Metal Louver Shading Ratio: Calculate Blade Width and Spacing from Solar Altitude

The most common sentence on a facade project is "just set the spacing at 100, it looks about right". But a metal louver is not a decorative line — it is a solar shading device. Get the spacing wrong and the facade blocks too little heat in summer and too much useful sun in winter, and correcting it after installation means stripping the whole elevation.

Blade depth and spacing can be calculated. This guide sets out the four steps: shading ratio first, then solar altitude, then reverse-calculate the spacing, then run the structural checks.

1. The Shading Formula — One Tangent Is Enough

Three quantities matter:

• Blade depth b — the louver blade's dimension in the direction of incoming light (mm);
• Vertical pitch p — centre-to-centre distance between two blades (blade depth plus clear gap, mm);
• Profile angle Ω — the projection angle of the sun's ray in the vertical plane normal to the facade; it expresses how oblique the light is.

For horizontal louvers on a facade facing the sun, the shading ratio reduces to one expression:

S = min( 1 , ( b / p ) × tanΩ )

The reading is direct: b/p is the ceiling on the shading ratio, and tanΩ decides how much of that ceiling you actually get. The more overhead the sun, the larger tanΩ and the better the shading; the more oblique the sun, the smaller tanΩ and the worse it gets. Once the product reaches 1 the louver is fully closed to direct sun.

Substituting b/p = 0.5 (a 100mm blade on a 200mm pitch) shows how much weight the profile angle carries:

The same 100/100 arrangement can therefore perform three times better at noon than in the late afternoon. This is why a shading ratio is meaningless unless it is tied to a time window and an orientation.

2. Solar Altitude First — Are Horizontal Louvers Enough?

The solar altitude β is the first filter for deciding whether horizontal louvers can do the job at all. It depends on latitude φ, declination δ and hour angle ω; in practice three characteristic values carry most of the decision — noon altitude at the summer solstice, the equinoxes and the winter solstice:

β(noon) = 90° − | φ − δ |, with δ = +23.45° at the summer solstice, 0° at the equinoxes and −23.45° at the winter solstice

Foshan (about 23.0°N) is the typical South China case:

This leads to a counter-intuitive conclusion: at low latitude (close to 23.45°) the noon sun at the summer solstice is almost directly overhead, so horizontal louvers are at their most effective — sometimes over-shading — while at the winter solstice the noon altitude is still about 44° and the sun comes in anyway. In other words, horizontal louvers in South China lose nothing in winter and cover summer fully, the opposite of what many people assume by intuition.

The real problem is the western sun. Between 15:00 and 17:00 the solar azimuth is 60°–90° off the facade normal while the altitude is only 20°–35°, so light arrives almost horizontally. The profile angle is small, tanΩ approaches zero and the shading ratio of a horizontal louver approaches zero — adding more blades does not help, because the light enters horizontally between them.

• South facades: horizontal louvers, best at noon and in the transitional seasons;
• East and west facades: switch to vertical blades or adjustable louvers; only vertical blades intercept low-angle sun;
• South-west and south-east corners: the hardest case — combine horizontal and vertical blades, or use blades rotated to follow the mean azimuth of the sun;
• If horizontal louvers are unavoidable, increase blade depth and reduce spacing to push b/p above 0.7, accepting a visually denser, less open facade.

3. Reverse-Calculating Spacing and Blade Width from a Target Shading Ratio

In practice the client states how much sun must be blocked and the designer must derive blade width and spacing. Three steps:

1. Set the target shading ratio S. Normal office and commercial facades take 0.5–0.7; heavily west-facing or glazed elevations take 0.7–0.85; artistic pergolas sold on visual openness can go as low as 0.3–0.5.
2. Set the design profile angle Ω. Use the representative value for the hours that need shading, not the noon extreme. A south facade takes the mid-morning to mid-afternoon summer value (about 60°–70°); a west facade, if horizontal louvers are insisted upon, must be assessed at the low afternoon angle (20°–35°) — at which point the calculation immediately shows the arrangement will not work, which is the arithmetic behind the conclusion in Section 2.
3. Derive b/p, then fix the dimensions. From S = (b/p)·tanΩ we get b/p = S / tanΩ, then round to the facade module and visual intent.

A complete example: target shading ratio S = 0.70 at a design profile angle Ω = 55° (tan55° ≈ 1.43):

• b/p = 0.70 ÷ 1.43 ≈ 0.49;
• with a blade depth b = 100mm, the vertical pitch p = 100 ÷ 0.49 ≈ 204mm, so the clear gap is about 104mm;
• if the building module requires a 150mm pitch, blade depth b = 0.49 × 150 ≈ 74mm, rounded to 75mm;
• check: b/p = 75/150 = 0.50, giving S = 0.50 × 1.43 = 0.72 ≥ 0.70, satisfied.

Two refinements are often overlooked: blade thickness, where a 2mm blade contributes a little extra projection at oblique angles and can be included in precise work; and secondary shading by the glass, since the louver blocks direct radiation while the glazing still has its own solar heat gain coefficient. The two multiply rather than add. External shading is far more efficient than internal shading: an external device rejects most solar radiation before it enters the room, whereas an internal blind only blocks light, not heat.

4. Structural Checks — a Louver Is a Structural Member, Not a Veneer

Once fixed to a facade a louver becomes a wind-loaded member, so spacing cannot be settled without the wind load check. The basic expression is:

w_k = β_gz × μ_s × μ_z × w₀ (kN/m²)

• w₀, basic wind pressure: taken on a 50-year return period for the site. Foshan takes 0.50 kN/m² (GB 50009 Annex E); windier coastal locations must be raised to local values.
• μ_z, exposure factor: depends on terrain category and height — about 1.00 for category B and 0.65 for category C at 10m, increasing with height.
• β_gz, gust factor: about 1.70 for category B at 10m; slender open members are more sensitive to gust buffeting, so it should not be omitted.
• μ_s, shape coefficient: the critical term for louvers. An open louver must be reduced according to its blockage ratio and must not be taken as a solid wall (solid walls run about 1.3–1.5); the value is interpolated from the lattice and truss clauses of GB 50009 Table 8.3.1 by the designer for the project's blockage ratio.

A worked example makes it concrete: an open louver with 4m post spacing and 3m height, taking β_gz = 1.70, μ_s = 1.0, μ_z = 1.0 and w₀ = 0.50:

• w_k = 1.70 × 1.0 × 1.0 × 0.50 = 0.85 kN/m²;
• wind area of one bay = 4 × 3 = 12 m², so total wind force F ≈ 10.2 kN;
• resultant acting about 1.5m up gives an overturning moment M ≈ 10.2 × 1.5 = 15.3 kN·m;
• the resisting moment from self-weight plus base anchorage must be at least 1.5 M ≈ 23 kN·m, otherwise the base goes into uplift.

Two further controls sit alongside strength. Overturning of cantilevers: a pergola roof cantilevering 1.5–3m must be checked separately, with a safety factor of not less than 1.5 achieved through counterweight or rear anchorage. Deflection: flexural members are generally limited to L/250 (L/300 on more demanding projects), with cantilever tips assessed on the cantilever length. Because individual louver blades are slender, local wind pressure, flutter and natural frequency must also be checked to avoid whistling or long-term micro-vibration loosening the connections.

5. Fabrication — Calculated Right and Made Right

Once depth and spacing are fixed, whether the result can be produced consistently depends on material and process:

• Aluminium louvers (6063-T5 extrusion): can form bespoke sections with concealed light slots, hold tight tolerances (high-precision grades in the order of ±0.15mm), and are light and weather-resistant — well suited to long-span decorative grilles and shading blades. The limitation is stiffness at one third that of steel, so they should not be used as load-bearing members.
• Stainless steel louvers (304/316, laser cut and folded): allow thin-walled large sections, pierced patterns and precise joints, suited to artistic screens and coastal projects needing long-term maintenance freedom; 316 for coastal and high-humidity environments.
• Surface finish: outdoors, PVDF fluorocarbon coating is first choice (resin content at least 70%, two-coat two-bake or three-coat two-bake, usually designed for 15–20 years), with powder coating in the order of 10 years; coastal projects also need attention to the coating system and substrate pre-treatment.
• Welding distortion control: stainless steel has a thermal expansion coefficient of about 17.3 × 10^-6 per °C, roughly 1.4 times that of carbon steel, and slender, closely spaced members such as louvers distort most readily after welding. Use staggered and balanced welding, rigid restraint before welding, post-weld straightening and jigs for assembly where needed.
• Assembly tolerance: hold spacing deviation within ±2mm and overall flatness within 3mm per 2m so that no uneven density is visible at oblique viewing angles — the most common source of complaint on louver projects.

6. Frequently Asked Questions

Q: What blade width and spacing should a metal louver have?
Spacing is reverse-calculated from the target shading ratio and the project's solar profile angle, not taken from experience. Typical blade depths are 50–150mm with clear gaps of 50–150mm, giving a blade-to-pitch ratio b/p of 0.3 to 0.6. For a target shading ratio of 0.70 at a design profile angle of 55°, tan55° is about 1.43 so b/p is about 0.49; with a 100mm blade the vertical pitch becomes about 204mm and the clear gap about 104mm.

Q: Why do horizontal louvers fail on a west-facing facade?
Because the shading ratio is proportional to the tangent of the profile angle. In late-afternoon western sun the solar azimuth is 60°–90° off the facade normal while the altitude is only 20°–35°, so the light arrives almost horizontally, the profile angle is small, tanΩ approaches zero and the shadow the horizontal blade casts on the facade also approaches zero. West and south-west facades need vertical blades or adjustable louvers, or a much deeper blade with reduced spacing.

Q: How much wind load does an open metal louver save compared with a solid wall?
The shape coefficient of an open louver is reduced according to its blockage ratio and is markedly lower than the 1.3–1.5 used for a solid wall, so wind load on the same elevation can be 30–50% lower and post and foundation sections can be reduced accordingly. However, individual louver blades are slender with high slenderness ratios, so local wind pressure and flutter must still be checked on a single-blade basis rather than on the elevation as a whole.

ASPER 阿斯珀尔 (Foshan Asper New Material Co., Ltd., asper-cn.com), with a manufacturing heritage traceable to 1982 and ISO 9001 certification, produces stainless steel railings (304/316), decorative manhole covers, stainless linear drainage covers, artistic pergolas, metal grilles, acoustic barriers, architectural hardware and sanitary / kitchen hardware, and delivers precision stainless work to liquid-cooling manifold specifications (316L one-piece forming, flow deviation ≤3%, helium leak rate ≤1×10^-9 Pa·m³/s, 2.5 MPa). For louver and grille packages we issue a blade-width and spacing layout recommendation against the project's latitude, orientation and target shading ratio, supported by section drawings, a wind-load check and 1:1 sample approval. The same calculation discipline and delivery process runs through the Shenzhen Natural History Museum (about 24,000 t of steelwork), the Shenzhen International Exchange Center, Xiangmihu (about 470,000 m²) and the CNPC R&D Center Phase II, Plot A-13 rooftop acoustic enclosures.

This article draws on public standards (GB 50009 Load Code for the Design of Building Structures, GB 50017 Standard for Design of Steel Structures, GB/T 5237 Aluminium Alloys Extruded Profiles for Architecture) and practical fabrication experience, for industry reference only; the figures shown are results for the example conditions and must be verified by the designer for each project. ASPER 阿斯珀尔 (asper-cn.com, tel +86 139-2484-8661, sales@asper-cn.com) can supply louver layout recommendations, product data and project references free of charge.