Passive Solar Overhang Calculator
Calculate the overhang depth needed to shade a window—and test how much glass is shaded at different dates and times. This tool accounts for latitude, longitude, UTC offset, façade orientation, solar altitude, solar azimuth, window height, and the gap above the window.
Size and Test a Fixed Horizontal Overhang
Enter the project location, the design moment, and the window geometry. Results update when you select Calculate overhang.
Results will show the solar position, façade profile angle, recommended overhang depth, tested shade coverage, and a seasonal comparison.
How the Passive Solar Overhang Calculator Works
A fixed horizontal overhang casts a shadow down a vertical wall. The vertical shadow length depends on the overhang projection and the solar profile angle relative to that wall. If the shadow reaches from the underside of the overhang to the bottom of the glass, the window is fully shaded at that moment. If it reaches only partway down the glass, the window is partially shaded.
Many simple roof overhang calculators use solar altitude alone. That shortcut is exact only when the sun is directly perpendicular to the façade. This calculator also considers the difference between solar azimuth and façade azimuth. It converts solar altitude into a façade-specific profile angle before calculating the shadow.
The solar-position equations estimate declination, equation of time, true solar time, hour angle, elevation, and azimuth from the location and local clock time. The tool then calculates the horizontal shadow angle and profile angle for the selected façade. This approach is appropriate for concept design and manual cross-checking, while detailed projects should still be tested with hourly climate data and three-dimensional shading software.
Calculator Inputs Explained
| Input | What to enter | Why it matters |
|---|---|---|
| Latitude | Decimal degrees from −89.9 to +89.9 | Controls the seasonal solar path and altitude. |
| Longitude | Decimal degrees; east positive, west negative | Converts local clock time to true solar time. |
| UTC offset | Standard offset plus daylight saving when active | A wrong offset shifts the calculated solar position by roughly 15° per hour. |
| Design date and time | The moment when the shading target should apply | An overhang sized only at noon may not shade morning or afternoon sun. |
| Façade azimuth | Clockwise from true north: N=0°, E=90°, S=180°, W=270° | Determines whether the sun is in front of the wall and corrects the profile angle. |
| Window height | Vertical height of exposed glazing | Full shade must cover this height plus the gap above it. |
| Gap above window | Overhang underside down to the top of glass | A larger gap requires a deeper projection. |
| Projection to test | Horizontal wall-to-edge distance | Used to estimate current shade coverage. |
Passive Solar Overhang Formula
For a vertical façade and a horizontal overhang, the calculator first determines the horizontal shadow angle, or HSA:
HSA = solar azimuth − façade azimuth
The angle is normalized to the range −180° to +180°. Direct sunlight reaches the front of the façade only while the absolute HSA is less than 90°. The solar profile angle is then:
Profile angle = arctan[tan(solar altitude) ÷ cos(HSA)]
If H is the window height, G is the vertical gap above the window, and αp is the profile angle, the minimum projection for full shade at the selected moment is:
Projection = (G + H) ÷ tan(αp)
For a tested projection P, the shadow drop is P × tan(αp). Shade on the glass begins only after this drop exceeds the gap. The estimated shaded fraction is therefore:
Shade fraction = clamp[(P × tan(αp) − G) ÷ H, 0, 1]
This is clean geometric shading. It does not estimate diffuse sky radiation, reflected radiation, glazing solar heat gain coefficient, thermal bridges, cooling loads, or the thermal response of the room.
How to Interpret the Results
Recommended projection
This is the theoretical minimum horizontal depth needed to shade the full glass height at the chosen moment. It is not automatically the best construction dimension. Add an appropriate design tolerance and test nearby hours, critical warm-season dates, wall thickness, gutters, fascia, roof pitch, and the exact underside geometry.
Tested shade coverage
This is the vertical percentage of the glass shaded by the overhang. A result of 100% means the direct solar beam is geometrically blocked from the complete window at that moment. It does not mean that the window receives zero solar energy, because diffuse and reflected radiation remain.
Seasonal comparison
The comparison table tests the entered overhang at 9:00, 12:00, and 15:00 local solar time around the two solstices and an equinox. Solar time is deliberately used for this quick comparison so that morning and afternoon positions are symmetrical around solar noon. Use the main calculation for a precise local clock time.
Why Façade Orientation Changes Overhang Performance
Horizontal overhangs are usually most effective on equator-facing windows: south-facing in the Northern Hemisphere and north-facing in the Southern Hemisphere. On these façades, the warm-season sun is often high enough for a practical projection to create useful shade, while lower winter sun can pass beneath it.
East- and west-facing windows are harder to protect with a simple horizontal overhang. The sun can be low and strongly oblique, particularly in the morning or late afternoon. The formula may then return a very deep projection, or the sun may be behind the plane of the selected wall at the tested time. Consider exterior shutters, vertical fins, sliding screens, awnings, pergolas, deep porches, or carefully placed vegetation.
Near the equator, the sun path can move to either side of the building during the year. In very high latitudes, low solar altitude can make full fixed shading impractical. Climate, sky conditions, surrounding obstructions, and the desired balance between heating and cooling remain essential.
Before sizing glazing or shading, review passive solar orientation. For the wider strategy, see the passive solar shading and overhangs guide and roof overhang design for passive solar homes.
Worked Overhang Design Example
Consider a south-facing window in the Northern Hemisphere with a glass height of 1.50 m and an overhang underside 0.20 m above the glass. If the calculated profile angle at the chosen warm-season design moment is 65°, the required projection for full shade is:
(1.50 m + 0.20 m) ÷ tan(65°) = approximately 0.79 m
An 0.80 m projection would therefore be close to the geometric full-shade threshold at that particular sun position. But it should not be accepted without testing other hours and dates. If the same façade receives an afternoon profile angle of only 40°, full shade would require approximately 2.03 m—showing why time and orientation matter.
Next, check whether the overhang admits desired winter sunlight. Use the seasonal table, then run the main calculator for winter dates and occupied hours. Combine the result with the Solar Angle Calculator, Window-to-Wall Ratio Calculator, and guidance on passive solar window placement.
Common Roof Overhang Sizing Mistakes
- Using solar altitude instead of profile angle. This ignores how oblique sun changes the shadow on a façade.
- Entering magnetic orientation. Solar geometry requires true geographic north.
- Forgetting daylight saving time. The UTC offset must match the selected date.
- Measuring to the wrong edge. Use the effective shading edge and the underside height, accounting for fascia and gutters.
- Testing only the summer solstice. The warmest period may occur weeks later because buildings and surroundings store heat.
- Assuming full shade means no heat gain. Diffuse sky and reflected solar radiation still reach the glass.
- Applying a south-window rule to east or west glass. Low-angle sun usually needs vertical or adjustable external shading.
- Ignoring trees and adjacent buildings. Real obstructions can change solar access substantially.
- Skipping winter checks. An oversized overhang may block useful passive heating and daylight.
- Treating a concept result as construction documentation. Structure, wind, snow, drainage, waterproofing, fire safety, and local codes require professional design.
Accuracy, Assumptions, and Design Limits
The solar equations used here follow the widely used NOAA fractional-year approximation for solar declination and equation of time. NOAA describes these as low-accuracy equations; the geometric output is appropriate for early design, education, and independent checks, not high-precision astronomical or bankable energy analysis.
The overhang model assumes a vertical planar wall, a rectangular vertical window, a level horizontal overhang parallel to the window head, a single effective outer shading edge, and unobstructed direct beam sunlight. It does not model overhang width at the sides, vertical fins, reveal depth, sloped awnings, nearby buildings, terrain, vegetation, clouds, diffuse radiation, multiple reflections, or daylight distribution.
For detailed design, test the full three-dimensional building with an annual hourly weather file and a validated daylight or building-energy model. Confirm structural dimensions, drainage, moisture control, snow and wind loads, fire separation, and code compliance with qualified professionals.
Frequently Asked Questions
What is the best overhang depth for passive solar design?
There is no universal depth. It depends on latitude, façade azimuth, window height, gap above the glass, climate, and the dates and hours you want to shade. Use the recommended projection as a starting point, then test warm- and cool-season performance.
Should I use June 21 to size an overhang?
Use the summer solstice as one reference, not the only design date. Solar altitude is highest near the solstice, but peak overheating may occur later. Test several dates across the local cooling season.
Does the calculator work in the Southern Hemisphere?
Yes. Enter a negative latitude and the actual façade azimuth. A typical equator-facing façade in the Southern Hemisphere points toward true north, approximately 0°.
Why is the recommended overhang extremely deep?
The solar profile angle may be low because the sun is low in the sky or far to one side of the façade. That is a signal that a horizontal overhang alone may be unsuitable. Consider vertical or adjustable exterior shading.
Why does the result say the sun is behind the façade?
The direct solar beam is striking the opposite side of the wall plane at that moment. The selected window receives no direct beam from that sun position, regardless of the overhang depth.
Can I use feet instead of meters?
Yes. Select Imperial. The geometry is dimensionless, so meters and feet produce the same proportions when all dimensions use the same unit.
Does 100% shade mean zero solar heat gain?
No. It means the overhang blocks the direct solar beam from the full vertical glass height in the idealized geometry. Diffuse sky radiation and reflected radiation may still contribute heat and daylight.
Technical References
Solar-position methodology: NOAA General Solar Position Calculations and the NOAA Solar Calculation Details. Passive solar design context: U.S. Department of Energy Consumer Guide to Passive Solar Home Design.
Disclaimer: This free calculator provides educational, early-stage estimates. It is not architectural, engineering, energy-code, structural, or safety advice.
