According to Union Corrugating, most roof types simply weigh too much to be installed over top of one another without adding additional structural support to the roof. Roofs are designed to support a predetermined amount of weight and the installation of subsequent layers of heavy roofing materials such as asphalt roofing shingles can cause structural damage. In these cases, removing the old asphalt roofing shingles is usually recommended, but this creates a large amount of landfill waste. In fact, asphalt roofing shingles make up over 3% of all municipal solid waste, or 6.8 million tons annually. Installing metal roofing over shingles can help solve these issues.
Because metal roofs are so light yet so strong, metal roofing panels can typically be easily installed over your existing asphalt roof shingles without the need to add structural reinforcement. The old asphalt roof shingles under the metal roof simply serve as an extra layer of insulation and water protection and pose no problems to the new metal roof. Of course, each metal roofing over shingles installation is different, so consult your licensed metal roofing contractor to confirm if your roof is suitable for a metal roofing over shingles installation.
A steel roof is a lifetime investment, lasting 50 years or more if it’s a good one, noted by Family Handyman. Asphalt roofs typically last 20 to 30 years, depending on the installation and shingle quality. So, the investment might be worth it, but the range of choices makes shopping for a steel roofing system more complicated than you might think.
If you want to install steel over asphalt, your best choice 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.
Finally, the frame creates a ventilation gap that helps prevent ice dams in the winter and cools the roof and the house in the summer. To vent the roof, holes are cut in the old roof above the soffit, which pulls air through the soffit vents underneath. Cool outside air then travels under the shingles to the vented ridge, pulling out moisture and heat.
Question 1: Can You Install Metal Roofing Over Shingles?
YES. One big advantage to metal roofing is its light weight, which means metal roofing can be installed over an existing shingle roof. However, there are some things that you will want to check out before doing it this way. For one, you will need to check with your local building department and ask if this is an approved installation method. More than likely they won’t just come out and say yes. They will want to make sure that the plywood that the existing shingle roof is attached to is in good shape, meaning it has no soft spots. The reason for this is so when the metal roof gets installed, that the screws have something to bite into. Can you image screwing a metal roof, or anything for that matter, into rotten wood? More than likely the screws won’t hold and that is something you definitely don’t want happening if you want your roof to stay on during one of those afternoon thunderstorms we get here in Florida.
- Tricounty Metals
Question 2: Why leave shingles when installing a metal roof?
Find a better roofing contractor. The old roofing should be removed, underlayment installed and furring strips screwed down for the metal to be fastened to. Don’t let the installer screw the metal to plywood sheathing. It won’t hold over time. If the old roof needs to be replaced why leave it there and not make sure there are no underlying problems before covering it over again? Also, the extra weight could be too much for your framing and cause cracking or other structural problems.
- Todd Shell, Angie’s List
- Some metal roofing contractors are trying to convince people that you do not need roof wrap under metal roofing – presumably so they can get their initial installed cost more competitive with other roofing materials. Don’t fall for that – metal roofs do leak at times, and without the roof wrap you have nothing to keep the water from saturating and rotting the sheathing and rafters, destroying your entire roof.
- Also, if your roofing is flat sheet raised seam (flat panels except with bulges up for the seams) rather than corrugated, the batts or nailing strips (though roofing is not fastened with nails) should be discontinuous – small gaps are needed every few feet to allow hot air to flow up to the ridge vent (which you should definitely have installed), otherwise it just becomes an oven between the roofing and the sheathing, plus the sheathing gets much hotter and makes your entire attic hotter in the summer.
- It saves having to fill up landfills with old shingles.
- It ultimately increases the thermal resistance (R Value) of the roof assembly, actually increasing energy efficiency compared to tearing off the old shingles. What happens is that some of the heat which does pass through the metal shingles now gets stopped by the R Value of the old shingles, rather than flowing direct into the attic. Otherwise, that heat will flow direct to the roof deck and into the attic.
- It allows the property owner to spend discretionary dollars on a better roof rather than on removing the old roof and disposing of it. Going over the old shingles also avoids the potential issues that can occur, like unexpected rainstorms, when old roofs are torn off.
- Unknown, Angie’s List