, cracking, and seasonal movement as humidity levels change.
Span depending on profile depth: Aluminum provides high span load capacity because engineered internal ribbing creates exceptional rigidity. Wood offers moderate span load capacity, requiring deeper and heavier profiles to achieve equivalent spans.
Impact on the number of pedestals: Aluminum results in fewer overall pedestals because longer joist spans require fewer support points. Wood requires more pedestals because shorter allowable spans mean supports must be placed closer together.
Weight: Aluminum profiles are very lightweight, making them easier to handle on roofs and elevated balconies. Treated wood is significantly heavier, especially when wet or freshly treated.
Rot and fire resistance: Aluminum offers zero susceptibility to rot and is non-combustible (commonly available Euroclass A1/A2 classifications). Wood is combustible and susceptible to wet and dry rot over time.
Initial cost: Aluminum represents a higher initial material investment. Wood involves a lower initial material cost.
Ease of fastening: Aluminum requires clip channel systems or specialized bimetal self-drilling screws. Wood allows for quick, direct fastening with standard wood and composite screws.
Matching the joist to the task
Neither material wins in every scenario. The right choice depends on the site's exposure, the terrace height, and the structural design.
When wood makes sense
Wood is a sensible, cost-effective choice when you are building a low terrace in a dry, well-ventilated environment on a stable concrete slab. If the terrace has a large clearance above the ground (at least 150 mm to 300 mm above ground level) and good airflow around the perimeter, the wood will dry naturally after rain.
If you use wood, invest in joist tape along the top edges and make sure the wood grade is C24 or structural grade 1. Plan the pedestal layout according to strict span tables instead of guessing, and avoid spans greater than 400 mm to 500 mm with low joists unless calculations confirm the deflection limit.
When aluminium is the better investment
Aluminium is the better choice for:
- Rooftop terraces and balconies: Where dead load limits matter, access is difficult, and fire regulations require non-combustible materials.
- Low-clearance terraces: Where the deck sits just 50 mm to 80 mm above the concrete slab. Moist air accumulates in this tight space. Wood will rot quickly here, while low-profile aluminium box profiles will remain intact.
- Premium hardwood and composite boards: If you are spending $10,000 on high-end composite or Ipe boards that will last 30 years, pairing them with a timber substructure that degrades in 12 years creates a costly mismatch.
Verdict before purchase
Before ordering materials, resist the temptation to price joists in isolation. A cheap timber joist that requires pedestals every 400 mm can easily cost more in total for fixings and labour than a rigid aluminium profile that allows pedestal spacing to be stretched to 800 mm or 1,000 mm.
Download the technical data sheets for both the proposed joists and the adjustable pedestals. Check our pedestal load capacities and head width specifications against the joist profiles, verify the permissible deflection at the expected service load, and calculate the total cost of the substructure as a single system. This ten-minute calculation on paper will save you hours of shimming, adjusting and levelling on site.
