Shingles alone should be fine, right?  They’re thick, heavy, and are designed to withstand everything the weather and sun throw at them.

True, but only partly.

Extreme conditions occur infrequently, but because they occur often enough, roofs need to be designed to function then as well.  Shingles alone can’t withstand some of the most extreme cases of weather.

Why?

Because they’re not seamless.

If you look at flat or low slope roofs, they have one top waterproofing layer that is designed to withstand all water from the outside.  This works because they’re functionally seamless.  Liquid-applied roofs dry into technically seamless membranes, and TPO or torchdown roofs are heat-seamed on location.

But shingle roofs are different.  They consist of many pieces of shingles layered on top of each other.  Adhesive helps seal them down, but the seal is never powerful enough to withstand the strongest winds or water pushing upwards along the roof’s slope.

Underlayment plays the secondary water-shedding role

Because of occasional extreme weather, underlayment catches the small amount of water that bypasses shingles and sheds it.  If not for underlayment, water would frequently reach the wood sheathing then leak throughout the house.

This raises the question: why are shingles even needed?  Can’t underlayment function as the top waterproofing layer as well?

The best underlayment certainly can last for weeks as the exterior layer, but only under normal conditions.  Strong winds will knock underlayment off, and the sun will tear it apart.

As a secondary layer beneath shingles, underlayment works wonderfully.  But it’s shingles that always take over 90% of the brunt of the sun and weather for many years.