Applying a coating to a composition shingle roof is not a good idea for several reasons.

First, most composition shingle roofs are not designed to be coated. Composition shingles are made with a specific type of asphalt that is designed to be durable and long-lasting without the need for additional coatings. Applying a coating to composition shingles can interfere with this design and may reduce the shingles' lifespan.

Second, applying a coating to a composition shingle roof can void the manufacturer's warranty. Most manufacturers of composition shingles offer a warranty that covers defects and failures that may occur with the shingles. Applying a coating to the shingles can void this warranty, leaving the homeowner without protection if any issues arise with the roof.

Third, applying a coating to a composition shingle roof can be costly and time-consuming. The process of applying a coating to a roof is labor-intensive and can be expensive. In addition, the coating must be applied correctly in order to be effective, which can require a significant amount of time and expertise.

Fourth, applying a coating to a composition shingle roof may not provide the desired benefits. Some homeowners choose to apply a coating to their roof in order to extend the life of the shingles or to improve the roof's appearance. However, in most cases, applying a coating to composition shingles will not significantly extend their lifespan, and the coating may change the shingles' appearance in a way that is not desirable.

Overall, applying a coating to a composition shingle roof is not a good idea due to the potential interference with the shingles' design, the risk of voiding the manufacturer's warranty, the cost and time involved, and the potential lack of benefits. It is generally better to rely on the durability and performance of the composition shingles without the addition of a coating.