1. Should I install a metal roof directly over-top of an asphalt shingle roof? No. Why?
- Premature Failure: Metal has a high degree of thermal expansion and contraction. As the roof heats up and cools down, it moves along both its length and its width. Placed directly against the granules on asphalt shingle, this movement will scratch the underneath of the panel and eventually expose the bare steel, thus potentially causing premature failure i.e. corrosion.
- Voided Warranty: The installation of a metal panel directly over top of an abrasive material, in most cases, voids the manufacturer’s corrosion warranty (this would apply to Galvalume panel only as galvanized steel does not carry a corrosion warranty).
- Shape: The panel will take the shape of the underlying uneven surface and, in many cases, make shape issues visible that were previously hidden.
2. What if I lay down an underlayment before putting the metal roof on?
- This is certainly a better option, but you will still have the same concerns with regard to Shape (see above) and, should the underlayment wear away or tear and expose the metal to the asphalt shingle, there exists the potential for Premature Failure which would result in a Voided Warranty.
- If it is decided that this is going to be the installation method, recommended underlayments would be Grace Tri-Flex and IKO RoofGard. They are impervious to moisture and highly tear resistant. MMR does NOT recommend the use of 15# or 30# roofing felt (tar paper) for this application.
3. Does that mean I should be stripping all asphalt shingle roofs before laying metal down? No. MMR recommends the stripping of the existing asphalt shingles, however we understand that time and budget constraints do not always cooperate. Here is the recommended best practice for re-roofing over an asphalt shingle roof:
- Roof Deck: Inspect the existing shingle roof to determine if it is suitable as a barrier against condensation. Remember that a metal roof will, to a certain degree, develop condensation on the underside of the panels. Those areas that show signs of previous water infiltration (underlying sheathing is soft or spongy) or damage (missing or deteriorated shingles) should be repaired accordingly and rendered watertight with the use of ice & water shield and an appropriate sealant (if necessary).
- Ridges & Hips: Remove the cap shingles to avoid having a hump. If leaving exposed with the threat of rain / snow, cover with tarp or roof underlayment.
- Roof Edges: Cut the overhanging asphalt shingles flush with the fascia / rake boards. This will also remove the existing drip edge. This allows you to install your eave / rake trim true to the house with a clean, tight fit to the fascia boards.
- Hips, Ridge & All Protrusions: 9 – 12 inches of width on all sides of using plywood, OSB or similar decking (thickness to match other lumber).
- Valleys: 16 – 24 inches of width on either side using plywood, OSB or similar decking (thickness to match other lumber).
- Gable & Eave Edges: 1″ X 3″ spruce firring or similar dimension light gauge steel framing, set back from edge by ” – ” so that it doesn’t kick the trim out.
- Interior of Roof Deck: 1″ X 3″ spruce firring or similar dimension light gauge steel framing, installed horizontally approximately 16″ OC (use every 2nd course of shingles are your guide).
4. All of this sounds like a lot of work. Why don’t I just strip the roof? That is a valid comment and many contractors / homeowners decide to do just that. Here are considerations for both options:
Leave Existing Roof In Place:
- Pro: No need to pay for dump fees;
- Pro: No need to pay for new roof underlayment so long as existing roof is suitable as a barrier against condensation;
- Pro: If using firring, creates an air barrier between the metal panel and the roof. This allows condensation that develops on the underside of the panel to dry more quickly;
- Pro: If using firring, reduces thermal transfer to the underlying roof deck as the metal panel is sitting up off the asphalt shingle;
- Pro: If using firring, allows for a cost effective leveling of the roof surface if required;
- Pro: Reduces the burden placed on landfills each year;
- Con: Does not allow a thorough inspection of the roof deck prior to installing new roof.;
- Con: Renders the installation of snow guards more difficult and potentially less secure;
- Neutral: Not accepted by all building codes (hurricane prone areas require installation to solid deck using specific UL-90 rated clips).
Remove Existing Roof:
- Pro: Allows you to inspect the condition of the roof deck and make repairs as needed;
- Pro: Provides for a smooth, flat roof surface on which to install;
- Pro: Allows for the proper and thorough installation of an ice & water shield;
- Pro: Allows a solid and uniform surface into which snow guards can be fastened if required;
- Pro: Provides a solid surface for all fasteners;
- Con: Added costs through dump & disposal fees;
- Con: Added costs through new roof underlayment.
5. Do you have any pictures that clearly show the strapping of a roof? Absolutely.