Second Lawyer of Shingles

[John Blake]

I’m a real estate agent not an inspector so forgive my ignorance for this question but I would like too know what the science and your practical experience is regarding putting on a second layer of shingles on an roof without removing the first? This is for roofing in southern Ontario. As you know this used to be common practice in the past.

[Gunnar Alquist]

John,

Here in CA, code allows a maximum of 2 layers of shingles on a roof. However, I believe that some manufacturers do not warranty shingles as a second layer. My own personal observations have been that second layers of shingles do not last as long as single layers, but this is just observation, not a scientific observation. If you are looking for specifics, I suggest you consult with the shingle manufacturer.

[Mike Moser]

For years the UBC allowed up to 2 overlays. This means 3 layers total.
The roof had to have proper support (snow load etc.) . In Minnesota 24" oc rafters with 1/2 " H clipped plywood was sufficent. In my 30 years of roofing ,3 layers was common.The manufacture's in ARMA all allowed this with no difference in warranty but recommended "nesting" as the installation procedure. All always stated the preferred was to tear off . You will find multiple layers for years until the existing stock of roofs are replaced. In all cases the areas used the deck strength as the most common criteria for allowing any layover. The "nesting" as well as the "racking " installation refered to in another post are still in most installation instructions, ARMA and NRCA manuals.

Mike Moser

[Ted Menelly]

I’m a real estate agent not an inspector so forgive my ignorance for this question but I would like too know what the science and your practical experience is regarding putting on a second layer of shingles on an roof without removing the first? This is for roofing in southern Ontario. As you know this used to be common practice in the past.

Well since you are asking for practicle experience I will tell you it is just plain dumb. You are just putting off till later something you should do now. As in removing all roof covering and starting a new.

The top layer of a 2 layer roof will never last as long for one. The chance of a leak around some of the flashing is much greater as no roofer putting a second layer of shingles ever does the flashing right. And third but certainly not least....you are going to pay dearly or someone will when it comes time to remove 2 layers instead of one. They just don't charge you for removing one layer. There is a hell of a lot more work. They will charge you double plus for that labor.

No matter what anyone may tell you (and my opinion is the only one that counts :p ) they are wrong to do 2 layers. The only thing you are saving is the cost of labor for removing that first layer. Buty even then they are going to charge a bit more to put that second layer on as they are going to make that feeble attempt to do all the flashing correctly which is time consuming.

If it makes you feel any better I have been in and associated with every aspect of construction fo 37 years so my opinion may be worth just a little anyway.

[Phil Brody]

As a contractor and inspector we charge $50/sq. for the rip $10 more if there is a 2nd layer so there is a $40/sq. savings instead of ripping every time. Depending on circumstances it might be worthwhile, but ripping to the deck is always preferred.

[Michael Greenwalt]

The option of using two layers vs. one will depend on the region and cost. It is not always dumb to place a second layer. It is true that there could be up to a 10% loss of life expectancy but if the original layer is in good condition then this isn't always a bad choice. The additional cost associated with removing the original layer is sometimes prohibitive to the estimated loss of shingle life. (20-30% cost increase vs 10% reduction in life expectancy).
The new IRC has a breakdown of regions for hail frequency purposes which dictates the necessity to only apply one layer. I don't know that this has been done for Canada (do you even get hail?). If you are not in a high probability region then two layers is still allowed.
A call to the shingle manufacture or thier representative can give you a good answer to your question and would be highly recommended. They can tell you the capabilities of thier product over an existing layer.

[Phil Brody]

I wonder how hail would impact the roof layer criteria ?

[Wayne Carlisle]

4. For asphalt shingles, when the building is located in an
area subject to moderate or severe hail exposure according
to Figure R903.5.

So you are not supposed to add layers in areas prone to moderate to severe hail exposure.

[Phil Brody]

Understand it's code just wondering the logic, when there is 2 layers is it more susceptible to damage ? or is there a greater safety risk ?

[Derek Kelly]

Hi, years ago I replaced a roof over existing, I will never do it again.I suggest strip it and go new.

[Wayne Carlisle]

In my opinion, yes, it is more suceptable to damage.

There will be voids between the layers where the top most shingle is not on a solid surface. Therefore just walking on a roof like this could cause damage.

[Phil Brody]

When putting a second layer over a curling or structurally unsound substrate is a perfect disaster for failure. I don't see voids any greater on the second layer than would be on the first. Just look at the edges on the rakes and see how the contact area is nearly as tight as the first.

[Phil Brody]

BTW the poster is located in Canada, heat is the #1 enemy, the ability to roof over could easily be driven geographically. I have seen many 3 tab 2nd roofs over 30 years old still good, many dirty but still functional White seems to be a big plus.

[mathew stouffer]

Phil,
Not second guessing, but more of an inquirey. I have always been told temperatue fluxuation is the biggest factor in a roofs life expectancy. For example, between seasons in Park City we can have 100+ degree temperature variations. What are you thoughts on this.

[Ted Menelly]

Phil,
Not second guessing, but more of an inquirey. I have always been told temperatue fluxuation is the biggest factor in a roofs life expectancy. For example, between seasons in Park City we can have 100+ degree temperature variations. What are you thoughts on this.


I have a thought.


Don't live in Utah :p

[Phil Brody]

I'm sure they are both the top evils along with wind, depending on the number of days there are big temperature swings vrs. consistently hot days with sun. A different type of failure although. In your climate I'd expect buckling and cracking while a consistently hot climate will dry out the petroleum based product quicker. I guess it would be shades of gray.

[Phil Brody]

What's the matter with Utah too far from Mexico ?

[mathew stouffer]

Yep lots of cracking around here. Thanks.

[Matt Fellman]

As with the others, I highly recommend stripping to the deck so you can check for damage and replace and problematic roof decking or other problems.

Aside from performance issues with the new roof there are also a variety of other things that come up. Gaps at siding to roof seams go away. Sure, you can trim the siding but then when the layers are removed you're in a real pickle. There's the overall weight problem. You're much more likely to have broken rafters or sags in the roof deck.

It's just an inferior way of doing things IMO.

[Jerry Peck]

Another thing to consider, and most installers do not consider this, is that the layer of shingle being covered much be in solid shape and not deteriorated, otherwise it does not make the required suitable base for the second layer of shingles to be applied to.

Being as most roofs are not replaced or recovered until long after they should have been, it is seldom that the existing layer of shingles makes a suitable base for the second layer of shingles. One must almost basically recover the roof before the roof needs to be recovered before recovering is allowed.

[John Kogel]

Nobody's mentioned laying asphalt over cedar shingle. That has been done a lot in my area - the price of new cedar made that not an option, the price of tearoff followed by new decking over the open strapping, ok what's the third choice? We can lay a new asphalt right over your old shingles. Often when that layer failed, they will have laid on a third layer. Now for the buyer of that house, there is only one option, and it will be costly. :(

[Jim Luttrall]

To my understanding, asphalt over wood shingles (no solid decking) is no longer allowed.
Also, insurance is going to be much more expensive if available at all for asphalt over wood shingles.