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another roofing ?

| Posted in Construction Techniques on September 5, 2004 04:43am

i was wondering if someone could give their opinion on a question that i have. i need a new roof and my sheathing underneath the shingles is 1 x 6 with minimal (less than a 1/4″) gaps. after the tear off can i felt and shingle or lay a layer of 3/8″ plywood over the 1 x 6 felt and shingle or tear everything off to the rafters and sheath the whole roof w/ 1/2″ ply and felt and shingle? any help would be greatfull.

THANKS TO ALL

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  1. User avater
    Dinosaur | Sep 05, 2004 04:58am | #1

    If the 1x6 is in good shape, there's no reason to tear it off or cover it with ply. There is nothing wrong with a board roof, whether t&g or plain edged. Since I can no longer buy T&G 1x6 spruce, I roof-deck small buildings with square-edged 1x6. This is a perfectly viable way to go, especially with the price of plywood these days.

    Unless your roof deck is in really rotten shape, leave it alone and just replace any rotted boards or sections with new 1x6. It's a lot easier and cheaper to do spot repairs on board roofs than it is on plywood.

    The main advantage of sheathing a roof with plywood is that it goes on faster. Remember, it's not the wood that keeps out the weather, it's the roofing. All the wood does is hold up the roofing so it can do its job....

    Dinosaur

    'Y-a-tu de la justice dans ce maudit monde?

  2. Piffin | Sep 05, 2004 04:58am | #2

    If your sheathing is solid and fairly smooth, you can lay shingles over it, but it will look better and last longer ( asphaltic shingles anyway)

    if you have a smooth surace. We often lay a 7/16"osb or a 3/8" plywood over the old board sheathing to skip the gaps. No need to tear off the old sheathing unless there is rot.

     

     

    Welcome to the
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    Knowledge FHB Campus at Breaktime.
     where ...
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    1. Miko | Sep 05, 2004 07:05am | #3

      What a bizzare coincidence.  I planned on posting this same question  tonight.  I have a house built in 1907 with a 12/12 pitch on the front(east) with sweeping (curved) eaves. In the middle are 2 dormers (north and south facing) 12/12 pitch with sweeping eaves.  At the back (west) the pitch changes to 4/12 up to about 18" from the peak where it is 12/12.    The 12/12 pitched front and dormers have 1x4 lathes with cedar shingles and covered with composition.  The gap between the lathes is about  3 1/2" to 4".  The rafters are old growth 2x4's (fir?).  The front has a knee wall and the rafters are uniformly spaced 24" on center.  The dormers 2x4 rafters are  spaced anywhere from 32" to 38" o.c. with sweeping eaves.  The 4/12 pitched back has 2x4 rafters haphazardly spaced 24" to 28" o.c. with 1x10's without a gap as the sheathing and composition on top.  I want to know if it is acceptable (and to code) to remove the composition and cedar shingles, lay 3/8" or thinner plywood or OSB (it has to bend at the sweeping eaves), and top with felt and composition.  

      1. User avater
        rjw | Sep 05, 2004 02:33pm | #10

        In my area they tear off all shingles and sheath over the "skip sheathing" with OSB, but I'm not sure of the thickness required; usually I see 1/2 (7/16?)

        Sojourners: Christians for Justice and Peace

      2. Hubedube | Sep 05, 2004 07:06pm | #11

        First, check your DIY basement's low heating question

        1. Miko | Sep 06, 2004 05:41am | #17

          What DIY basement heating question?

      3. Piffin | Sep 06, 2004 03:33am | #13

        How codes compliant that is will depend on your local codes. I would be extremely hesitant to do that way without adding rafters since the sopans between is so great. The 3/8" ply won't do that without noticeable sags. you could do it by doublking up the ply with adhesive between and be solid there, but then you are adding a lot of weight to the 2x4 rafters. depends on local variances and snow loads 

         

        Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

    2. User avater
      jonblakemore | Sep 05, 2004 07:19am | #5

      Piffin,

      Why do you recommend sheathing over the 1x6's? Are you concerned about vertical variance or the spaces themselves? Sheet goods are supposed to be space 1/8", I can't imagine that gaps of <1/4" would be a problem. 

      Jon Blakemore

      1. Piffin | Sep 06, 2004 03:43am | #14

        It is pretty much standard in the trade to call for patching any spacings over 3/8" to 1/2" with metal scrap. loose knots also. usually when deck sheathing is laid in green, by time for re-roof, they have shrunk to leave spaces abiout that size - though most I work over is 1x8 and 1x10 or more. I commonly see spacings of nearly an inch from shrinkage.

        If this roof was laid with 1x6 KD then it probably didn't shrink so much and really is only up to 1/4" spacings. it isn't the vertical alignmet that I concern myself wit, but the movement. Nail shingles down to board that allow movement - especially if there is lack of support under the asphalt for 1/2" or so - at hose seams, then the shingles will be broken from loading of foot traffic, snow, winds, etc long before the warrantee is up. most manufacturers mention smooth, solid or sound surfaces under the shingles when laid. That is one reason they sometimes disallow roofing over old plys of shingles.

        Anyway - if we have to spend a lot of time working patches of metal over lots of spread open seams and loose knots, it is far easier and cheaoper to simply add a ply of 3/8" over all. Another thing I like is when the roof is over a hundred years old, it gets pretty loose as far as wood fibres goes. Adding the layer of olywood lets the nails have something to hold onto. 

         

        Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

  3. Miko | Sep 05, 2004 07:18am | #4

    Hey mightyJIM;  I would only be concerned if there was a large span between the rafters and low pitched roof .  Also are you in an area with heavy snow fall?  This weight , over a large span, could  cause the 1x's to sag and create a problem over many winters.  This is probably why they use clips between sheets of non- tongue and groove plywood.

    1. User avater
      Dinosaur | Sep 05, 2004 08:23am | #6

      He has no need to worry about the 1x6 sagging between rafters unless the span is well over 24", which is quite unusual. Today many codes permit 7/16 OSB on 24" centers for low-pitch roofs, although I personally think that spec insufficient, and would deck such a roof with 5/8 ply. 1x6 has approximately the sag resistance of 3/4" ply, perhaps a mite less, but it is certainly equal to 5/8 in this respect. If it is unplaned 1x, as is commonly found in many older roofs, it's sag resistance is even greater.

      Piffin's comment about getting a better looking job with asphalt shingles over a smooth roof deck is correct--spaces and unequal heights can indeed telegraph through the felt and shingles on a low-pitch roof. But this is mostly an æsthetic issue, not a structural one.

      H-clips are a quick and easy substitute for 2x blocking when sheathing a roof with sheet goods. But they, too, can telegraph through the shingles, and they do not support the edge as well as a continuous piece of wood backing the joint. On the other hand, they interfere less with ventilation.

      Dinosaur

      'Y-a-tu de la justice dans ce maudit monde?

      1. Miko | Sep 05, 2004 09:49am | #7

        Hey Dino:  Thanks for the reply.  What about my idea of going over my 1x4's with ply or OSB?  I need some flex in watever I choose because of the sweeping eaves.  Which has more play to it  and will the building codes allow this added weight on the 2x4 rafters?

        1. seeyou | Sep 05, 2004 02:25pm | #9

          You need to check your local codes, but the sheet goods aren't going to add much weight to the structure. You'll probably be adding more weight by switching from cedar to asphalt. Those old growth, full 2x4's are probably stronger than new nominal 2x6. Don't tear the 1x4 off. If the swoop is more radical than 1/2" OSB or ply can bend to, use 2 layers of 1/4" on the swoop and switch to 1/2" when you get above it.

        2. Piffin | Sep 06, 2004 03:45am | #15

          You might be OK going over the strapping/furring strips with 3/8". I first thought you would be going all the way down to rafters. 

           

          Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

        3. User avater
          Dinosaur | Sep 06, 2004 05:22am | #16

          Board sheathing is ideal for swept or curved eaves, obviously, but if there is a good reason to add a layer of sheet goods on top of the existing boards, you can bend ½" ply to a reasonable curve if you kerf the inside of the curve about an eighth of an inch deep every 1-2" all through the section you want to bend. It's real handy to have a nail gun for this kind of work because of the large quantity of nails you have to sink, LOL....

          Here's how it's done:

          View Image

          If you haven't got a second carp to stand on the sheet to help you bend it, deep-throat Sandvik bar clamps are useful. My lead carp weighed more than either me or the second carp, so he got to do the standing part here....

          The curved piece of 2x spanning from the rafters of the lower roof to those of the porch roof is called a coyau in French; I don't know what it's called in English, sorry.

          On the code question, I couldn't answer not knowing what code you're subject to, but if you've really got 2x4 rafters, I'd say code or no code you'd be pushing things. Try to re-roof without adding any more material than you absolutely have to. Use scrap flashing as Piffin suggests to cover any egregious gaps; it doesn't weigh much for the square footage it covers.Dinosaur

          'Y-a-tu de la justice dans ce maudit monde?

          1. Miko | Sep 06, 2004 06:05am | #18

            After further review it looks like the thing to do is put 3/8" plywood over the 1x4 (strappping)  on the 12/12 portion of the roof and keep the 1x10's on the 4/12 portion of the roof, replacing the damaged lower section with 3/4" plywood  H-clipped to the 1x10s and add support to these 2x4 rafters.  The dormers prevent  12/12 front and 4/12 back of roof from meeting so the different thickness of sheathing (1 1/8' front and 3/4" back) should not be a factor.  Thanks for the great advise and photo and thank you mightyJIM for letting me piggyback on your post.

  4. seeyou | Sep 05, 2004 02:19pm | #8

    Rather than pay for sheet goods, upgrade to a heavier dimensional shingle. They hide little imperfections better. I would, however, hand nail them. That way you can feel it the nail hits the gap and adjust accordingly.

  5. Shep | Sep 05, 2004 07:49pm | #12

      One thing to be aware of with board sheathing is that if the roofer uses nail guns, the nails might hit the gaps between the boards and not really hold the shingles. A roofer I know only hand nails roofs like that. Some older houses I've seen have had some pretty wide gaps between the boards ( 1/4-1/2")

      Depending on where you are and how much wind you get, this might or might not be a big problem. If in doubt, I would add a layer of plywood

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