A west-facing living room in Thrissur: shading, layout and roof harvest

Geometry. For a wall facing the sun's azimuth, a horizontal projection of depth d drops a shadow of d·tan α on the wall, where α is the solar altitude. The whole window is shaded when

$$d \ge \frac{h}{\tan\alpha} = \frac{1.5}{\tan 30^\circ} = \frac{1.5}{0.577} \approx 2.6\ \text{m}$$

Judgement. A 2.6 m chajja is not a serious proposition: it is structurally heavy in RCC, it darkens the room for most of the year, it collects debris and it still fails in March and May, when the afternoon sun is lower or more northerly. It also does nothing about the heat that the neighbour's west wall re-radiates.

Better response. Cut the west glazing rather than try to shade it:

• Move the staircase and toilets to the west wall as a thermal buffer, and put the living room on the north or east face.
• Use deep vertical fins or adjustable louvres on any unavoidable west opening; vertical fins work against a low, due-west sun where a chajja cannot.
• Add a verandah or balcony with a perforated screen, light-coloured external finishes, and roof insulation or a ventilated terrace finish to control the ceiling heat gain.

Constraints to satisfy: entry from the north road; a hot west wall 3 m from a neighbour's building; monsoon wind from the west/southwest; heavy driving rain; every bedroom needing cross-ventilation.

Proposed arrangement

• Entry and car porch on the north, the only public edge; the porch doubles as shade for the north face.
• Staircase, toilets and store along the west wall. They need little daylight, tolerate heat and act as a buffer between the afternoon sun and the habitable rooms. The 3 m gap to the neighbour is too narrow to rely on for light or breeze, so treat the west as a service edge.
• Living and dining on the north and east, opening to a shaded verandah; north light is diffuse and the east face is cool in the afternoon.
• Bedrooms on the east and south-east, with openings on two opposite walls so air can move along the prevailing west/southwest monsoon wind, entering through shaded west openings and leaving on the east.
• A small internal courtyard or a top-lit stairwell to drive stack ventilation when the outdoor air is still, which is common in April.
• Deep eaves and a sloping roof with wide overhangs rather than a flush flat roof: 3000 mm of rain demands generous overhangs and clear rain-water disposal away from the plinth.

Annual harvest

Roof plan area = 12 × 10 = 120 m². Annual rainfall = 3.0 m.

$$V = 120 \times 3.0 \times 0.8 = 288\ \text{m}^3 = 288{,}000\ \text{litres}$$

The 0.8 factor is a runoff coefficient allowing for evaporation, first-flush diversion and filter losses; a first-flush device alone typically rejects 1–2 mm per shower.

Demand

5 persons × 150 L/day = 750 L/day → 273,750 L/year. On an annual balance the roof nearly covers household demand.

But the balance is the wrong test. Rainfall is concentrated in two monsoons, so the question is the length of the dry spell to be bridged. A 60-day dry spell needs 45,000 L; that is an expensive tank. A practical recommendation is a 15,000–20,000 L sump (20–27 days of autonomy), sized by the dry spell the family is willing to pay for, with mains or a well as backup.

Limits and details. Peak intensity of, say, 100 mm/h gives 120 × 0.1 = 12 m³/h ≈ 3.3 L/s, which sets the downpipe count (two 90 mm downpipes are ample); the tank needs a filter, a first-flush diverter, mosquito-proofing and an overflow to a soak pit. Water stored over months must be kept dark and covered.