Thanks for your input Mongoct. You made some interesting points--didn't convince me though ; ) I just hope my city allows another layer of shingles! My current roof is great so I wouldn't be worried about the warranty.

Re:flashing--the current stuff is fine, but I want it replaced with a different color so that would still be done anyway.

If the whole job is quick and easy, that's fine with me I would just hope the price reflects that!

A house I sold in 1987, built around the turn of the 20th Century, had five layers of shingles. No leaking.

If you want to take your chances and save a few bucks, your choice.

Shingling over flat(single thickness shingles) is a very common procedure. As stated, most codes set the layers at three. Over shingling two or three layer shingles(double ir triple thickness shingles) is not as advisable.

Now, why is that number three?

Basically, it is because of the weight of the material versus the structure of the roof support members and the decking. Most codes call for 24" on center spacing and 1/2" thick sheathing(decking). Roof joists were usually at least 2x6 and had to have cross connecting rafters.

That was fine when 1/2" plywood was well constructed, or when actual boards were allowed for sheathing.

Newer styles of trusses allow for 2x4 construction and many newer 1/2" ply sheets have much more flex. I have seen code compliant roofs sag between roof members immediately after the shingles were installed. And that was using single thichness shingles(minimum weight)

Your house was built within the time frame of which I speak. The older houses which had successful layers installed were built with the older framing and shingles.

Why has your roof lasted without problems? Obviously because it was well built initially. And the materials used were capable of lasting 25 years.

Now, go out and look at your roof. Are any sections/areas sagging? Go up into the attic and inspect the entire underside of the decking and the structure. Any problems?

Just because moisture has not come into the living area does not mean there is no moisture problem.

If there are absolutely NO problems, and the existing shingles are single thickness, and you are going to install single thickness new, and you are willing to face possible problems in the next decade that might force the tear off/repair/replacement of the roof---by all means do a second layer.

However, if you consider the tear off as insurance, you should be good for the next 25 with few worries.

Worthy and Handy mac,

Thank you both for your input---extremely helpful! I will go up in the attic and explore the condition. The roof has had a few tabs come off in storms, but we immediately had them replaced. Otherwise, the roof looks fine---there is quite a bit of the gravelly stuff coming off the existing shingles though.

I do think our house was well built compared to what has been going up in the past decade. I'm going to shop around and see about getting more estimates, but when I call to set them up I'll ask if they will even consider putting a second layer on---if not then they don't need to come.

OK, I just talked to a real live roofer : ) He's been in business for 18 years. He said that he usually does tear offs, mainly because "industry standards" have changed from 20-30 years ago, and shingle companies aren't wanting to fully warranty shingles put on over the top of existing shingles.

I asked about going into the attic to see if there's any sign of damage and he said if I've seen no evidence of water damage on the interior of the house (stains, bubbling/peeling paint, etc.) that there's no reason to look in the attic. He said I might see something that looks like water damage up there, but its really just signs of moisture/evaporation. He's going to come and take a look next week----should be interesting!

You actually do not know if the roof deck has any damage from water until you tear off the old shingles.

The only thing you know is that water has not yet made it through the deck to the attic.

Replacing flashing will go a lot faster in conjunction with a tear off.

Get a 'per square foot' repair cost in any proposal, in case any part of the roof deck needs repair.

I know this thread is old, but in researching this topic I find a whole bunch of alarmists saying "oh my gosh if you don't tear it off you'll never know about those rotten roof decks!" These alarms are most loudly sounded by the groups who will make money from the tear off. I have not come across one person who said "man, I wish I had never done that layover..." Rather it's just people saying "well you never know..."

I know, I know--there are people who have had legitimate issues with this, and not discounting that. But it seems highly overblown.

And in fact, a layover is a common and successful roofing technique that has been done for decades with low incidence of problems as far as I can tell.

I'd never put a second layer of shingles, much less three, on my own house. I don't want the extra weight. I don't want to put a new roof on without knowing if there's damage to the sheathing. I'll get a flatter roof. I'll get a cooler roof that will last longer.

exactly my point--your comments are speculative, not based on experience. as seem to be everyone else's on the web. there has been no research done showing that layovers don't last as long as tear-offs...this is just more conjecture.

Soooo, it really comes down to, are you going to kick yourself in the butt if you go with the double layer and something happens causing you to regret it or not. Sounds like you won't.

ChrisInSD, I can't find any research linking a second, or even a third, layer of shingles with premature failure due to increased heat retention. So, if everything else is alright, I wouldn't object to a second layer. I'd still feel uneasy, though, about a third layer because of weight, flatness, and worries about the condition of the decking after twenty-five or thirty years. Thank you for making me think.

Why would you leave potentially unsound material under your brand new roof? Or put up with added unevenness? When I've gotten roof estimates in the past the cost difference has been essentially trivial, no more than 10% of the cost of the job to take off the old before applying the new. That's not enough to accept the possible or even imagined consequences in my book. Older roofs often don't have a good ice/water barrier at the eves and in the valleys, getting that installed by itself would justify the tear off in my mind.

Toolbelt -- I think that's pretty well stated. I would modify and say I would definitely kick myself in the butt if my decision turned out to be a mistake (who wants to save a penny and lose a pound). But that I think the chances of loosing the pound are 5-10%, not 70% as the internet would lead you to believe.

Mag -- Thanks, I think that makes sense. Like you, I wouldn't do a 3rd layer. I understand that's been banned by many building codes over the years. But a 2nd layer is still up to code almost everywhere because, if the roof hasn't failed and is flat and not curled up etc, then you should be good to go.

RWiegand -- The cost difference in my case is more than 10%. If it was indeed trivial I wouldn't be evaluating the options so carefully. I'm OK with imagined consequences, as long as they aren't real :) I would insist on replacing all of the flashing and new underlayment. Ice barrier is something to consider for sure on an older roof, but I live in San Diego where the temperature never approaches 32 degrees.

This post was edited by ChrisInSD on Mon, Jan 5, 15 at 13:04

Is your current roof 3-tab or architectural?

3 tab

There is a huge difference between the heights between 3 tab and architectural shingles. Most architectural shingles have more than one layer in the actual shingle. That means each shingle weighs more than a 3 tab type---up to twice the weight. Overlaying those means more voids under the new shingles because of how the shingles are constructed.

The practice of overlaying shingles started when 3 tab shingles were the norm.

If a roof with architectural shingles were overlaid with new architectural types, you could have 4 layers of weight in total.

That was part of my point after asking the question, thanks handy, The other thing to consider is that an architectural shingle will not be as flat and laying potentially a 3-tab shingle over and architectural shingle may also give a very wavy or uneven appearance to the shingles.

Agree with these comments. I think tear offs will naturally become impossible given the trend of using dimensional shingles.

3 tabs are flat and easy to layover. The dimensional shingles would "telegraph" their outline through the top layer causing it to appear uneven I suspect.

You could do 3 tab, layover the architectural shingles, then tear off and replace with 3 tab so you can eventually do another layover. ;) Should last you about 100 years...

Uh, the top half of architectural shingles are single layer, so, let's see, a second layer would actually be three layers in some places, four layers in others? Perhaps the best question to ask is just how long do they make roofing nails? ;-)