Joist System - How Composite Decking Substructure Works

What is a joist system?

Before the first composite board appears on a balcony, you need to prepare what goes underneath it - the substructure. A joist system is a grid of profiles (joists) laid on the balcony surface, onto which panels are mounted. It is the foundation of the entire floor - its quality determines the durability, stability, and final appearance.

In traditional construction, joists are associated with wooden beams under a house floor. On a balcony, the principle is the same, but the materials and installation techniques are completely different. A balcony is an outdoor environment: rain, frost, sun, wind. The substructure must withstand all of it.

In this article, we explain how the joist system for composite decking works, what types of joists are available, why spacing matters, and how BalkonSetup handles installation - without drilling into the floor surface.

Types of joists - aluminium, composite, or wood?

There are three main types of joists used under composite decking on the market. Each has its own advantages and limitations.

Wooden joists (pressure-treated)

The cheapest option, but also the shortest-lived. Pressure-treated wood lasts 5 - 8 years in outdoor conditions, after which it begins to warp, crack, and lose its load-bearing capacity. On a balcony, where moisture accumulates under the panels, degradation occurs faster than on a ground-level terrace.

• Pros: low price, easy to work with
• Cons: rotting, deformation, need for replacement after a few years

Composite joists (WPC)

Made from the same material as the decking boards - a blend of wood and polymer. They do not rot, crack, or require maintenance. Their durability is comparable to the panels themselves (15 - 25 years). At BalkonSetup, we use composite joists as the standard substructure element.

• Pros: moisture resistance, no rotting, dimensional stability
• Cons: higher cost than wood, heavier weight

Aluminium joists

The most durable and lightest option. Aluminium does not corrode (it forms an oxide layer), does not expand thermally as much as composite, and is the ideal choice where minimal weight matters - for example, on older balconies with limited slab load capacity. At BalkonSetup, we offer aluminium joists as a premium option.

• Pros: longest lifespan, lightweight, zero water absorption
• Cons: highest cost

Joist spacing - why 30 cm is the standard

Joist spacing is the distance between their centres. It is one of the most important substructure parameters because it affects:

• Load capacity: the smaller the spacing, the higher the floor's load capacity per square metre
• Panel stability: too large a spacing = boards sagging under weight
• Material usage: smaller spacing = more joists = higher cost

For composite decking, the optimal spacing is 30 cm (measured centre to centre). This provides sufficient support without excessive material use. For balconies with unusual shapes (e.g. L-shaped), each zone can have a different spacing adjusted to conditions.

Mounting tape instead of screws and anchors

Traditional joist installation requires drilling into the balcony floor - expansion plugs, screws, chemical anchors. On a balcony, this is problematic for several reasons:

• Waterproofing damage - every hole in the floor surface is a potential leak point
• Noise - a hammer drill in an apartment building is a problem for neighbours
• Irreversibility - holes in the floor surface remain forever
• Building management restrictions - many housing associations prohibit interference with balcony structures

The solution? Specialised mounting tape. In the BalkonSetup system, joists are laid on industrial double-sided tape that provides adhesion to the surface without drilling. This is not ordinary office tape - it is a professional product with adhesion strength of several dozen kilograms per linear metre.

How does it work in practice?

1. The balcony surface is cleaned and degreased
2. Mounting tape is applied to the bottom of the joist
3. The joist is pressed against the floor surface - the tape creates a durable bond
4. The weight of the entire floor (panels + joists + clips) additionally presses the joists against the surface

The result: stable installation, zero holes, possibility of disassembly in the future.

Mounting clips - invisible board connections

Composite panels are not screwed to the joists. Instead, mounting clips are used - elements that hold the boards in place while allowing for their thermal movement (expansion and contraction).

In the BalkonSetup system, we use two types of clips:

• Starter clips (metal) - installed in the first row of panels, they lock the starting edge of the board on each joist
• Joining clips - installed between consecutive rows of boards, they hold the edge of one board and the beginning of the next simultaneously. This is the most numerous element in the entire system

Joining clips are our proprietary product - designed and tested by the BalkonSetup R&D department. They underwent multi-stage durability testing: static loads, thermal cycles, and simulation of years of use. The result is an element that reliably holds boards for years while allowing their natural thermal movement.

The clip system eliminates visible connections - screws, nails, or bolts. The floor surface is smooth and uniform, and each panel can be removed if needed without damaging adjacent boards.

Under-floor ventilation - why it matters

Joists lying on the balcony surface create a natural ventilation space under the panels. Rainwater flows through the gaps between boards and is then drained under the substructure towards the drain. At the same time, air circulates freely beneath the panels, preventing moisture build-up.

Why is ventilation under the boards so important?

• Moisture drainage: rainwater that gets under the panels can freely evaporate thanks to air circulation
• Mould prevention: standing water and lack of air create the perfect environment for fungi and mould
• Faster drying: after rain, a floor with ventilation dries significantly faster than one lying directly on the surface
• Waterproofing protection: no water stagnation = lower risk of damage to the waterproofing layer underneath

In the case of balconies with a slope (which is standard), water naturally flows towards the drain, and the remainder dries thanks to air movement beneath the panels.

Thermal expansion - gaps that protect the floor

Every material changes its dimensions with temperature. Composite boards are no exception - on a hot summer day, a 240 cm panel can expand by as much as 2 - 4 mm. That is not much, but if the floor has no room for this "movement", it will start to buckle or crack.

That is why the joist system includes expansion gaps:

• Between panels and walls: 5 - 10 mm clearance around the perimeter of the balcony
• Between panels in a row: clips automatically maintain a gap of approximately 5 mm between boards
• Between zones (L-shaped balcony): an additional expansion gap at the bending point

Mounting clips are designed so that they do not block the thermal movement of the panels. A board can freely expand and contract while the clip holds it in place without rigidly locking it. This is one of the reasons the clip system is better than screws - a screw blocks thermal movement, leading to cracks.

Load distribution - how joists spread the weight

A balcony has a limited load capacity - a typical balcony slab supports 200 - 300 kg/m2. A composite floor on joists weighs approximately 15 - 25 kg/m2, which is a small fraction of the allowable load.

The role of joists in load distribution:

• Uniformity: the weight of furniture, plant pots, or people is distributed across the entire joist surface rather than at single points
• Linear support: each panel rests on several joists simultaneously - no overload points
• Flexibility: the joist + clip + panel system creates a "floating floor" that transfers dynamic loads (walking, moving furniture) without damage

For people living in older buildings where balcony load capacity may be lower, we offer aluminium joists - they are significantly lighter than composite ones.

DIY or professional installation?

The joist system with tape and clips is simple enough that DIY installation is entirely feasible. Many people successfully lay composite flooring on their balcony themselves - especially on rectangular balconies with a small area.

What to watch out for with DIY installation?

• Even joist spacing: a difference of even 1 cm can cause panels to miss the joists
• Levelling: joists should lie in one plane, otherwise the floor will "wave"
• Cutting precision: with unusual shapes (L, angles), each panel must be cut precisely to the outline
• Expansion gaps: do not forget them - in summer, missing gaps = buckled panels

How does BalkonSetup do it?

1. LiDAR measurement - a precise scan of the balcony with millimetre accuracy
2. Computer simulation - joist and panel layout in a dedicated simulator (waste optimisation, raw length selection)
3. CNC cutting - every joist and panel is machine-cut, not hand-cut
4. Installation with level control - joists are laid using a laser level

Regardless of whether you choose DIY installation or commission professionals - a well-designed substructure is the key to a durable and aesthetically pleasing floor.

What does a finished substructure look like?

A completed joist system on a balcony is a grid of parallel profiles attached with tape to the floor surface. From above, it looks like "scaffolding" waiting for panels. A few characteristics of a properly laid substructure:

• Joists run perpendicular to the direction of the panels
• Spacing is uniform across the entire surface (accurate to within 2 - 3 mm)
• Edge joists are shifted half their thickness inward (edge margin)
• An expansion gap is left at the walls
• Joists are levelled - the height difference between adjacent profiles does not exceed 1 mm

Once the panels are laid, the substructure is invisible. The only things that reveal its presence are the gaps between panels (approx. 5 mm) and a ventilation gap at the edge of the balcony - both are intentional and correct.

Summary

The joist system is an invisible but critical element of every composite floor on a balcony. From the choice of joist material (composite or aluminium), through spacing (optimally 30 cm), the installation method (tape instead of screws), the clip system, to expansion gaps and ventilation - every detail matters.

At BalkonSetup, each of these elements is calculated by computer, machine-cut, and installed with precision. The result is a floor that not only looks great but functions properly for years - regardless of weather and temperature.