Roofing, To tear-off or not to tear-off

"A different roofer said we could just install another layer on my roof and not take off the current shingles.  Do you do that too?"

​​Should a roofer remove the first layer of shingles before installing a new shingled roof?  
 
Many roofers offer the option of installing a layer of shingles over a single layer of existing shingles on a residential home. The IRC codes also allow one lay-over roof on a single-family residential home.  This means if a home has only one layer of shingles, it is safe for the homeowner to install a full layer of shingles over that roof.  
 
Some people would argue that this just increases the water and weather resistance when it’s done properly.  This is incorrect.  Shingles are designed to work in a system.  Modern asphalt shingles require a layer of underlayment; tar-based felt, synthetic underlayment, or rubberized synthetic underlayment is typical. A roofer must remove a few shingles to even see if an underlayment was installed on the first installation.  If not, this is already an issue.  
 
Also, flashings and counter flashings are difficult, if not impossible, to install properly on a lay-over.  When the shingles land against an end wall or side wall, a flashing is required.  Also, drip edge is required in many cities that have adopted IRC 2015 codes.  If the original roof does not have drip edge, the original roof must be cut back and drip edge applied to the wood below. This causes the appearance of drooping on the ends and edges.  
 
My practice of installing a professional and durable roof is to remove the entire roof system down to the plywood, inspect all of the wood decking and repair or replace any issues.  Once the decking is solid and strong, we install a full layer of synthetic felt underlayment with ice and water shield in every valley, penetration, and flashing area. This layer of protection is sufficient to withstand rain by itself.  
 
Once the underlayment is properly applied, the drip edge is installed on all eaves and rakes (the drip edge keeps water from running down and back around the shingle to the wood beneath, forcing the water flow to move out away from the home).
 
Starter shingles are used to create a starting edge for all shingles.  The starter should be applied to all eaves, rakes, and gables.  Some roofers cut corners here and use cheap shingles, they only use starter shingles on the eaves, or some don’t use starters at all.  

Finally, use a good shingle.  Do not cut corners on brand or material.  If it’s done right, this could be the one and only time you buy a roof.  Do it right. Remember, a roof is the most important thing you will install on your home.  
 
Ventilation is arguable.  Some prefer gable vents and no other ventilation. Some also argue for box (or "turtle") vents along the ridge.  I prefer a solid ridge vent with soffit vents installed.  If your home doesn’t have soffit vents, your roofer should take that into account and advise you to either install an after-market soffit vent system or utilize a different ventilation system to create good airflow through and under the shingles.  Ventilation will increase the life span of shingles and help keep them looking good longer.  
 
Once the vents are chosen and installed, hip and ridge caps should be installed.  Do not allow a roofer to use standard shingles and cut them for ridge caps. The shingles are not meant for this purpose and they aren’t flexible enough for the bends they must make.  Use designed hip and ridge shingles for this purpose and enjoy a long-lasting, worry-free roof.  
 
Many upgrades are available.  Ask your roofer about these.  Some shingles offer streak resistance while others offer hail resistance.  Class IV shingles are more durable and proven to withstand higher winds and hail storm damage.  Some insurance companies even reward homeowners who install Class IV with a discount.  
 
Metal roofs are another great option.  From the classic rib-style metal panel to standing seam metal roofing, the metal panel roofing is by far the best roof a homeowner can install.  It’s the most durable and longest lasting option available.  Metal shingles also exist and offer a metal roof with a shingle appearance.  This helps those homeowners with HOA restrictions who want a metal roof.  Be sure to ask about these options.  
 
Many options exist.  Be sure to see as many as possible and make your home your protected castle. I have plenty of ideas to share.

It’s scary out there.  If you as a homeowner don’t know what to look for, you could be taken advantage of by an inferior roofer. Read company reviews, talk to former clients, get referrals from friends, or walk onto a job site to inspect the work your potential company is performing. DO YOUR HOMEWORK!