What is usually the rain hitting direction considered during designing of houses so as to consider issues due to the same?

For design, purposes most rain “hitting” is considered vertical. In high wind areas, a 45-degree projection can be used much like shadow projections.

More typically, we are concerned about water shedding and ponding.

Water shedding is where the roof, or other elements, direct the water, often concentrating the flow to specific areas. Normally, we want to direct water away from door openings, and window openings to a lesser extent. Gutters and diverters will help with this but can become overwhelmed during heavy rain.

Ponding is a buildup of water in an unwanted area. This happens a lot on flat roofs or dead-end valleys of roofs. Plugged up drains or insufficient drainage tends to be the cause. This increases the weight on the roof very quickly and can cause leaks by overwhelming or undermining the waterproofing system.

So, again the sort answer: vertically.