Leaving existing asphalt shingles in place underneath a new metal roof installation is sometimes permitted by local building codes as a single-layer overlay, but it is generally discouraged by professional contractors. Retaining old shingles adds unnecessary dead weight to the roof structure, traps residual heat that can degrade metal panels prematurely, hides underlying plywood deck rot or moisture issues, and prevents proper inspection and fastening of the new roofing underlayment and furring strips.
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