Light
The distribution designed in Casa sobre el Arroyo does not strictly follow the usual distributions following a house's orientation. In fact, the northern face of the building is where the kitchen and bathrooms are located, rooms that usually are placed by the south, in the facade with less sunlight incidence.
In the schematic analysis above, we can discern between natural light in winter and natural sunlight in the summer:
- Winter: The leaves of deciduous trees in the land have fallen, letting the sun rays get to the building through the naked branches. The sun is lower, so the light gets deeper into the house's interior, although it is a weaker light. The northern face gets a lot of direct sunlight, while the southern area of the building gets indirect light and some of the soft light from sunrise and sunset.
- Summer: The trees in the land have grown new leaves that act as a natural parasol, protection that is needed due to the big crystal areas in the perimeter. The trees protects the facades of the house from strong sun rays, but not the flat rooftop, from where some sun heat could have gotten into the building. The light inside the house in this season is mostly shaded, indirect light.
Analysis of sunlight incidence in the interior of the house in section A-A' (plan below).


Using the sunlight analysis above, we can make a plan of the average natural light that gets in the house all around the year.
This natural light in the house can be labeled in 4 types:
1. Strong direct sunlight (yellow): The sun incides the house on its highest point, when the sunlight is more powerful.
2. Weaker direct sunlight (orange): The sun comes in at its lower points (sunrise and sunset). As the sun is the lower, the light comes further inside due to its angle.
3. Indirect sunlight (red): The sunlight does not come in, but there is diffused and ambiental lighting that slightly illuminates the interior.
4. Half-light (purple): These are the darkest areas of the house, poorly illuminated and only by disperse indirect light.
This plan and light types cannot be applied the same way in winter than in summer. In winter, it would be more near to reality, while in summer there would predominantly be lights of types 3 and 4, as vegetation would avoid direct sunlight.

Sunlight incidence analysis.
Being labeled the types of natural light that gets in the building, we can make a deeper analysis of its use:

Piano and dining room, restored state.
Starting with the type 1 (strong direct sunlight), we can see where the sun gives its most powerful light, near mid-day (its highest point). It is where the architect decided to locate the bedrooms, bathrooms, kitchen, and the study room.
The study is where the northern light affects the most, maybe Amancio wanted it to be well illuminated most time of the day as well as a comfortable temperature to work there. This could be the case of the kitchen too, perhaps he wanted the service employees to have great working conditions. In the other hand, for the bedrooms, the way they are positioned could be to avoid waking up his parents early in the morning in case they wanted to sleep in, or to let them go to bed before sunset without the sun disturbing them.
The bathrooms might have been located also in the north face mainly as a consequence of the bedrooms placing.
In the type 2 (weaker direct sunlight), there's the piano zone at the east, and the living room at the west. The piano zone is at the north-east side of the building, meaning that it has natural sunlight from sunrise to mid-day. It is the most privileged area of the house, and it makes sense because the piano zone is a very important piece of the house, as Alberto Williams needed a comfortable place to compose his music. The living room, on the other hand, is placed at the west side of the building, where the sun only gets in from the evening to sunset.
Following with the type 3 (indirect sunlight), that occurs in the southern face of the house and in other areas where direct light turns into indirect light due to reflection or dispersion. This happens at the interior of the rooms, and in part os the living room, but mostly in the dining area. It is the most disadvantaged place of the house, but maybe the architect considered this dining area would be used sporadically, only if guests came over, as there is a smaller table for 2 near the living room.
Finishing with this analysis, in type 4 (half-light), there are the darkest places of the building: the corridors, and the staircases. There could be a problem with the staircases if below the first floor there were no windows, but the arches and ribs in the structure let the light in, as there are glass panels installed in them as fixed windows. In this areas there are no major problems due to the natural lighting.

The staircase, original state.
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