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The correct overhang depth is site-specific. Size the horizontal projection and its height above the window against the sun altitude for the summer hours you want shaded and the winter hours you want exposed. A projection that works in one city can over-shade or under-shade a similar window elsewhere because latitude, wall orientation, window height, and seasonal sun angles all change. The practical method is to draw the window and overhang in section, obtain the site’s solar positions, and iterate until the shade line meets both seasonal targets.
Why one overhang dimension cannot work everywhere
At middle latitudes, the summer sun is higher in the sky and the winter sun follows a lower, more oblique path. That seasonal difference allows a fixed horizontal overhang to shade glazing in hot months while admitting lower winter sun. The U.S. Department of Energy explains the seasonal change through Earth’s 23.5° axial tilt; the page does not establish a publication year for that explanation. See DOE’s Solar Radiation Basics.
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Solar altitude is only one part of the geometry. The window’s height, the distance from the overhang underside to the top of the glazing, and the projection from the wall determine where the shade line falls. A four-foot-high window with a 14-inch gap needs a different projection from a four-foot-high window with a 20-inch gap, even at the same latitude. Nearby roofs, trees, porches, and neighboring buildings can change the result again.
NREL’s Passive Solar Technology Basics describes overhangs as “Control—devices such as roof overhangs used to shade the aperture area during summer months.” That control works best when the aperture is close to true south; NREL’s passive-solar approach uses an orientation within 30 degrees of true south. Treat that as a design condition, not a universal rule for every climate or building.
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Therefore, do not select a depth from a generic rule such as “one foot per foot of glass.” Measure the actual opening, select the dates and hours that matter to occupants, and calculate from the local sun path.
How a horizontal overhang creates the seasonal shade line
In a vertical section through the wall, sunlight arrives at an altitude angle above the horizon. The outer edge of the overhang casts a descending shadow toward the wall. A higher summer sun produces a shorter vertical reach below the overhang; a lower winter sun produces a longer reach and can pass beneath the projection to the glass.
For a first-pass, two-dimensional check, use these variables:
- P: horizontal overhang projection from the wall.
- G: vertical gap from the underside of the overhang to the top of the window.
- H: window height.
- α: solar altitude in the vertical plane that is perpendicular to the window wall.
The vertical drop of a ray at the overhang edge is approximately P × tan(α). The shade depth measured down the glazing is approximately max(0, P × tan(α) − G). This simple relationship assumes a level horizontal overhang, a vertical wall, and sun aligned with the section. It is a sizing aid, not a complete annual simulation.
To shade the entire window at a selected summer condition, the calculated shade depth should be at least the window height: P × tan(αsummer) − G ≥ H. To keep the overhang from shading the glass at a selected winter condition, the vertical drop should not reach the top of the window: P × tan(αwinter) ≤ G. Most projects choose intermediate targets, such as shading the upper 80 percent in summer while leaving a defined strip of winter glass exposed. Write those targets down before adjusting dimensions.
Collect the information before drawing
Record the following for each opening. A separate worksheet is needed for windows that differ in height, orientation, or overhang elevation.
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- Window geometry: rough-opening or finished-glazing height and width, sill height, frame depth, and whether the glass is divided into different vertical sections.
- Overhang geometry: available projection, underside elevation, fascia thickness, and whether the projection is continuous across the opening.
- Orientation: wall azimuth relative to true south, not just a compass label such as “southwest.” A small deviation can matter when the sun is low.
- Site obstructions: trees, adjacent buildings, chimneys, railings, roof forms, and terrain that block or reflect sunlight.
- Seasonal objectives: dates and occupied hours for full shade, partial shade, or solar admission. Solstice noon is a useful illustration, but it may not match the hours when the room overheats.
- Glazing properties: solar heat-gain coefficient, visible transmittance, tint, and any interior or exterior shading already planned.
A tape measure is essential. A digital angle finder inclinometer can help verify wall, roof, and overhang angles during layout, but it does not calculate the sun path and cannot replace location-based solar-position data.
Step-by-step method for sizing the projection
1. Confirm that a horizontal overhang is the right first strategy
Start with glazing that faces close to true south. NREL’s passive-solar guidance places its described aperture strategy within 30 degrees of true south. If the wall is substantially east or west of south, the sun reaches the glass from the side, and a horizontal projection sized from a noon section will miss important morning or afternoon exposure.
For east- and west-facing windows, plan on a separate strategy: exterior operable vertical shades, fins, screens, or well-placed landscaping can intercept low-angle sun more effectively. Building Science Corporation discusses orientation-specific solar control and the way window solar gain affects building heat flow in BSD-011: Thermal Control in Buildings.
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2. Define the seasonal design conditions
Choose at least one summer condition to shade and one winter condition to admit. Record the date, local clock time, and the amount of glazing to be shaded or exposed. “Summer” and “winter” are too broad for a construction dimension: a west-facing room may overheat at 4 p.m. even when a noon condition looks acceptable.
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Solstice noon is a common illustrative pair because it clearly shows the altitude difference. Add spring and fall checks when the building relies on passive winter heat; an overhang that blocks summer sun can also block useful spring heat, while a design favoring winter gain may admit unwanted fall sun on warm days. NREL’s Passive Solar Design Strategies guide emphasizes these shoulder-season trade-offs.
3. Obtain the sun positions for the actual site
Use a solar-position calculator with the project’s latitude, longitude, date, and local time. NREL’s SOLPOS tool calculates apparent solar position from date, time, and location. Record solar altitude and azimuth for every selected condition.
The altitude used in the simple section calculation is the sun’s angle in the plane normal to the wall. When the sun is off to one side, its effective angle in that section differs from the raw altitude. For a wall that is not close to south, or for critical shading work, use a three-dimensional sun-path or building-energy analysis rather than applying the noon altitude directly.
4. Draw the section and trace the shade line
Draw the wall vertically at a scale large enough to show the window, the overhang underside, and the full projection. Mark the measured gap G and window height H. From the overhang’s outer edge, draw a ray at each selected solar altitude back toward the wall. Where the ray intersects the wall is the shade-line height.
Increase or decrease P until the summer ray reaches the desired depth while the winter ray remains above the glazing, or reaches only the amount of winter shading you have accepted. Check the actual overhang elevation: moving the projection upward changes the gap and can be as important as adding inches to the projection.
5. Test more than one hour
Repeat the trace for the occupied hours that drive comfort, not just noon. Test morning and afternoon conditions, then check the dates around the spring and autumn equinoxes. The noon-solstice construction is a starting point; it does not guarantee shade for every unwanted hour or annual energy savings.
Also test the sun from both sides of the opening when the wall is angled from true south. A continuous overhang may shade the center of the glass while side portions remain exposed. Window reveals, mullions, balconies, and neighboring projections should appear in the section or plan used for the final check.
6. Coordinate the result with glazing and the whole building
Shading cannot be evaluated independently from the glass and the building’s heating and cooling loads. Building Science Corporation notes that south-facing windows in passive-solar houses may be selected with a higher solar heat-gain coefficient to admit beneficial winter gain. A deep projection over low-gain glass may provide little winter benefit; a shallow projection over high-gain glass may leave too much summer load.
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Coordinate the overhang with ventilation, thermal mass, interior blinds, and mechanical-system assumptions. The objective is not maximum shade at every season; it is the combination of solar control and useful winter admission that fits the room’s actual operation.
NREL’s location examples: useful benchmarks, not universal specifications
The NREL design guide illustrates the method with a four-foot-high window and different city conditions. The guide is an older document, and the retrieved page does not establish its publication year. Its figures are dated illustrative examples, not current code requirements, engineering approvals, or guarantees of comfort.
| Location | Illustrative projection | Overhang bottom above window top | Window used in example |
|---|---|---|---|
| Akron-Canton, Ohio | 26 inches | 15 inches | 4 feet high |
| Dillon, Montana | 32 inches | 15 inches | 4 feet high |
| Tulsa, Oklahoma | 19 inches | 14 inches | 4 feet high |
| Las Cruces, New Mexico | 14 inches | 11 inches | 4 feet high |
| Flagstaff, Arizona | 21 inches | 14 inches | 4 feet high |
| Prescott, Arizona | 17 inches | 13 inches | 4 feet high |
The spread—from 14 inches in the Las Cruces example to 32 inches in the Dillon example—shows why copying a depth from a nearby-looking project is unsafe. The examples pair projection with vertical placement; using only the projection discards half of the geometry. Use the guide to understand the seasonal-altitude method, then recalculate for the project’s latitude, orientation, window, and desired hours.
Orientation-specific decisions
South-facing glazing
A horizontal overhang is most predictable on glazing close to true south because the sun crosses the wall’s vertical section in a way the projection can intercept. Even there, verify morning and afternoon conditions and account for any azimuth difference from true south.
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Morning sun arrives at a low angle and can pass beneath or around a projection sized from noon geometry. Use exterior vertical elements, adjustable screens, or landscape design in addition to—or instead of—a fixed horizontal overhang.
West-facing glazing
Late-afternoon sun is similarly low and often coincides with high indoor temperatures. A horizontal roof edge alone is rarely a complete west-window solution. Prioritize exterior operable vertical shading and obstruction analysis, then use a horizontal element only as part of the combined design.
North-facing glazing
Do not assume a south-window overhang formula applies. Direct sun exposure, reflected light, and the building’s latitude determine whether a projection is useful. Model the actual wall orientation and selected hours.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Shoulder seasons, obstructions, and partial shade
Fixed devices have no seasonal switch. If the projection is deep enough to block the high summer sun, it may also block spring sunlight when heating is still useful. Conversely, reducing the projection to maximize winter gain can admit fall sun during a warm spell. Decide which condition has priority for the room and document the compromise rather than calling one dimension “optimal” in every season.
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Obstructions can either help or hurt. A deciduous tree may provide extra summer shade and lose leaves for winter admission, while an evergreen or a neighboring roof can remove winter sun permanently. Measure obstruction angles from the window and include them in the same sun-position checks. A clear overhang calculation is not a substitute for a site survey.
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Partial shade is often the realistic target. You may choose to protect the upper glass while allowing winter rays to reach a lower portion, or to shade only the hours when the room is occupied. State the target as a percentage or a measured portion of glazing, then use the shade-line equation and section drawing to check it.
Construction and verification checklist
Once the geometry is selected, verify that the built detail can deliver it:
- Dimension the projection from the finished wall face to the controlling outer edge, and state whether fascia, trim, gutters, or screens are included.
- Dimension the vertical gap from the actual underside of the overhang to the top of finished glazing, not to rough framing.
- Check that structure, connections, and drainage can support the projection and local weather loads; the solar calculation does not size those components.
- Coordinate flashing and waterproofing where the overhang meets the wall and roof.
- Confirm that operable windows, insect screens, maintenance access, and fire-safety clearances remain usable.
- Re-measure the completed work. A change in wall thickness, trim, or window replacement can alter both projection and gap.
- Photograph or sketch the final geometry and retain the selected dates, times, and solar-position inputs for future alterations.
For a new build or major renovation, a designer can combine the three-dimensional site model, glazing properties, and annual weather data. This is especially valuable when the wall is far from south, the opening is unusually tall, or neighboring obstructions change with the season.
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The U.S. Department of Energy’s Efficient New Home tour includes the McElmo Canyon Custom and Isler Residence as built examples where overhangs help control high summer sun. They illustrate the principle in complete buildings; they do not establish a transferable projection for another site or window.
Common sizing mistakes and their fixes
Using a universal depth rule
Problem: A rule copied from another house ignores latitude, orientation, and window geometry. Fix: Recalculate with local solar positions and the measured projection and gap.
Measuring only the projection
Problem: Two overhangs with the same projection can cast different shade lines when their vertical gaps differ. Fix: Record both projection and underside-to-window-top distance.
Checking only solar noon
Problem: Morning and afternoon low-angle sun can enter even when noon is shaded. Fix: Test occupied hours and both shoulder seasons.
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Problem: A horizontal projection misses low-angle side sun. Fix: Add exterior vertical shading or landscape control and use a three-dimensional analysis.
Promising energy savings from dimensions alone
Problem: Shading performance depends on glazing, climate, obstructions, ventilation, and operation; no single dimension proves a savings percentage. Fix: Treat the overhang as one part of the envelope design and model the whole building when performance matters.
What the final design should document
A construction-ready shading note should identify the wall orientation relative to true south, each window’s height and width, the projection, the vertical gap, the selected summer and winter dates and hours, the solar-position source, and the intended shade fraction. Include the assumptions about trees and adjacent buildings and identify any separate east- or west-facing treatment.
That record makes the result reproducible. If the window size changes or a roof edge moves during value engineering, the team can rerun the same checks instead of relying on an unqualified rule of thumb.
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