Gutter guards should not be installed under shingles, as this can interfere with the roof's structure and may void the roof warranty. Instead, gutter guards should be attached directly to the gutters to ensure proper water flow and prevent damage to the roof.

Installing gutter guards under shingles can cause roofing issues and void warranties. The best practice is to attach the guards to the gutter system without affecting the roof structure, ensuring that water flows properly and the roof remains intact.