How do I size a roof overhang for passive solar control?

To size an overhang for a south-facing window, you need the noon solar elevation angle at your latitude for the summer and winter solstices. The overhang depth that just shades the bottom of the window at the summer solstice noon equals the window height divided by the tangent of the summer solstice noon elevation, minus the window height divided by the tangent of the winter solstice noon elevation (to confirm winter sun access). In practice, designers use a ratio called the overhang projection factor: projection depth divided by window height. For most Canadian latitudes between 44° and 53°N, this ratio falls between 0.4 and 0.7 depending on whether you want full shade at the solstice or at a shoulder date.

Will an overhang block winter sunlight on south windows?

A correctly sized overhang allows full direct sun on south-facing windows in winter because the sun is much lower in the sky. At a Canadian latitude of 44°N (Toronto), the noon sun elevation drops to about 23° in December, while in June it rises to about 70°. An overhang designed to just shade the window at the summer solstice allows the low winter sun to reach the full window height from October through February at most Canadian latitudes. This seasonal selectivity is the fundamental principle of passive solar design: the overhang acts as a natural thermostat, blocking unwanted summer gain while admitting useful winter solar heating.

🏠 Overhang Sizing

Overhang Sizing Calculator

Calculate the roof overhang projection depth needed to fully shade south-facing windows in summer while admitting full winter sun. Uses solar geometry for any Canadian latitude and window height.

🏭 Canadian Location

🔆 Window & Overhang Inputs

Window Height i

Gap from Window Top to Overhang i

Shade Target Date i

Winter Sun Access Date i

🏠 Overhang Sizing Results

Toronto

Overhang projection (P)—

Window height (H)—

Projection ratio P/H—

Summer solstice elevation—

Dec. solstice elevation—

Shading Analysis

📊 Monthly Window Shade Fraction (%)

Monthly Shading Detail

Month	Noon Solar Elevation	Shadow on Window (m)	Window Shade Fraction	Design Condition

Export:

How to Use the Overhang Sizing Calculator

Select your Canadian city

Each city loads its latitude, which determines the solar elevation angles at different times of year. Higher latitudes have lower winter sun and therefore need shallower overhangs for the same window height, but the summer sun is also lower, which complicates the trade-off.

Enter window height and gap above

Measure the vertical height of the glazed area only — from the top of the window frame to the sill. The gap above is the distance from the window top to the underside of the overhang. This affects how much shadow the overhang casts on the window at different sun angles.

Choose shade start and winter sun end dates

The shade start date determines when the overhang first achieves full shade on the window. Earlier dates require deeper overhangs. The winter sun access date is when you want full direct sun to start entering the window in autumn. These two dates bracket the overhang design.

Review the monthly shading profile

The results show the overhang projection depth, the P/H ratio for your design library, and a month-by-month chart of how much of the window is shaded at solar noon throughout the year. Use the solar heat gain calculator to quantify the cooling load reduction from this shading.

Sizing Roof Overhangs for Canadian Passive Solar Design

A roof overhang above a south-facing window is one of the most effective and cost-free passive solar design strategies available to Canadian building designers. When properly sized, it acts as a seasonal switch: blocking the high summer sun that would overheat the space, while admitting the low winter sun that provides free heating. The geometry is determined entirely by the solar declination at your latitude — no moving parts, no controls, no energy consumption.

The geometry of overhang shading

At solar noon, the sun is due south and at its highest elevation angle for that day. Whether the overhang shades the bottom of the window depends on whether the shadow cast by the overhang edge reaches the window sill. The shadow length down the window face from the overhang equals the overhang projection divided by the tangent of the noon solar elevation, minus the gap between the window top and the overhang soffit. To shade the bottom of the window completely at noon on the target date, this shadow length must equal or exceed the window height. Rearranging: projection depth P = (window height H + gap G) × tan(elevation angle). The larger the elevation angle, the more P must grow for the same window height. At lower Canadian latitudes (43 to 45°N), summer noon elevations of 68 to 70° require larger P/H ratios than at northern latitudes (51 to 54°N) where summer noon elevation is only 59 to 62°.

The trade-off between summer shade and winter sun

The overhang sizing problem has two constraints moving in opposite directions. Deep overhangs shade more of the window in summer but can also block some winter sun. Shallow overhangs admit more winter sun but provide less summer shade. The design goal is an overhang depth that achieves full shade on the target date (such as June 1) while still allowing full direct sun on the window on the winter access date (such as October 1). This calculator solves both constraints simultaneously for your specific latitude, window height, and gap, and the monthly chart shows clearly where the sun transitions from fully shaded to fully accessible across the year.

The projection factor P/H and its use in design

Canadian passive solar guidelines often express overhang sizing as a projection factor — the overhang horizontal projection divided by the window height. For most Canadian latitudes between 44° and 52°N, the optimal P/H ratio for June 1 shade with October 1 winter sun access falls between 0.45 and 0.65. This ratio is useful because it scales directly with window height: a 1.5 m window needing a P/H of 0.55 requires a 0.83 m overhang projection, while a 2.0 m window with the same P/H needs a 1.10 m projection. Once you have the P/H ratio for your location and design dates, you can apply it to every south-facing window on the project without recalculating each time.

Limitations of horizontal overhangs

A horizontal overhang provides full shade only at solar noon on the design date. In the morning and afternoon hours on the same day, the sun is to the east or west of south and at a lower elevation angle, so it can pass below the overhang and enter the window from the sides. To block early morning and late afternoon sun in summer, the overhang should extend laterally beyond the window edges, or vertical fins should be added to the sides. The solar shading calculator models the effect of existing or proposed shading geometry on annual solar gain. For west-facing windows where afternoon sun is the primary cooling concern, a combination of a modest horizontal overhang and a vertical fin or louvre system is more effective than a deep horizontal overhang alone.

Construction considerations for Canadian climates

Roof overhangs in Canada must handle snow accumulation, ice damming, and freeze-thaw cycling. Deep overhangs (over 600 to 700 mm) typically require structural support through cantilever framing, knee braces, or rafter extensions. The soffit should be ventilated if the overhang encloses part of the roof assembly to prevent moisture accumulation. In areas with heavy snowfall, a slight downward pitch on the overhang surface (1° to 2°) helps shed snow rather than retaining it as a load. These construction details are beyond the scope of this tool; always consult a structural engineer for overhang projections exceeding 600 mm in Canadian construction.

Frequently Asked Questions

To size an overhang for a south-facing window, you need the noon solar elevation angle at your latitude for the summer and winter solstices. The overhang projection depth P = (window height H + gap G above window) × tan(solar elevation at target shade date). For most Canadian latitudes between 44° and 53°N, the projection factor P/H falls between 0.45 and 0.70 for a June 1 shade start with October 1 winter sun access. This calculator computes the exact depth for your specific inputs and shows the monthly shading profile so you can verify the design achieves your seasonal comfort goals.

Sizing for the summer solstice (June 21) gives the minimum overhang depth that achieves full shade on that specific day. However, the hottest and most uncomfortable period in Canada is often late July through August, not the solstice. Sizing to fully shade the window through August requires a slightly deeper overhang than solstice sizing. Conversely, sizing for a shoulder date like May 1 to October 1 means the overhang shades the window for five months, which also maintains shade in late September when cooling loads can still be significant. This calculator lets you choose your target shade period so the overhang depth matches your actual comfort and cooling goals.

A correctly sized overhang allows full direct sun on south-facing windows in winter because the sun is much lower in the sky. At a Canadian latitude of 44°N (Toronto), the noon sun elevation drops to about 23° in December, while in June it rises to about 70°. An overhang designed to just shade the window at the June 1 shade date allows the low winter sun to reach the full window height from October through February at most Canadian latitudes. This seasonal selectivity is the fundamental principle of passive solar design: the overhang acts as a natural thermostat. The monthly shading chart in this calculator shows exactly which months have full shade vs. full winter sun access for your specific inputs.

A horizontal overhang directly above a south-facing window provides shading at solar noon, when the sun is due south and at its highest elevation. In the morning and afternoon hours, the sun is to the east or west of south and at a lower elevation, so it can enter under the overhang from the sides even when the overhang provides full noon shading. To shade a window throughout the day, the overhang should extend laterally beyond the window edges by approximately the same dimension as its projection depth. Alternatively, vertical fins on the sides of the window block morning and afternoon sun. Use the solar shading calculator to model side shading in addition to the overhang.