How to Install a Metal Roof Over Shingles



How to Install a Metal Roof Over Shingles

How to Install a Metal Roof Over Shingles – What to consider when putting a steel roof over asphalt shingles. Need a new roof? Covering your existing asphalt shingles with metal roofing might be worth the expense. Here are some pros and cons.

I frequently am asked whether homeowners and contractors should remove roof shingles from a home’s roof before a new metal roof is installed. My answer often surprises people. However, it is based upon experience with tens of thousands of installations over many years.

Can you put steel roofing over asphalt?

Can you avoid tearing off your old asphalt shingle roof by putting a steel roof over it? Maybe. Most codes now allow no more than two layers of roofing before requiring a complete tear-off. So, if your roof has two layers, it may require a complete tear-off, even though steel is lighter than asphalt. Codes vary, so call your local building department to get the straight story.

What’s involved

Anatomy of a steel roof

This is what it looks like when you install a steel shingle roof system over an existing asphalt shingle roof.

Detail 1: Venting—the eaves

To vent the roof, holes are cut in the old roof above the soffit, which pulls air through the soffit vents underneath.

Detail 2: Venting—the ridge

Cool outside air travels under the shingles to the vented ridge, pulling out moisture and heat.

A steel roof is a lifetime investment, lasting 50 years or more if it’s a good one. 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 (Detail 1) above the soffit, which pulls air through the soffit vents underneath. Cool outside air then travels under the shingles to the vented ridge (Detail 2), pulling out moisture and heat.

The vast majority of installations of the metal roofing systems produced by my company are over existing shingles. The low weight of metal roofing is one thing that encourages this. Steel shingles weigh about 1/4 what asphalt shingles weigh and aluminum shingles weigh about 1/8 what standard shingles weigh. I would say that in most cases, less weight is being added to the home by the new metal roof than what the current shingles have lost due to granule wear and oil evaporation. In other words, the total roof weight was probably greater when the last layer of shingles was installed than it will be when the metal roof is completed.

Pros and cons

Stone-coated steel shingles

Best Stone-coated steel shingles on a Budget

Stone-coated steel shingles HD Images

Material cost per 100 sq. ft.: $200 to $350
Labor cost per 100 sq. ft.: $150 to $450
Weight per 100 sq. ft.: 120 to 140 lbs.
Wind resistance: 120 mph
Resistance to hail damage: Excellent
Over an asphalt roof? Yes
Do it yourself? No

Painted steel shingles

Best Painted steel shingles on a Budget

Painted steel shingles Hd Images

Material cost per 100 sq. ft.: $125 to $250
Labor cost per 100 sq. ft.: $150 to $650
Weight per 100 sq. ft.: 65 to 85 lbs.
Wind resistance: 120 mph
Resistance to hail damage: Very good
Over an asphalt roof? Yes
Do it yourself? Yes

Standing seam steel

Standing Seam Steel on a Budget

Best Standing Seam Steel HD Images

Material cost per 100 sq. ft.: $300 to $550
Labor cost per 100 sq. ft.: $400 to $1,200
Weight per 100 sq. ft.: 140 to 160 lbs.
Wind resistance: 80 mph
Resistance to hail damage: Good
Over an asphalt roof? No
Do it yourself? No

Standard asphalt shingles

Best and Cheap Standard Asphalt Shingles

Standard Asphalt Shingles HD Images

Material cost per 100 sq. ft.: $30 to $85
Labor cost per 100 sq. ft.: $75 to $250
Weight per 100 sq. ft.: 240 to 300 lbs.
Wind resistance: 60 to 80 mph
Resistance to hail damage: Good
Over an asphalt roof? Yes
Do it yourself? Yes

I like going over the old shingles for 3 reasons:

It stops the need 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. The benefit of this is during the summer.
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 potentially damaging issues that can occur, like unexpected rainstorms, when old roofs are torn off.

Still, ultimately, it is the property owner’s decision but I would have no qualms about going over a layer of old shingles in most cases. It’s not uncommon to go over multiple layers either actually. On older homes, of course, I do put the caveat out there that if there are signs of an existing weight issue or old leaks, those need to be addressed and that may require removing the old shingles. If there are concerns about the integrity of the roof decking, then pull-out resistance tests using the roof fasteners can be performed.

One other thing to keep in mind is that building codes, where active, usually require no more than two layers of roofing of any type on a structure. There have been instances of building inspectors waiving that requirement but that is not an easy thing to get accomplished.

How to Install a Metal Roof Over Shingles Brown Roof

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Steel roofs can be noisy and unattractive in residential settings. However, if the steel roof is installed over an existing asphalt roof, noise shouldn’t be a problem. And, as you can see from the photos, if you pick a good-looking style from the many choices available, your house definitely won’t look like a cattle barn.

A steel roof is expensive, but if it lasts the life of your home, it might be worth it. The type we show here costs about twice as much to have installed as a new asphalt roof. But some steel shingle manufacturers offer do-it-yourself systems that cost about the same as an installed asphalt roof. These systems aren’t attached to a frame, but instead install directly on the old roof. Consequently, they don’t have the same ventilation advantages.

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