The overhang depth for a passive solar house is not a design preference — it is a calculation. The geometry of the sun's arc across the sky at your latitude determines exactly how much horizontal projection above a south-facing window is required to block summer sun while admitting winter sun. This is one of the oldest calculations in architecture and one of the most reliable.
The Solar Geometry at the Foundation of the Calculation
The sun's altitude angle at solar noon changes with the seasons. At the summer solstice, it reaches its maximum height above the horizon. At the winter solstice, it is at its lowest. The difference between these two angles is what makes a fixed overhang a functional shading device.
At Denver (39.7 degrees north latitude):
- Summer solstice noon sun altitude: approximately 73.3 degrees
- Winter solstice noon sun altitude: approximately 26.3 degrees
The spread between these two is about 47 degrees. That spread is what a fixed overhang exploits. The overhang projects far enough to intercept the high summer sun while the low winter sun passes beneath it and enters the glazing.
The Basic Formula
For a horizontal overhang at the top of a south-facing window:
Overhang projection = window height / tan(summer noon altitude)
At Denver, tan(73.3°) = 3.35
For a 1.8-meter-tall window:
- Overhang projection = 1.8 / 3.35 = approximately 0.54 meters
This means a 54-centimeter horizontal projection above the window will provide full shade at summer solstice noon and allow full solar exposure at winter solstice noon.
Adjusting for Real Performance Goals
The basic formula optimizes for the solstice moments. In practice, you want the shading to extend a bit further into the shoulder seasons:
- Summer cooling concern: you want shading to begin in late May and continue through late September, not just on June 21
- Winter heating benefit: you want solar access to begin in late September and continue through late March
Adjusting to shade the window from April 21 through August 21 (a common design target), using the noon altitude of approximately 61 degrees on April 21 at Denver latitude:
- Overhang projection = 1.8 / tan(61°) = 1.8 / 1.80 = 1.0 meters
This is nearly double the solstice-only calculation. The practical resolution is often an intermediate overhang dimension (0.7 to 0.75 meters) combined with interior shades or exterior roll-down screens for the early and late summer periods when the overhang alone is insufficient.
Latitude Comparison: Denver vs Mexico City
The section as narrative: the section drawing through the south facade at two latitudes tells two different stories.
At Mexico City (19.4 degrees north latitude):
- Summer noon altitude: approximately 87 degrees (nearly vertical)
- Winter noon altitude: approximately 43 degrees
- A shallow overhang of 0.3 meters provides substantial summer shading because the summer sun is nearly overhead
At Denver (39.7 degrees north):
- Summer noon altitude: 73 degrees
- Winter noon altitude: 26 degrees
- A 0.54-meter overhang achieves the equivalent summer shading at solstice
At higher latitudes (45 degrees north, northern Colorado or Montana):
- Summer noon altitude: approximately 68 degrees
- Winter noon altitude: approximately 21 degrees
- Required overhang depth increases again; the lower winter sun angle also means deeper penetration of winter solar gain into the room
In MÉTODO, we design in both Mexico and the Mountain West. The overhang depth calculation runs every time, because the latitude difference between a project in Mexico City and a project in Denver changes the geometry by a factor that affects every solar-facing element of the design.
East and West Overhangs: Why They Do Not Work the Same Way
The horizontal overhang calculation only works for south-facing surfaces (in the northern hemisphere). East and west-facing glazing receives sun at low angles in the morning and afternoon — when the sun is near the horizon. A horizontal overhang at the top of an east or west window provides almost no shading benefit because the low-angle sun passes under it.
For east and west glazing, shading must come from:
- Vertical fins that block the low-angle sun from the sides
- Deep setbacks within the building mass
- Overhanging trees or site features
- Interior or exterior roller shades
This is one reason we minimize east and west glazing in passive solar designs and concentrate glazing on the south face where the geometry works.
The Section Drawing as the Verification Tool
The verification of the overhang calculation happens in the section drawing. Draw the summer solstice noon sun angle as a ray from outside the building through the section. If the ray is blocked by the overhang before it reaches the glass, the shading is adequate. Draw the winter solstice noon sun angle. If it passes below the overhang and enters the glazing to reach the thermal mass on the floor, the design is working.
This takes minutes with a protractor and a scaled section. There is no substitute for drawing it.
Próximos pasos
Overhang design is resolved in the section drawing during schematic design — before structural and framing decisions are made. We include sun angle analysis in our early design deliverables for passive solar projects.