planmero
enLanguage

11 / 13 · Sun & louvres

Sun Angle and Louvre Shading Calculator

Choose a city or enter coordinates, set the date and time and describe your louvres: slat width, spacing and whether they are horizontal, vertical fins or a louvred roof. You get the sun position from the NOAA solar algorithm, the smallest tilt that stops direct sun, the cut-off angle of the current setting and a chart of the whole day.

Your sizes

Time zone: America/New_York (UTC−4)

Louvres

Direction you look when you stand with your back to the wall, in degrees from north (S = 180°).

Fits on one page — choose Landscape if your browser asks.

Drawing

Side section

sun 26°31.3°wall

Tilt needed over the day

0°30°60°90°03691215182124now 18:00
Tilt neededSun gets through at current tilt

Result

Tilt that blocks direct sun
31.3°
Sun elevation
25.7°
Sun azimuth
279.7°
Sun on the louvres
yes
Horizontal shadow angle
9.7°
Profile angle
26°
Open slats block the sun above
43.2°
At the current tilt sun passes between
-43.2° … 43.2°

Direct sun gets through at the current tilt now.

The drawing shows the slats at the needed tilt. Rays that pass a gap are drawn to the wall.

Direct sun only: diffuse and reflected light still get through. Slat thickness is ignored and the section assumes long slats.

Sun position: NOAA Solar Calculator algorithm (Meeus), refraction included.

Geometry only, not a structural design — fixing and wind load follow the louvre manufacturer and local rules.

Build your own rotating louvres

SAM KIT is a kit of rotating mechanisms for wooden or aluminium slats 19–20 × 99–100 mm: one pair turns five slats, tilting by hand from 0° to 140°.

SAM KIT at Flowgarden →

Flowgarden is the shop of the same team behind Planmero.

How it works

The sun position comes from the NOAA Solar Calculator algorithm (equations from Jean Meeus, Astronomical Algorithms): elevation including atmospheric refraction and azimuth clockwise from north. Summer time is taken from the city's time zone; for your own coordinates enter the UTC offset of your clock.

For a wall, the horizontal shadow angle is the sun azimuth minus the direction the wall faces. If it is within ±90° and the sun is up, the sun shines on the wall. The profile (vertical shadow) angle is atan(tan elevation / cos horizontal angle) — the angle horizontal slats and overhangs actually 'see'.

In the section across the slats, parallel rays are stopped completely when the shadows of neighbouring slats touch: width × |sin(slat angle − sun angle)| ≥ spacing × cos(sun angle). The calculator finds the smallest tilt from fully open that meets this, and the range of sun angles that still passes at your current tilt.

An overhang or solid pergola roof shades the wall down to projection × tan(profile angle) below its edge; once that reaches the floor, the floor stays shaded out to projection − height ÷ tan(profile angle) from the wall.

What the calculator assumes

  • Direct sun only. Diffuse sky light and light reflected off the slats or the ground still get through, and slat thickness and curvature are ignored.
  • The section is two-dimensional: slats, overhang and pergola are taken as long compared with their depth. Near their ends the sun gets round the side.
  • The sun position is accurate to a small fraction of a degree for years 1800–2100; terrain, buildings and trees around you are not considered.
  • This is a geometry tool, not a structural design. Fixing, wind load and the louvre system itself must follow the manufacturer's instructions and local rules.

Questions

What is the profile angle and why not just use the sun's elevation?
Horizontal slats and overhangs only care about the sun's angle in the vertical section across the wall. When the sun is off to the side, that angle is steeper than the elevation — tan profile = tan elevation ÷ cos horizontal angle — so a low western sun can still look high to a south wall.
Which tilt should I use in summer?
Look at the day chart for a summer date: it shows the tilt each hour needs. For horizontal slats on a south wall the midday sun is usually blocked even with open slats; east and west walls need steep tilts in the morning or afternoon.
Why does it say no tilt can block the sun?
If the slats are narrower than their spacing, a low sun always finds a gap, even with the slats closed. Use wider slats or a smaller spacing — slats wider than the spacing can close completely.

Planning aid only. It does not size structural members or check loads — follow the manufacturer's instructions and your local building rules.