A specification guide aluminium slat fencing

Aluminium Slat Fencing: A Specification Guide to Gap Spacing, Span and Finish

19 October 2026

The horizontal slat screen has become one of the most widely specified boundary treatments in contemporary residential architecture. It appears on project pages from Melbourne to Rotterdam, usually photographed on the day of handover, with the slats crisp and the shadow lines even. What those photographs rarely show is the same screen four summers later, when the top rail has bowed, the powder coat has chalked on the north face, and the gaps that were meant to read as a rhythm have gone slightly irregular because nobody allowed for thermal movement.

None of that is a failure of the material. Aluminium slat screening is an unusually forgiving system when it is specified properly. It is light, it does not corrode in the way steel does, it can be finished in almost any colour, and it can be fabricated to tolerances that make a 20-metre boundary read as a single gesture. The failures come from a handful of decisions made early: slat geometry, span, fixing method and finish grade. All four are easy to treat as contractor’s choice and expensive to revisit.

What follows is the set of decisions worth making deliberately.

How to Calculate Privacy From Slat Width and Gap Spacing

The most common specification error is choosing the gap by eye. A 20mm gap looks generous on a sample board held at arm’s length. On a boundary, viewed obliquely from a neighbour’s upper window, that same gap can disappear entirely, or stay wide open, depending on geometry nobody checked.

The useful way to think about it is the sightline angle. A slat screen is opaque at any viewing angle steeper than the ratio of slat depth to gap. A 65mm-wide slat with a 50mm face depth and a 20mm gap blocks everything viewed at more than about 22 degrees off perpendicular. Stand square to it and you see through; walk past it and it closes. That is why slat screens feel private when you are moving and oddly transparent when you stop.

So the question is not “how much privacy do I want” but “privacy from where”. Screening a courtyard from a two-storey neighbour means the critical sightline comes from above, which favours a deeper slat profile or a narrower gap. Screening a street-facing frontage from passing pedestrians means the critical sightline is roughly horizontal, which allows a far more generous gap and a lighter screen.

Recommended Gap Spacing by Privacy Level

How to Calculate Privacy From Slat Width and Gap Spacing

Practical starting points:

Full privacy, any angle: gap of 10mm or less, or a lapped profile with no straight-through path. At this point you have a fence that happens to have slat articulation, not a screen.
Standing privacy, oblique transparency: 15 to 20mm gap with a 50mm-plus slat depth. The most common residential compromise and usually the right one.
Articulation and airflow, limited privacy: 30 to 40mm gap. Works for pool surrounds, service screens and anywhere the aesthetic matters more than the concealment.

Making the Slat Module Resolve

The second geometric decision is modularity. Slat and gap together form a module, and that module should divide cleanly into the fence height you have actually got, not the height on the drawing. A 1.8m screen at a 65mm slat and 20mm gap gives 21.2 modules, meaning a 13mm orphan course at the top or bottom. Either adjust the gap to 19mm, or decide in advance where the irregular course sits and make it deliberate. Put it at the bottom, behind planting, every time.

Horizontal vs Vertical Slat Fencing: Which to Specify

Horizontal slats dominate current practice because they visually lengthen a boundary and sit comfortably with the long low massing of contemporary houses. They also collect debris on every slat face, shed water onto the slat below rather than away from the screen, and, when used as a pool barrier, present a ladder.

Vertical slats drain, self-clean in rain, and avoid the climbability problem almost entirely. They read as more formal and they shorten a boundary visually, which is sometimes exactly what a narrow site needs. On sloping ground they also handle level changes far more gracefully: vertical slats can be raked or stepped with the fall, where horizontal slats must step, producing the staircase profile that looks deliberate in a render and clumsy in reality.

If the screen is doing a safety job as well as a visual one, the orientation decision may be made for you. Australian pool barrier requirements under AS 1926.1 treat closely spaced horizontal members as climbable, and a barrier with horizontal elements less than 900mm apart faces a substantially higher minimum height as a result. European and North American codes handle this differently but most address climbability in some form. Check the barrier code before the aesthetic is locked, not after.

Post Spacing, Span and Deflection in Slat Screens

The single most visible long-term failure of slat screening is deflection, the slight sag or bow that makes a run look tired even when nothing has actually broken. It is a function of three things: slat thickness, post spacing, and wind.

Aluminium slats are typically roll-formed or extruded in wall thicknesses from around 1.0mm up to 2.0mm. The thin end of that range is perfectly adequate at 2.4m post centres and marginal at 3.0m, particularly on a solid or near-solid screen where wind load is highest. Every reduction in gap increases the load the screen has to carry, which is the trade-off nobody mentions: the more privacy you specify, the stiffer the structure has to be. A screen at a 10mm gap is close to a solid wall as far as wind is concerned.

Reasonable defaults for a 1.8m residential screen: 2.4m maximum post centres, 1.4mm minimum slat wall thickness, and a stiffened or doubled top course on any run over about 6 metres. Exposed sites, and anything above 2.1m in height, warrant engineering input rather than a rule of thumb. The cost of an extra post every 2.4 metres is trivial next to the cost of re-straightening a 15-metre boundary.

Powder Coat Finish Grades for Exterior Aluminium

Aluminium does not rust, which leads to the assumption that the finish is cosmetic. It is not. The powder coat is the entire weathering system, and the difference between finish grades is the difference between a colour that holds for fifteen years and one that chalks visibly in four.

The standards worth naming in a specification differ by market. AAMA 2603, 2604 and 2605 apply in North America, Qualicoat Class 1, 2 and 3 in Europe, and the Dulux Duratec and similar super-durable ranges in Australia. The hierarchy is the same everywhere. The entry grade is an interior-and-sheltered product. The mid grade is appropriate for most residential exterior work. The top grade, built on fluoropolymer or super-durable polyester chemistry, is what coastal and high-UV sites actually need.

Three finish decisions that repay attention:

Specify the grade, not the colour. A colour name with no standard attached will be supplied in whatever grade is cheapest that week.
Darker colours work harder. A dark slat on a west-facing boundary in a high-UV climate runs hot, which accelerates both coating breakdown and thermal movement. It is still the right choice aesthetically much of the time, just specify up a grade.
Woodgrain finishes are a sublimation process over powder coat, and the grain layer weathers on its own schedule. They are convincing and they have improved enormously, but they are not a like-for-like substitute for the plain colour in terms of longevity.

Thermal Movement in Aluminium Slat Runs

Aluminium expands roughly 23 micrometres per metre per degree Celsius. On a 6-metre slat run through a 40-degree annual temperature swing, entirely ordinary in a continental or Australian climate, that is about 5.5mm of movement.

Five millimetres is enough to buckle a slat that has been hard-fixed at both ends, pop a concealed fixing, or open a visible gap at a junction. The remedy is not complicated: fix one end, allow the other to float, and detail junctions and corners with a movement allowance rather than butting them tight. But it has to be drawn. Left to site, every fixing gets screwed home and the screen fights itself for the next decade.

This is also an argument for concealed fixing systems over face-screwing. Concealed fixings, slats clipped or slotted into a channel within the post, hide the fastener, allow a degree of slip, and avoid the row of screw heads that dates a screen faster than anything else. They cost more in fabrication and they are considerably less forgiving of out-of-plumb posts, which means the groundwork has to be right.

Sourcing Aluminium Slat Fencing: Kit Systems vs Fabrication

Slat screening sits in an awkward procurement space. It is sold as a kit system by fencing suppliers, fabricated to drawings by metalworkers, and increasingly offered as a hybrid where a standard system is modified on site. The three routes produce visibly different results, and the drawn detail should match the route.

Kit systems are fast, cost-predictable and dimensionally rigid: you get the manufacturer’s slat profiles, their post centres and their gap options. Custom fabrication gives complete control of profile, gap and junction detailing at a significant cost premium and a longer lead time. The hybrid route, a standard extrusion with custom spacing, raking or junctions, is where most good residential work actually lands, and it depends entirely on whether the installer can fabricate as well as assemble.

That capability varies enormously by market. In Perth, Western Australia, where high UV, sandy ground conditions and a long coastline make both finish grade and footing design unusually consequential, local specialists such as FencrGatr handle aluminium slat fencing as a fabrication job rather than a kit assembly, which is what makes custom gap spacing and raked runs on sloping blocks viable at residential budgets. The broader point travels: ask prospective installers whether they cut and finish in-house before drawing anything that depends on it.

Footings and Ground Conditions for Slat Fence Posts

A slat screen is only as straight as its posts, and posts are only as true as their footings. The failure sequence is almost always the same: an undersized footing in reactive or sandy soil, a post that moves a few degrees over two wet seasons, and a screen that now reads as visibly out of plumb even though every slat is intact.

Minimum footing depth should be driven by soil classification and post height, not by habit. A 1.8m screen in stable ground might be adequately footed at 600mm; the same screen in sand, in reactive clay, or at 2.1m-plus height needs considerably more, and may need a wider diameter rather than simply a deeper hole. Where a screen sits above a retaining structure, the retaining design has to account for the fence’s wind load, a detail routinely missed, and one that transfers a surprising moment into the top of the wall.

Five Slat Screen Failure Modes to Design Out

The orphan course. Slat module that does not divide into the actual height, resolved on site by whoever is holding the drill.
The hard-fixed run. No movement allowance, leading to buckle or fixing failure within a few seasons.
Under-specified finish on a hot elevation. Chalking and colour drift on the west and north faces while the rest holds.
Gap chosen for the sample board, not the sightline. Privacy that evaporates at the one angle that mattered.
Post centres carried over from an open screen to a near-solid one. The same spacing that worked at a 40mm gap deflects visibly at 10mm.

Key Takeaways for Specifying Slat Screens

Slat screening rewards precision in a way few boundary treatments do. The system is simple enough that every decision shows: a 2mm inconsistency in gap is legible from the street, and a screw line is legible forever. That sensitivity is exactly why it has become the default architectural boundary, and exactly why it is worth drawing properly rather than scheduling as “slat fence, colour TBC”.

Get the geometry from the sightline, the span from the wind load, the finish from the standard rather than the swatch, and the movement allowance onto the drawing. The screen will then do what the handover photograph promised, four summers later.

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