Installing a Metal Roof Over Existing Shingles

Re-covering, sometimes called a roof-over or overlay, means installing new metal roofing on top of an existing asphalt shingle roof instead of tearing it off. Metal is light, roughly half the weight of asphalt shingles by one industry estimate, so it is often possible to add it without overloading the structure. Industry associations note that metal can often be installed over an existing roof, subject to local rules.

Code and permit considerations

Building codes in many jurisdictions limit how many roof coverings may be stacked and expect the existing roof and deck to be sound. Requirements vary by country, state, province and city, so the local building department or code should always be checked before planning a roof-over.

Technical checks

• Deck condition: soft, rotten or delaminated decking must be repaired; fasteners must reach sound wood.
• Flatness: curled or uneven shingles can telegraph through flat panels and increase oil canning. Some installers add battens or a new underlayment layer to create a flat plane.
• Underlayment and separation: a new underlayment or slip sheet is usually installed; manufacturers specify compatible products.
• Ventilation and condensation: trapping moisture between layers must be avoided; ridge and eave ventilation details may need upgrading.
• Flashings: new flashings at walls, chimneys and penetrations are normally required rather than reusing old ones.
• Fire and wind ratings: ratings apply to tested assemblies, which may or may not include a recover layer.

Pros and cons

A roof-over saves tear-off labour and disposal and keeps the house covered during work. Disadvantages include hiding deck problems, limits on future re-covering, and the extra care needed to keep panels flat.