How should underlayment be placed in asphalt shingle roofs?

Underlayment plays a crucial role in asphalt shingle roofing systems as it serves as an additional layer of protection against water infiltration. When considering how the underlayment should be placed, extending it over any drip edge is the correct approach. This ensures that any water that may run down the roof will be directed over the underlayment and onto the drip edge, rather than allowing it to seep under the shingles or underlayment, which can lead to leaks and damage.

The purpose of the drip edge is to effectively channel water off the edges of the roof and away from the underlying structure. By having the underlayment overlap the drip edge, it provides an uninterrupted barrier that enhances the overall waterproofing of the roof assembly.

In contrast, placing the underlayment underneath all existing layers may not effectively address water intrusion at critical points like eaves and valleys. Restricting the underlayment to only extended eaves or using it solely as a top layer would compromise the roof's integrity, leading to potential leaks. Layering and positioning the underlayment thoughtfully ensures that the roof is better equipped to handle water intrusion, thereby prolonging the roof's lifespan and minimizing maintenance needs over time.