How deep should a roof overhang be for summer shade and winter sun?
An overhang over a south-facing window (north-facing in the southern hemisphere) can shade the glass in summer and let the low winter sun in. Enter your latitude, the window heights and the eave height: the calculator gives the overhang from the wall for full shade at the summer solstice and the longest overhang that still lets the winter sun reach the whole window. It works from the sun altitude at solar noon [1].
Updated 2026-10-30 · every regulation and price links to its source at the bottom of the page. A guide, not a construction design.
Size the overhang for your window
Measure heights from the finished floor of the window's storey: the bottom and top of the window opening and the underside of the eave (the fascia or soffit edge). The defaults are a typical window 0.85–2.10 m under an eave 2.95 m above the top floor.
The overhang is measured from the facade. The glass sits deeper by the outer reveal, so the shadow reaches it as if from a longer overhang — the calculator takes the reveal off.
In the app, with the same data
The result above is yours, free and without signing in. In the app the same data gives you more:
- Overhang advice for the south-facing windows of your own plan
- See your house in 3D — outside with the roof, inside and as a walk-through
- The plan as PDF and DXF to show a builder or an architect
No retyping: the values from this page are put into a new plan.
Sign up free — your data moves into the plan →I already have an account — sign in
What you typed stays in your browser until you move it into a plan yourself.
How it works: the sun altitude at noon
At solar noon the sun's zenith angle equals the absolute difference between latitude and solar declination [1]; the altitude is 90° minus that. The declination peaks at +23.45° on about 22 June and drops to −23.45° on about 21–22 December [2].
An overhang of depth W shades the wall down to W × tan(altitude) below its edge. So for shade at June noon to reach the bottom of the glass, the projection over the glass must be at least H ÷ tan(summer altitude), where H runs from the eave underside down to the sill. For December noon sun to reach the top of the glass, the projection must not exceed D ÷ tan(winter altitude), where D is the gap from the window head up to the eave.
Why noon: on an equator-facing wall in summer, the morning and evening sun comes in from the side and hits the facade more steeply, so the overhang shades deeper then than at noon; in winter it is the other way round. June noon therefore gives the shortest overhang that shades all day, December noon the longest that still lets the sun in (our geometry, an example calculation).
What to measure: sill, head and eave
New Zealand's passive-design guidance works the same way: overhang depth = height from the sill to the eave × a factor for the nearest city, and the gap from the window head to the eave underside at least that height × a second factor — otherwise the top of the window never gets winter sun [3]. Its table covers four cities; this calculator derives the factors from any latitude.
The common mistake is to use the window height alone. The shadow has to travel from the eave edge to the bottom of the glass, so what counts is the distance from the eave down to the sill: 2.10 m for a 0.85–2.10 m window under an eave at 2.95 m, not the 1.25 m window height.
Worked example: latitude 49.84° and 46.48°
Window 0.85–2.10 m, eave underside 2.95 m above the floor, window in the middle of a 42 cm wall (21 cm reveal), facing south.
| 49.84° N | 46.48° N | |
|---|---|---|
| Sun altitude at noon: June / December | 63.6° / 16.7° | 67.0° / 20.1° |
| Projection over the glass for June shade | 1.04 m | 0.89 m |
| Overhang from the wall for full shade | at least 0.85 m | at least 0.70 m |
| Overhang that still lets December sun on the whole window | up to 2.60 m | up to 2.10 m |
East, west and in between
Horizontal shading — an overhang or eave — works best on the equator-facing wall; east and west walls need vertical devices, and south-east or south-west walls a mix of horizontal and vertical (egg-crate) shading [4]. In the morning and evening the sun is low and shines almost straight in: no overhang stops it.
Shading devices are external, internal or between panes; fixed ones are canopies, eaves, pergolas and balconies, removable ones awnings and blinds [5]. A roof eave is the simplest fixed external one — nothing to open or close.
Frequently asked questions
How deep should a roof overhang be over south-facing windows?
Deep enough for the June noon shadow to reach the bottom of the glass: the height from the eave down to the sill ÷ tan of the summer noon sun altitude. At 49.84° N, for a 0.85–2.10 m window under an eave at 2.95 m, that is about 1.04 m over the glass, or 0.85 m from a wall with a 21 cm reveal.
What is the sun angle at noon in summer and winter?
90° minus your latitude, plus 23.45° in June and minus 23.45° in December [2] [1]: at 50° N about 63° and 16.5°.
Will a deep overhang make the room dark in winter?
Not if there is a gap between the window head and the eave: the low winter sun passes under it. The calculator checks the gap and shows how much of the glass gets winter sun.
Does this work in the southern hemisphere?
Yes: enter a negative latitude and read «south» as «north» — the window faces the equator. New Zealand guidance uses the same method [3].
Sources
- en.wikipedia.org (INT) · common practice (not a rule) · checked 2026-10-30 · source
- PV Education — Declination Angle, Declination Angle (INT) · common practice (not a rule) · checked 2026-10-30 · source
- level.org.nz, Eaves: Calculating overhang depth; Calculating window head height (NZ) · common practice (not a rule) · checked 2026-10-30 · source
- repositary.knuba.edu.ua (UA) · recommendation · checked 2026-10-30 · source
- repositary.knuba.edu.ua (UA) · recommendation · checked 2026-10-30 · source