Why is there more sun exposure on south and west sides of roof?

[Henry Holt]

I am reading J Scott's book on House flipping, and one of the comments in the book when he discusses examining the roof is as follows:

"South and West faces of roofs are generally most prone to wearing out, so these should be the parts of the roof that are examined most closely."

This question stems purely from being curious, by why do the South and West faces receive more sun exposure?

I figured it would be that the East and West sides of the house would receive the most direct sunlight, because of the sun rising and setting in those directions...  What am I missing?

[Michael Gansberg]

Hi @Henry Holt,

I love science questions 🌞

The vast majority of the time, the Sun is in the southern part of the sky if you live in the northern hemisphere- which most of us do. Insolance, which can be thought of as the quantity of sunlight, is increased when a surface is perpendicular to the sun's rays, and it is minimized when a surface is parallel to the sun's rays. Think about it this way- if you went outside at noon without a shirt, your head and shoulders bear the brunt of the sunlight if you're standing up(they're perpendicular to the sun when it's directly overhead,) but your torso bears the brunt if you're lying down. A roof sloping down and "pointing" southward will catch a high quantity of sunlight throughout the year, but a northern sloping roof will have a surface which is more parallel to the incoming rays, thus will absorb less energy from the sun.

The east vs west is a more difficult conundrum. They should absorb similar quantities of energy from the sun- my best guess is that the western facing roof absorbs that light when it's warmer out(i.e., later in the day.) This higher temperature could weaken the roofing material(it could disrupt the cross-linking of the molecules that make up parts of the roof,) thus causing the western exposure to be more susceptible to solar damage- though that's just an educated guess.