People have been installing metal roofs over layers of asphalt shingles for decades – in fact, it used to be common practice and likely would have been considered wasteful in terms of time and energy not to do it. This practice is done less and less with asphalt on asphalt shingles, yet many people still find it acceptable to bury a shingle roof under a metal roof.
There are pros and cons to most methods for home improvements, and roof-overlays are no exception. On the positive side, not tearing off the old asphalt shingle roof when installing metal roofing can save time and money. It can also arguably increase the R-value (insulation characteristics) of the roof and act as an increased sound barrier. On the flip side, not tearing off the additional layers of the current roof increases the load (weight) of the roof, void or reduce manufacturer warranties, may impact the aesthetics of the new metal roof, and keep serious construction issues hidden.
Here are the benefits (or what might look like benefits) and disadvantages of installing a metal roof over an asphalt shingles roof, along with things you might not have considered and, ultimately, whether you should do it or not.
Saves Time And Money
The most common reason for doing a roof overlay is that it saves money. The homeowner can save money on both the labor costs and the landfill fees. If this is the primary motivation behind burying the existing shingle roof underneath your new metal roof, you should put a pencil to paper and do the math on what you actually save.
For example, in Texas, crews are only paid $10 per square (100 SF of roof) for tear-off, and the customer is charged $15-20 per square. An average asphalt roof is 30 squares – so the labor savings are anywhere from $300-$600. The landfill charges for the additional layer are generally around $200. The result is that the homeowner is not saving thousands of dollars as they might think, but rather a few hundred dollars. We have to ask ourselves, is the tradeoff worth it?
If your motivation is more the effort required to tear off the old roof, you can’t argue with that — tearing off an existing shingle roof is hard and dirty work. It’s definitely easier to bury the old top under another, although you will still want to install new underlayment. However, tearing off the existing roof is not as time-consuming as you might think, and the work itself is quite satisfying.
Increases R-Value And Sound Barriers
It’s also decidedly true that additional roof layers provide “free” increased R value and acts as a sound barrier. Asphalt shingles have an R value of around .44, and wood shingles have an R value of .97.
Installing foam insulation boards under roofing materials can be price prohibitive. Another option is to spray foam on the underside of the roof deck. Both of these options cost more but result in far better R-values.
Generally, the goal for a roof is to have an R-value of at least 30. If you are after the soothing pitter-patter of the rain on a stormy night, you might not want much R-value at all!
Increases Roof Load
Shingles vary in how much they weigh, but usually you can bet on them weighing between 150 and 250 pounds per square. Metal generally weighs about 150 pounds per square. By doing a roof-overlay, the weight of the roof can almost double when you have two layers on it. If the house is built to handle the load, it is not an issue, and honestly, adding this amount of weight is pretty minimal for most structures. Metal weights significantly more than asphalt per square foot. If there is any doubt, consider consulting with a structural engineer.
Reduces Manufacturer Warranties
Every manufacturer will be different, but it’s worth reading the manufacturer warranty to see if they will provide a warranty on a roof installed over a shingle roof. Berridge is one metal manufacturer with specific requirements for the roof sheathing – if the old roof was not torn off, there would be no way to verify that the sheathing met the requirements for the warranty.
There are also underlayment requirements that state that it must be in “good condition,” which means tears, holes, dried out, wrinkling, etc., have been repaired or replaced. Again, this would be difficult to guarantee on an overlaid roof.
Along this same vein of thought, you should also check with your local building department/HOA to ensure that they do not have any stipulations against a roof overlay.
Roof overlays can appear wavy or bumpy after the installation. This is less pronounced when metal is the top layer of roofing (rather than a second layer of shingles), but it can still be evident that the roof plane is not consistent. This, in turn, increases oil canning. Oil canning is the naturally incurred waves when a metal roof is fastened to the sheathing or purlins. The fasteners need to be appropriately driven (not under or overdriven), and the natural occurrence is a slight indent in the roofing material.
If the installer has to install over a layer of shingles, “perfect” depth will be more difficult to judge consistently because the installer is now dealing with a “shingle sandwich” instead of just the base layer and the roofing material.
*If you're not looking to replace a roof but give it an updated look, read this guide on whether you can paint shingles*
Covers Up Unknown Problems
The biggest reason not to do a roof overlay is that you never know exactly what you are covering up. Leaks in roofs can go undetected for years, resulting in rotten sheathing and even rotted rafters and trusses, not to mention mold. As a roofing contractor, due to these liability and warranty reasons, I will not ever do a roof overlay for a customer – the risk is just too significant.
If there is a leak issue present that is not resolved and continues to occur, to fix it, you will have to remove the metal and shingle layers to find the base layer that is the issue. This process can be costly – especially if the metal layer is a standing seam. With standing seam metal, you cannot simply remove one or two panels because of how they are interlocked – thus, the roof often must be torn off back to the edge of that plane of the roof.
If you decide to do an overlay, be cautious about trapping condensation under the new metal roof. You might consider laying down furring strips to provide a gap between the two roofs for better airflow and ventilation.
Should You Install Metal Roofs Over Asphalt Roofs?
Overall, yes, you can do an overlay of a asphalt roof with a metal roof, and lots and lots of people do it. In my experience, the risk is not worth the minimal savings incurred by burying the old roof, and I would personally advise against it. Roofs can be extremely expensive investments, especially metal roofs (metal roofing panels are expensive). At the end of the day, you will have a far better product if you do it right and exert the extra time and money before you install a metal roof.
Similar to the debate over installing architectural shingles over 3-tab shingles, it's better to just tear off first.