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METAL ROOFING: RE-ROOFING QUESTIONS & ANSWERS

Building contractors are always looking for installation methods that will provide themwith savings on labor and materials expenses. Here are some important answers tofrequently asked questions.

1. Should I install a metal roof directly overtop 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 itslength and its width. Placed directly against the granules on asphaltshingle, this movement will scratch the underneath of the panel andeventually expose the bare steel, thus potentially causing prematurefailure i.e. corrosion.

Voided Warranty: The installation of a metal panel directly over top ofan abrasive material voids the manufacturer’s corrosion warranty (thiswould apply to Galvalume panel only as galvanized steel does not carry acorrosion warranty).

Shape: The panel will take the shape of the underlying uneven surfaceand, in many cases, make shape issues visible that were previouslyhidden.

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 wearaway or tear and expose the metal to the asphalt shingle, there exists thepotential 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 30 and IKORooftop Guard. They are impervious to moisture and highly tearresistant. MMR does NOT recommend the use of 15# or 30# roofing felt(tar paper) for this application.

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3. Does that mean I should be stripping all asphalt shingle roofs before laying metaldown? No. MMR recommends the stripping of the existing asphalt shingles,however we understand that time and budget constraints do not alwayscooperate. Here is the recommended best practice for re-roofing over anasphalt shingle roof:

Roof Deck: Inspect the existing shingle roof to determine if it is suitableas a barrier against condensation. Remember that a metal roof will, to acertain degree, develop condensation on the underside of the panels.Those areas that show signs of previous water infiltration (underlyingsheathing is soft or spongy) or damage (missing or deteriorated shingles)should be repaired accordingly and rendered watertight with the use ofice & water shield and an appropriate sealant (if necessary).

Ridges & Hips: Remove the cap shingles to avoid having a hump. Ifleaving exposed with the threat of rain / snow, cover with tarp or roofunderlayment.

Roof Edges: Cut the overhanging asphalt shingles flush with the fascia /rake boards. This will also remove the existing drip edge. This allows youto install your eave / rake trim true to the house with a clean, tight fit tothe fascia boards.

Firring (strapping):i. Hips, Ridge & All Protrusions: 9 - 12 inches of width on all sides

of using plywood, OSB or similar decking (thickness to matchother lumber).

ii. Valleys: 16 – 24 inches of width on either side using plywood,OSB or similar decking (thickness to match other lumber).

iii. Gable & Eave Edges: 1” X 3” spruce firring or similar dimensionlight gauge steel framing, set back from edge by ¼” – ½” so that itdoesn’t kick the trim out.

iv. Interior of Roof Deck: 1” X 3” spruce firring or similar dimensionlight gauge steel framing, installed horizontally approximately 16”OC (use every 2nd course of shingles are your guide).

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4. All of this sounds like a lot of work. Why don’t I just strip the roof? That is a validcomment and many contractors / homeowners decide to do just that. Hereare considerations for both options:

Leave Existing Roof In Place Strip Existing Roof

Pro: No need to pay for dump fees. Pro: No need to pay for new roof

underlayment so long as existingroof is suitable as a barrier againstcondensation.

Pro: If using firring, creates an airbarrier between the metal paneland the roof. This allowscondensation that develops on theunderside of the panel to dry morequickly.

Pro: If using firring, reducesthermal transfer to the underlyingroof deck as the metal panel issitting up off the asphalt shingles.

Pro: If using firring, allows for acost effective leveling of the roofsurface if required.

Pro: Reduces the burden placed onlandfills each year.

Con: Does not allow a thoroughinspection of the roof deck prior toinstalling new roof.

Con: Renders the installation ofsnow guards more difficult andpotentially less secure.

Neutral: Not accepted by all

Pro: Allows you to inspect thecondition of the roof deck andmake repairs as needed.

Pro: Provides for a smooth, flatroof surface on which to install.

Pro: Allows for the proper andthorough installation of an ice &water shield.

Pro: Allows a solid and uniformsurface into which snow guards canbe fastened if required.

Pro: Provides a solid surface for allfasteners.

Con: Added costs through dump &disposal fees.

Con: Added costs through newroof underlayment.

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building codes (hurricane proneareas require installation to soliddeck using specific UL-90 ratedclips).

1. Do you have any pictures that clearly show the strapping of a roof? Absolutely.

(Please refer to the following pages)

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