Your new homeowner’s metal roof is undeniably beautiful. However, the biggest surprise you heard from them is they spent less than a month to replace their old asphalt shingle roof. Is this possible? As long-time roofing servicers of London ON with experience and knowledge, we can attest it is possible. On the other hand, we do not attest it can maximize the material’s performance. A metal roof over shingles problems can be quite a handful and creates many compromises.

However, we’re not saying you cannot achieve great results. Then again, there’s a reason why local building codes in London ON have strict regulations to prevent metal roof over shingles problems they’ve heard many times. The Spruce highlights one of the most common metal roof over shingles problems in the last few years.

Metal Over Shingles Problems: What You Need to Know

Metal roofs can be installed over your existing roof without tearing off shingles, provided local building codes allow it. While shingle removal is the preferred route, tear-off is messy and raises the cost of the job.

A potential problem with this type of installation is trapped water vapor. If trapped between the metal roofing and old roofing, moisture can build up and cause mold and rot. But roofers can install a vented metal roof that eliminates this potential problem. Or, installing the new metal roofing over furring strips (1 x 3s or similar) will raise the metal and provide a ventilating air pocket between layers.

Make sure to consult local buiding codes before having a metal roof installed directly over old shingles. Some jurisdictions may require full tear-off whenever a new roof is installed. (Continued)

Roofing underlayment plays a huge role in preventing any form of leaks and infiltrations. It protects your attic from poor insulation too. Now, imagine a layer of faulty asphalt shingle roofs supporting a roof-wide metal roof. Roofing contractors will try their best to prevent leaks, but there is no such thing as zero risks. When the leak starts, it will flow on the aged asphalt shingle roofing and the poor underlayment beneath it.

Professionals advise homeowners to select the best roof underlayment specifically for use with metal roofs. Englert Inc has a great post about metal roof underlayments that help prevent metal roof over shingles problems.

Choosing the Right Underlayment for Metal Roof Installations

Underlayment products for use beneath standing seam metal roof systems should be carefully selected to provide a roof system that performs optimally throughout the life of the building. Several considerations must be addressed in the roofing system design, including ensuring that the four barriers needed in any building enclosure i.e., water barrier, thermal insulation barrier, vapor retarder, and air barrier are provided and are in the correct location for a given climate.

Long term, in-service roof performance can be affected by several important factors including: the metal roofing system and its expected service life, the climate at the building location, roof slope and geometry, and ambient conditions (temperature/relative humidity) within the building.  Improper selection of the roof underlayment may allow roof leakage or entrap moisture due to vapor diffusion or air exfiltration, which may accelerate deterioration of concealed components, shortening their service life and necessitating repairs.  Further, to provide longevity the underlayment should match the intended service life of the metal roofing system. (Continued)

Lastly, it does save you plenty of money and time to tear off your old roofing shingles. However, perhaps the biggest concern of metal roof over shingles problems is the cost. “But my neighbor said they saved a lot of money.” The only choices you have with metal roof overlays is to use either designer metal roof shingles or standing seam variants. Using affordable yet durable and efficient corrugated roofing is out of the question.

The Family Handyman discusses this process and the costs associated in an excerpt below.

The Real Costs of Installing Metal Roof Overlays

If you want to install steel over asphalt, your best choice for installing metal roofing is to use a steel shingle system like the one shown instead of the more common (and more expensive) standing seam steel. In a standing seam roof, long, single panels run vertically all the way from the eave to the ridge. In contrast, steel shingles are small panels installed in horizontal rows much like other shingles.

Better steel shingle systems, like the one shown, install over a framework of 2x2s. This framework has three big advantages. First, because the frame is screwed directly to the rafters and the interlocked shingles are screwed to the frame, the roof can withstand winds of up to 120 mph without damage. Second, the frame creates a flat and straight surface even when the shingles are badly curled or the roof ridge is bowed. (Continued)

Metal roof over shingles problems encompasses so many of its advantages that we cannot advise any London ON homeowner to proceed with it. Instead, allow Cub Roofing or professional local roofers with experience to tear off your roof and make the best and most accurate installation possible. Contact us today to know more how we will proceed with your project now!