Roofing over existing shingles – Is two shingle layers a good idea?


multiple layers of roof shingles

Roofing over existing shingles - A good idea?

There are always a handful of potential customers throughout the span of a year that ask us if we can install a second layer of shingles on their roof without removing the first layer. As a rule, we do not install a second layer of shingles on a home. We, as a local roofing contractor in the Albany, NY area have made this decision after seeing first hand how a second layer of shingles is really not the best option for the homeowner as well as a huge liability for the roofing company. Although it may seem like a good idea at the time, this almost never ends up being a good solution in the long run for the homeowner as well as the home itself.

Below is a list of benefits as well as disadvantages to installing a second layer of shingles on your home. As you can see from a glance, the disadvantages far outweigh any possible advantages by a significant amount.

Why roofing over existing shingles may be a good idea

Although there are many disadvantages that exist when you have a second layer of shingles installed on your roof, there are a few advantages that are often considered by the homeowner. The major one, although not by much, is the cost savings perspective. We go into detail below by showing you both the advantages of having a second layer installed on your roof as well as the disadvantages that may go along with the install.

1. Lower cost for the homeowner

According to Home Reference, a complete tear off roof installation costs about 25% more than installing a roof overlay. This cost savings comes from a few different areas. Since the original layer is not being removed, there is little waste. The only waste that is incurred will be coming from the cut-offs from the new shingles, the ridge caps and other random waste resulting in low dump fees. On top of saving on dump fees, there is no labor cost related to the tear off of the original roof and the labor involved in cleaning up the shingles from the ground.

2. The convenience factor

In an ideal situation, not having to remove the old shingles is convenient for the roofing contractor since little cleanup is needed and no tear off is necessary. It is also convenient for the customer since there is little risk of damage to landscaping or flowers due to shingles being removed and less hazard around the house from all of the added roofing debris.

3. The Time Factor

Not having to go through the tear-off process allows for the roofer to get though the installation process much quicker. As long as the current roof is not having active leaks and no repairs are necessary, the work is completed much faster. The roofing contractor is also skipping the underlayment installation so that also saves time and is beneficial from a cost savings perspective.

Did you know?

According to Metropolitan Engineering Consulting and Forensics, the main cause of roof systems failure is not due environmental factors like wind damage. Rather, the most significant source of shingle damage occurs from the initial installation of the shingles. According to their observations, "shingle fastening errors, incorrect fastener placement, poor attachment at the eaves" were some of the main errors during the initial roof installation that led to roof system failure.

Why is installing a second layer of shingles a bad idea?

Once you get over the few initial benefits of getting the second layer of shingles installed, you will quickly see that those few benefits are far out-weighed by the disadvantages. Listed below are 14 disadvantages that may have you rethink having a second layer of shingles installed on your roof.

1. Manufacturers Warranty Voided

Most, if not all roof shingle manufacturers will void any type of shingle warranty included in a roof installation if they are applied over another shingle layer. If by chance they do warranty a second shingle layer, they will do so only if a certified roofing contractor performs the work and does the roof installation according to their provided manufacturer specs. Often times it will be easier to just do a tear off than installing a second layer following the manufacturer specs. You can read the fine print on the manufacturers shingle warranty for the particular shingle you want to install and you will see how quickly the warranty is voided even if the shingles are not being applied over a second layer.

2.Does not fix ice damming

If you have experienced ice damming or leaks of any nature on your roof, adding another layer of shingles on top of the current layer of shingles may not solve the problem. Ice damming typically occurs near the eaves where the cold air from the outside of the home meets the warm air from the attic or the outside wall of the home. Rapid freezing and thawing will cause ice to form on and beneath the shingles and cause leaks to occur when it melts. The only way to fix this is to ensure that you have an adequate amount of insulation in your attic that will eliminate warm air from entering the attic cavity and have the proper ice and water membrane installed underneath the shingles.

Since the roofer is simply adding another layer this does not solve the underlying underlayment issue and the ice damming and leaks will return even after the installation of a second roof layer. The ice damming will continue to occur when the conditions are ripe for ice damming in the winter.

3. Shorter Shingle Lifespan

Installing more shingles onto your roof adds another heat absorbent layer during the summer. That heat, over time will shorten the lifespan of the shingle causing it to warp, blister and shed granules. It will also take on the shape of the shingle layer beneath it over time. If the new shingle is thicker, it may not be as noticeable. If moisture gets trapped beneath the two layers, the trapped moisture can break down the shingle. As the moisture makes its way beneath the surface, it will cause deck rot and the developing of mold. Often times, the lifespan of the second layer is halved.

4. May not be allowed by code

While most townships allow for a second roof layer to be installed on the home, some do not. The added weight of the second layer of shingles combined with snow sitting on the roof can add a significant amount of weight and stress on the roof rafters. The roof rafters, especially on newer homes where the contractors try to get away with the most cost effective option for framing, are not designed for that amount of weight.

For example, in the Greater Albany, NY area, all municipalities will allow for a maximum of two shingle layers on the roof. Although, just because it is allowed does not mean that it is the route to take for the roof replacement on your home.

5. Tons of additional weight, literally!

If an average bundle of architectural shingle weighs between 65 and 80lbs, an average roof size of 20 squares(2,000 sq.ft.) will add 3,900lb - 4,800lbs of additional weight on the house structure with the installation of a second layer of shingles. That extra weight is a lot of stress on the framing of the house. Below is a picture of a roof that Home Evolution was hired to completely rebuild after it collapsed due to the extra stress on the framing caused by multiple layers of roofing material combined with trapped moisture in between roof layers.

roof collapsed due to added weight of multiple roof shingles and moisture

6. The Heat Factor

Although lighter shingles tend to absorb less heat from the sun when compared to darker shingles, the fact still remains; shingles absorb heat. Asphalt, one of the main components of a shingle will always absorb heat. If you’ve ever walked barefoot on an asphalt driveway on a hot summer day, you have an idea of how hot the shingles can get on the roof itself. By adding a second layer, you now have two layers that are constantly absorbing heat. That heat absorption will cause both layers of the shingles to deteriorate more rapidly than it would installed as one layer over roof decking.

7. Trapped moisture on the roof

Any moisture that becomes trapped in-between the shingle layers will eventually cause damage to the roof decking, insulation as well as the shingle itself. Keep in mind, just because you install a new layer of shingles over the old layer, that does not stop the old layer from deteriorating even further underneath the new shingles. Once that moisture reaches to the decking, you can expect the decking to rot and decay, insulation to get wet in the attic cavity and even rot the roof trusses that provide support for the roof.

8. Extra roof preparation

Shingles are designed by the manufacturer to be installed on a flat surface in order to seal properly as well keep the surface waterproofed. A roof with 3 tab shingles installed on it is still fairly flat as 3 tab shingles are non dimensional and maintain a fairly flat surface. If the roof has architectural shingles installed on it however, new shingles need to be thicker and the first course of shingles need to be “rip cut” in order for them to sit properly on top of the old shingles. Even though there is a new layer of shingles installed, any imperfections on the original layer like blistering, dips and gaps will eventually begin to show as the top layers seal down and begins to take on the shape of the older shingles.

9. Roof & shingle repairs still necessary

Roof vents, pipe boots and flashing throughout the roof still need to be replaced or repaired in order to waterproof the roof properly. Avoiding this will almost guarantee roof leaks. Any necessary repairs to the original layer of shingles still need to be repaired before the second layer is applied.

Since shingle repairs are still necessary, this often creates a problem. There is usually a small margin of price difference between a complete roof replacement with shingle tear off and a shingle over. If extensive repairs need to be done, or unforeseen damage arises during the installation, the homeowner as well as the roofing contractor have two choices to make;

1. Do the necessary repairs (comes with additional cost)
2. Do the shingle over and not do the necessary repairs.

If the necessary repairs are done, most likely this will bring you up to the same cost, or even more than having a complete tear off done with the job. Avoiding the necessary repairs will drastically diminish the lifespan of the new roof as well as cause leaks. Either way, the customer ultimately pays the true cost of having the roof done in the form of upfront cost or delayed cost when a leak occurs. If the roof ever leaks due to poor workmanship from a roof-over, chances are there is no workmanship warranty involved and the homeowner is stuck with paying for repairs, again.

10. Underlayment and roof decking can't be replaced.

As technology has progressed, new underlayments have come out on the market that are composed of synthetic materials to replace felt paper as a water barrier. Ice & Water membrane is now a standard part of any roof installation in colder climates. Ice buildup can occur near the eaves and in valleys during the cold winter months. Since this is fairly new technology, most older roofs still have regular felt paper installed on them.

11. Curb appeal

If the original layer of shingles installed on the roof were architectural shingles, the second layer will always take on the shape of the original shingles. If the original layer of shingles had some blistering, curling, dips or other imperfections on them, chances are they will eventually appear on the top layer. Also, if there are dips on the plywood or areas where the plywood has lifted, that will also be visible even with a new layer installed over it.

12. Home Resale Value

If you ever plan to sell your home, the second layer of shingles will always show up on the inspection report done by any potential buyer. This will then be used as a bargaining chip to either take a haircut on the selling price of the home or force you to replace the roof at your own cost in order to sell it. Either way, the money you save by not doing a tear-off in the first place will be offset by the cost of replacing a roof again with 2 layers of shingles on it!

Insurance companies are now cracking down on multiple roof layers and will either only insure one layer on the homeowners insurance policy or will not issue the homeowners insurance policy at all if they see that there are two layers of roofing materials on the roof. Two layers of shingles are an increased liability for the insurance company to insure.

13. Future roof repairs

If the roof ever leaks, repairing two layers of shingles instead of one can be a nightmare. It is often difficult to pinpoint a roof leak on a single layer of shingles as water tends to travel along beams, rafters and on the roof decking. It becomes very labor intensive to repair two layers of shingles in order do the repair properly. Once you remove both layers of shingles to perform the necessary repairs, you will then need to install both of the layers back on. Installing only one layer back on will show a dip on the roof where the repairs were performed and potentially cause sitting water in the repaired area causing unwanted leaks.

14. The liability factor

Although you may think that there is less liability involved in not having to remove the original roof shingle layer, that is not necessarily true. Leaving the original layer on the roof creates more liability for the homeowner for several reasons. Replacing the roof the next time will be costly, repairing any potential leaks will be costly, and the added weight sitting on the roof will be a liability in itself. You also run the risk of loosing insurance coverage and take a hit on the resale value of the home if you ever plan to sell it.

From the roofing contractors perspective, installing the second layer of shingles without removing the first opens them up to liability from a workmanship perspective. If the roof ever leaks, the labor involved in fixing the leak properly will be extensive and expensive.

So is roofing over existing shingles really a good idea?

Although there are many disadvantages to getting a second layer of shingles installed onto your roof, ultimately it is the decision of the homeowner to make. If choosing to install the second layer, be sure to discuss the entire process with the roofing contractor performing the work to ensure that everything will be done to ensure that the roof lifespan can be maximized, even with a second layer. Getting everything in writing, including workmanship warranties can make the process a smooth one and clear up any misunderstanding as to what will and will not be done, as well as what will be covered under warranty in the case that a leak occurs after the roof is installed.