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New sheathing over old ashpalt shingles?

[email protected] | Posted in General Discussion on July 23, 2012 01:26am

We have an “Economy Dwelling”, that has 3/8 plywood for the roof sheathing.  And when the new shingle roof was installed about ten years ago, they didn’t pull the backer off the sticky on the tabs.  The building is in a high wind area, and all of the tabls are breaking off. 

I requested quotes to strip off the existing shingles and felt, and then install 5/8 sheathing, ice & water, new felt, and new asphalt shingles. 

One of the guys who gave me a cost proposal for the new roof proposed to install the new sheathing over the top of the existing roof, and save teh cost of the tear off. 

Is this a good idea?  There won’t be money in the budget to touch this roof again for twenty years. 

I know I could do it, and it would probably last the ten years left untill I retire, but I don’t like to work that way. 

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  1. cussnu2 | Jul 23, 2012 02:58pm | #1

    You aren't supposed to pull the backer off the sticky tabs.  Read the mfg instructions and you will find this out.

    http://www.gaf.com/documents/clear_plastic_release_film..._leave_on_or_take_off__-_steep_-43-821-v2.pdf

    Adding another layer of plywood OVER shingles and plywood and then adding additional shingles sounds like a fly by night solution and could well exceed the design loads for your roof framing.

  2. DanH | Jul 23, 2012 11:13pm | #2

    Right -- the tabs aren't supposed to be pulled off (run full speed from any contractor who says otherwise.)  Tabs breaking off probably is due to cheap shingles, but could be exacerbated by the lousy sheathing. (Although another possible cause is installation late in the season when the sealing strips don't have a chance to be sun-activated before cold/windy weather sets in.)

    3/8 is definitely too flimsy for roof sheathing, though.  Pay the money to do it right and rip off the old shingles.

  3. sapwood | Jul 24, 2012 11:57am | #3

    Installing new sheathing then new shingles over an existing shingled roof is a really really bad idea. I think that one qualifies as the #1 bad idea ever posted on Breaktime. 

  4. DanH | Jul 25, 2012 06:19am | #4

    Note that I have seen this done, and I would consider doing it on an old shed, or maybe even an old house you knew was going to be torn down in 5 years.  But it will look like shit, give you at most half the life of doing it right, and really bump up tear-off costs when it needs to be redone.

    It's only something I'd consider as a stop-gap to get an old rickety house through to the end of it's life.  Much better, if cost is a big issue, would be to use a roof coating to stretch out the existing roof a few more years.

  5. cussnu2 | Jul 26, 2012 09:03am | #5

    It always interesting when an original poster goes silent after the first post.  Have to wonder if they didn't get the answer they wanted so they went elsewhere to ask again.

    1. DanH | Jul 26, 2012 10:19am | #6

      Maybe Jigs is just speechless, in awe of our enormous literary accomplishment.

      1. [email protected] | Jul 26, 2012 12:04pm | #7

        No, Jigs is just busy.

        Haven't had much time to relax.  I'm trying to get contracts in place and money committed before it gets "borrowed" by the folks that fight fires. 

        I was pretty sure of what the answers would be, before I posted, but, thought I would double check. 

  6. Piffin | Jul 27, 2012 08:00pm | #8

    DEFINITELY NOT a good idea

    Also what is this about pulling baaker off?

    The strips are there to keep the shingles from sticking together  in the package. You do NOT need to pull thjem off.

  7. Piffin | Jul 27, 2012 08:00pm | #9

    DEFINITELY NOT a good idea

    Also what is this about pulling baaker off?

    The strips are there to keep the shingles from sticking together  in the package. You do NOT need to pull thjem off.

    1. [email protected] | Jul 30, 2012 06:51pm | #10

      The site is high, cool and windy.

      I know that the manufacturers say not to remove the backer.  But local experience, has made removing the backer the standard practice. 

      The roof temps won't hit 100-degrees, the wind blows constantlly, and the site is in the high desert so the wind tends to carry dust. 

      The local installers either peel of the backer and stomp on the shingle, or put a bead of adhesive at the top of th shingle, and a dollop of adhesive per tab, and stomp on the shingle.  The shingles won't ever adhere if you don't do one or the other, because it never gets warm enough for the adhesive to flow out from under the backer and adhere.  If you don't do something to adhere them, in ten years the tabs break off, in the winter when the winds is blowing 30 to 40 miles an hour, and the temperature is at about 10 below. 

      I didn't think laying new sheathing over the old roof was a good idea, but the low bidder did propose it, and I wanted to get feedback from knowledgable nuetral parties, before I just said no. 

      1. DanH | Jul 30, 2012 08:30pm | #11

        The plastic strip prevents the shingles from sticking together in the bundle.  Once the shingles are spread out and fastened, the adhesive strip touches an area with no plastic strip, and the plasic strip (which should remain attached to the back of the shingle, not the adhesive strip) touches an area with no adhesive.  Superstition may say to remove the strip, but all it does is create more trash to blow around in the wind -- it serves no useful purpose.  (Unless, perhaps, you have super-cheap or out-dated/improperly stored shingles where the strip is sticking to the adhesive vs the back of the shingle.)

        "Failure to bond" is certainly a problem.  Around here, shingles that don't get hit by full sunlight for 2-3 days before temps drop below maybe 50 are at risk of blow-off.  In these cases the manual application of adhesive may be necessary.

      2. Piffin | Jul 31, 2012 10:26pm | #12

        Peeling the strips will bo absolutely NOTHING to help the shingles seal. That strip is tp prevent them from sealing while stacked in the package. As soon as you ofset them the 5"+ to install them the sealing material is removed that distance away from the strip.

        Local practice? BS

        Too bad you don't have roofers there if that is the case.

        Adding fresh sealant in areas like you describe IS a valid method to ensure wind resistence. It is what we all did before sealdowns were standard fare. I stioll recal when you had the option to buy sealdowns for an extra dollar a square

  8. kevinbrown966 | Aug 06, 2012 03:28am | #13

    Yes, I would say that a new roof is necessary as 20 years down the road, you will have to replace your shingles anyway so, why not now.  

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