What installation method is used for asphalt shingles in a valley without metal flashing?

The correct choice involves placing an 18-inch strip of roll roofing over the underlayment in the valley. This method is commonly used in roofing to help direct water flow and prevent leaks in valleys, which are areas on a roof where two slopes meet. By using a strip of roll roofing, it provides an additional layer of protection that helps shed water away from the seams of the shingles, reducing the risk of water infiltration. This strip acts as an alternative to metal flashing, which is often utilized for the same purpose.

While it might be tempting to consider other options, they do not align with best practices in roofing. Installing shingles directly without any underlayment does not provide sufficient waterproofing in the valley area, and using a 36-inch strip of only shingles would not effectively address the vulnerability of the valley, as it lacks the moisture barrier that roll roofing provides.