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  1. #1
    Robert Alexander's Avatar
    Robert Alexander Guest

    Question OSB on top of cedar shake roof

    I have a home being rehabbed. After peeling back 3 layers of asphalt composition shingles, the original roof is cedar shake. Rafters are 24" on center (real 2x6 rough cut - old 1920's home). There are firring strips that were installed onto the cedar shake to provide a level surface to install the comp shingles.

    I am looking at two scenarios: (a) leave the cedar shake installed with firring strips and install 5/8" OSB on top of this, then install new 30-year architectural shingle roof or (b) complete tear-off down to the rafters and install 3/4" Plywood for support and install new roof.

    The only concern I have with option "a" is using long enough nails to properly secure OSB sheathing to rafters and dealing with valleys (uneven surface after sheathing put on top of shake. I am thinking rolled paper thickness can compensate for this.

    All thoughts appreciated.

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    Last edited by Robert Alexander; 05-08-2008 at 05:18 AM. Reason: typos
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  2. #2
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    Default Re: OSB on top of cedar shake roof

    I vote for "b". Bite the bullet and do it right.


  3. #3
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    Default Re: OSB on top of cedar shake roof

    I can see nothing but problems with (a).

    Starting with: How many layers of roofing do you have?

    First, the reason for limiting the number of layers of roofing is weight - two layers will add more weight to the trusses/rafters than one layer, history has shown that two layers are okay.

    So, let's go back to (a) where you have one layer of roofing (the wood shakes, and another layer of roofing (the 5/8" plywood), and you are talking about adding a third layer of roofing (the asphalt shingles) on top of that?

    As I said: "I can see nothing but problems with (a)."

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  4. #4
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    Default Re: OSB on top of cedar shake roof

    Your roof structure was more than likely not designed to carry that kind of load. (Even at what you have now)

    I would have it completely stripped off down to the rafters and then have the OSB installed, then the comp.

    rick


  5. #5
    Richard Rushing's Avatar
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    Default Re: OSB on top of cedar shake roof

    No Jerry.... read that again.

    He said, "After peeling back 3 layers of asphalt composition shingles, the original roof is cedar shake". That means there are already 4 layers of roof covering!!!

    This is something I can tell you about (for sure).... If you install plyboard or OSB over the existing covering (after removal of all 3 layers of composition shingle, you are still left with wood shake) and as sometimes happens, you have a hurricane come through S Carolina and remove your roof, you will more than likely be buying the whole roof yourself (with no insurance coverage due to an improper install).

    Don't do it!!!! If you do, we will have to put a contract out on you and have a fellow inspector in SC come over there and whack your pee-pee.

    RR


  6. #6
    Ron Bibler's Avatar
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    Default Re: OSB on top of cedar shake roof

    Take it off. take it all off. da da da. ta da da da.

    I vote for Plan #B Read my post on I fired my client yestarday.

    Best

    Ron


  7. #7
    Eric Mayberry's Avatar
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    Default Re: OSB on top of cedar shake roof

    Two problems:
    1. The IRC does not allow composition shingles to be overlaid over a shake or shingle roof ( I recall fire concerns). Cant see how OSB decking, then shingles would make much of a difference.
    2. Will be difficult to insure (I know they wont offer windstorm insurance in Texas with shingles over shakes). Its gonna be hard to convince an inspector that the new decking is there as it will be hidden from view by the cedar shingles from below and the composition shingles above.

    Just my two cents.


  8. #8
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    Default Re: OSB on top of cedar shake roof

    Save those shakes. They make excellent kindling.

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    Default Re: OSB on top of cedar shake roof

    Installing new osb on top of the old cedar is dumb. Spend the money and to a complete tear-off. Beyond that, long enough nails aren't your issue. The real issue is load calc for the existing rafters. Since you plan on using Arch shingles you should reduce the remaining dead load as much as possible. Arch shingles are much heavier than standard 15 or 20 year.

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  10. #10
    Robert Alexander's Avatar
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    Default Re: OSB on top of cedar shake roof

    Load not a problem. Some jurisdictions actually "legal" from research I did. Easy fix though. I called the local county code office and roof requires a tear-off to wood slats. I have called for a complete tear-off (to the rafters). Contractor still wanted to fight me on it so I ordered a code inspection for tomorrow morning. Man is he going to have a bad day. (-;

    Thanks for the feedback.

    P.S. This is same guy who reduced 4" DWV in home to 3" and had four 90 degree bends on pipe between were it left crawlspace to septic tank! That would have to be some agile t*rds to get around those bends without clogging. And inlet tee extended 3 feet into the tank! Blockage waiting to happen six ways to Sunday.

    First home I personally inspected that actually had several layers of shingles installed over cedar shake roof. Learn something new every day. I love it.

    Last edited by Robert Alexander; 05-08-2008 at 06:28 PM.

  11. #11
    Robert Alexander's Avatar
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    Default Re: OSB on top of cedar shake roof

    Quote Originally Posted by Eric Mayberry View Post
    Two problems:
    1. The IRC does not allow composition shingles to be overlaid over a shake or shingle roof ( I recall fire concerns). Cant see how OSB decking, then shingles would make much of a difference.
    2. Will be difficult to insure (I know they wont offer windstorm insurance in Texas with shingles over shakes). Its gonna be hard to convince an inspector that the new decking is there as it will be hidden from view by the cedar shingles from below and the composition shingles above.

    Just my two cents.
    Eric, if you find the actual IRC code for comment 1, can you forward to me? Many thanks.


  12. #12
    David Banks's Avatar
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    Default Re: OSB on top of cedar shake roof

    Quote Originally Posted by Robert Alexander View Post
    Eric, if you find the actual IRC code for comment 1, can you forward to me? Many thanks.
    Not Eric but.
    IRC 2003
    R907.3 Recovering verses replacement. New roof coverings shall not be installed without first removing existing roof coverings where any of the following conditions occur.
    1: Where existing roof or roof covering is water soaked or has deteriorated to the point where the existing roof or roof covering is not adequate as the base for additional roofing.
    2: Where the existing roof covering is wood shake, slate, clay,cement or asbestos cement tile.
    3: Where the existing roof has two or more applications of any type of roof covering.


  13. #13
    Richard Moore's Avatar
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    Default Re: OSB on top of cedar shake roof

    What everyone else said regarding OSB over the existing shakes...

    The only thing is that I keep seeing "down to the rafters" for the tear off. Around here it is usual to leave the original one-by skip sheathing in place, and then install the OSB or plywood over that (our "better" roofers seem to prefer plywood). The weight of the one-by's is negligable and I think it actually makes for a stronger decking. It's by far the norm around here for any home 1900-1980's that originally had a wood shake or wood shingle roof. See photo.

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  14. #14
    Robert Alexander's Avatar
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    Default Re: OSB on top of cedar shake roof

    Richard,

    Good point and great picture! I have actually instructed my contractor to keep the skip sheathing in place for support and install the 7/16 OSB on top of it like your picture illustrates.

    Many thanks for the input.

    I am keeping the cedar shake for kindling (no kidding). It would be even better if I could soak them in water and grill fish over top of them (like cedar planks). (-;


  15. #15
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    Default Re: OSB on top of cedar shake roof

    Quote Originally Posted by Robert Alexander View Post
    Good point and great picture! I have actually instructed my contractor to keep the skip sheathing in place for support and install the 7/16 OSB on top of it like your picture illustrates.
    Robert,

    Here is why that is not the best idea (the best idea is to take the spaced sheathing off and install the OSB/plywood directly on the rafters):

    - The OSB or plywood would not be supported entirely along their ends, unless ... you went to the trouble to cut strips of the same thickness as the spaced sheathing and securely nailed those strips to the top of the rafters.

    - The nails used would have to be enough longer to compensate for the extra 3/4" or more of wood thickness of the spaced sheathing.

    - Even with the above being done, the structural panel sheathing will lose some of the load it is transferring to the structure (the rafters) with the nails through the spacers between the spaced sheathing and through the spaced sheathing (the sheathing now would have an indirect connection to the rafters).

    Jerry Peck, Construction / Litigation Consultant
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  16. #16
    Richard Moore's Avatar
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    Default Re: OSB on top of cedar shake roof

    Here is why that is not the best idea (the best idea is to take the spaced sheathing off and install the OSB/plywood directly on the rafters):
    So...I went looking for professional opinions on this: After searching a roofing forum and visiting 20 or so web-sites, I cannot find a single place where it suggests removing the skip sheathing is the preferred method. The opinions on the forum vary from "it's not necessary" to "you'll have a structurally stronger roof with the skip sheathing left in place". Nowhere on the form could I find a recommendation to remove the skip sheathing.

    The other various web-sites include a number of city guidelines demonstrating the proper nailing methods to apply OSB or ply over the skip sheathing. None of those suggest the preferred method would be to remove it. In fact Robert's own call to his local AHJ resulted in "I called the local county code office and roof requires a tear-off to wood slats".

    - The OSB or plywood would not be supported entirely along their ends, unless ... you went to the trouble to cut strips of the same thickness as the spaced sheathing and securely nailed those strips to the top of the rafters.
    Agreed, but the gaps aren't that large and with 2 nails minimum per slat my gut feeling is that wouldn't be an issue.

    - The nails used would have to be enough longer to compensate for the extra 3/4" or more of wood thickness of the spaced sheathing.
    Of course...any decking requires the proper nails. (see the links below for recommended nailing.)

    - Even with the above being done, the structural panel sheathing will lose some of the load it is transferring to the structure (the rafters) with the nails through the spacers between the spaced sheathing and through the spaced sheathing (the sheathing now would have an indirect connection to the rafters).
    I don't believe that trapping a piece of solid wood between the decking and the rafters (with the right nails) would result in a weaker connection. And...no one is suggesting you nail in the areas with the spaces directly below.

    Jerry, I don't know how often you would see a skip-sheathing roof re-decked down there, and your local codes may be different in hurricane country. Up here in the Great Top-Left, the vast majority of older homes had wood shingle roofs and many "newer" homes had shake roofs. I can't tell you that the skip sheathing is never, ever removed as I'm not able to always determine the original, but I do know that I see it left in place a LOT, both on the homes I inspect and just walking or driving by ongoing re-roofs! The one thing I definitley can say is that OSB (or ply) over skip feels a damn sight more solid underfoot than "new" homes without it. I recently had about half of my own, 1927, roof stripped and re-done as part of a remodel (the rest was new) and now have ply over the skip in those areas. I'm very comfortable with the results.

    I guess, ultimately, Robert should discuss it with his roofer.

    A couple of city examples...
    http://www.ci.clovis.ca.us/UMAP/User...ofing-Plan.pdf
    http://www.ci.pittsburg.ca.us/NR/rdo...ngbulletin.pdf


  17. #17
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    Default Re: OSB on top of cedar shake roof

    Quote Originally Posted by Richard Moore View Post
    Jerry, I don't know how often you would see a skip-sheathing roof re-decked down there, and your local codes may be different in hurricane country.

    Richard,

    They used to install wood shakes and wood shingles down there (in South Florida) on spaced sheathing, until Hurricane Andrew came along and twisted everything apart which was not thoroughly rigid enough to transfer loads to the walls and foundations (and some that were).

    After that, the only sheathing allowed was structural panel - OSB or plywood, 5/8" minimum.

    And, and roof which had spaced sheathing for which the roof covering was replaced had to have the spaced sheathing removed and re-sheathed with those structural panels.

    Guess my bias of wanting the roof structures to stay intact during windy conditions sticks with me.

    Maybe Brandon, resident PE, can add some more insight?

    Jerry Peck, Construction / Litigation Consultant
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  18. #18
    Larry Wilson's Avatar
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    Default Re: OSB on top of cedar shake roof

    Thanks for the input.
    Larry W.
    South Carolina


  19. #19
    John Lampert's Avatar
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    Default Re: OSB on top of cedar shake roof

    This post finally prompted me to get registered. We are renovating a 1930's home with the original metal shingle roof over firing strips. If we had used asphalt shingles, OSB or plywood decking would have been installed over the strips. But, we chose a metal roof for is long life and high solar reflectivity. It was installed over the original firing strips and satisfied the manufactures requirements. By the way this house had zero insulation and this is the main subject of my reply. I try to keep abreast of the latest techology on home constuction and energy efficiency and am applying it as part of the renovation. Section R806.4 of the 2006 IRC allows for unvented conditioned attic assemblies. In upstate SC and western NC (Zone 3), air-impermeable (closed cell) insulation can be applied directly to the underside of the roof deck. With our rough sawn 2x6 rafters, we sprayed 3" of foam at R-7/per inch plus 3" fiberglass at R-13, for a total of R-34. Also, we sprayed the dormer side walls and gable ends with 1" foam plus fiberglass. This area will eventually be finished for living space. We have not used the HVAC zone in this area and have found that it stays within 4 degrees F of the first floor. So even if it was just an attic, there would be major energy savings and the life of an asphalt roof would be longer due to a lower deck temperature. I could go on and on about all the other things, but I will spare you at this time.


  20. #20
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    Default Re: OSB on top of cedar shake roof

    Quote Originally Posted by John Lampert View Post
    This area will eventually be finished for living space.
    John,

    From everything else you posted in your post, I would hope you investigated this too, but ...

    "This area will eventually be finished for living space." means the ceiling joists will then become floor joists, which are required to have a higher per square foot load capacity that 'just ceiling joists used for supporting gypsum board'.

    I will assume that you have already addressed that, and, if you are planning on installing gypsum board on the rafters, that you have looked into their load capacity rating as well.

    Welcome to THE inspectors board.

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  21. #21
    Aric Outlaw's Avatar
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    Default Re: OSB on top of cedar shake roof

    Quote Originally Posted by Robert Alexander View Post
    P.S. This is same guy who reduced 4" DWV in home to 3" and had four 90 degree bends on pipe between were it left crawlspace to septic tank! That would have to be some agile t*rds to get around those bends without clogging. And inlet tee extended 3 feet into the tank! Blockage waiting to happen six ways to Sund.
    WHY WHY! As the professionals who find and evaluate conditions, materials, workmanship etc. and expect the client to take our word as such are we discussing option A to begin with? Who would do this? We write this stuff up and now an inspector wants to construct in this manner? Then, again WHY in the heck would you continue to use the contractor described above to do the work? Why not hire the 12 year old down the street?


  22. #22
    John Lampert's Avatar
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    Default Re: OSB on top of cedar shake roof

    "This area will eventually be finished for living space."

    Jerry,

    Relative to the ceiling rafters that will become floor joists, I plan to sister each member with a new joist in the areas considered "living spaces". Since the old joists are rough sawn, this will also help provide a more level floor. With a center ridge height of only 8"-8" and an 8.5:12 roof pitch, much of the floor area will not be used. We added two shed dormers with a 3:12 pitch to provide adequate height for two bedrooms. These rafters are 2x10's with a 12" gluelam ridge beam. The new bathroom is centered over the center supporting wall. Only part of the original 2x6 rafters will require a finished surface. The ridgid closed cell foam insulation acually stiffens the roof structure, but I have not seen any valid data on this, so it's just a bonus.

    John Lampert


  23. #23
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    Default Re: OSB on top of cedar shake roof

    "Relative to the ceiling rafters that will become floor joists, I plan to sister each member with a new joist in the areas considered "living spaces".

    Hmmm, let's see how many ways this will go wrong...
    - going to match the existing joists in size or is there actually knowledge present to determine correct new joist size, for load req?
    - bathroom, I wonder if it is going to have a tub that fills with water, a terazzo shower base, or a poured and tiled 2 person shower base? That doesn't weigh much
    - doesn't sound like those new joists are going to run ext wall to ext wall
    - I wonder if they will even run load bearing wall to load bearing wall or ext wall; might be helpful
    - I didn't realize it would be so easy to install new joists after the insulation is installed.
    - Sister joists any idea how that is done properly or do you just shoot a box of nails into everything?
    I'm really not trying to sound too insulting but there are so many holes in your game plan, I can't but help shake my head knowingly and wait for the disaster 'what do I do now' post.

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  24. #24
    John Lampert's Avatar
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    Default Re: OSB on top of cedar shake roof

    (I'm really not trying to sound too insulting but there are so many holes in your game plan, I can't but help shake my head knowingly and wait for the disaster 'what do I do now' post.[/QUOTE])

    Markus,

    Check my original post and note that the foam insulation is applied to the underside of the roof decking, not in the floor joist cavities.

    The floor joists for the upper level are full-dimension old growth softwood that is very dense. The new joists to be sistered will span from exterior wall to center supporting wall and will be joined using construction adhesive and carriage bolts and/or lag screws. Shims will be installed to support the new joints at the supporting walls. Also, I may have misrepresented the use for these rooms as "living spaces". They actually will be bedrooms.

    As to the bathroom, the tub is a light-weight Sterling 4-piece unit and, yes, if a person wanted to waste water with a tub bath, the floor joists would be stressed beyond 40psi for a short time.

    That you for your generous, kind hearted remarks.

    John Lampert


  25. #25
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    Default Re: OSB on top of cedar shake roof

    Quote Originally Posted by John Lampert View Post
    Also, I may have misrepresented the use for these rooms as "living spaces". They actually will be bedrooms.
    Bedrooms are "living spaces" too.

    As to the bathroom, the tub is a light-weight Sterling 4-piece unit and, yes, if a person wanted to waste water with a tub bath, the floor joists would be stressed beyond 40psi for a short time.
    It's not the weight of the tub, it's the water in it. The best thing to do is to install a shower, no tub, that eliminates the 'will the tub ever be filled with water' issue.

    Jerry Peck, Construction / Litigation Consultant
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  26. #26
    John Lampert's Avatar
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    Default Re: OSB on top of cedar shake roof

    Quote Originally Posted by Jerry Peck View Post
    Bedrooms are "living spaces" too.

    It's not the weight of the tub, it's the water in it. The best thing to do is to install a shower, no tub, that eliminates the 'will the tub ever be filled with water' issue.
    Jerry,

    The only reason I differentiated between bedrooms(sleeping areas) and living spaces is that the IRC does so. Floor live loads for sleeping areas are based on 30 psf and living areas at 40psf. These are not large rooms. The span from exterior to center wall is about 12 feet. In the bathroom, rough-in for a tub unit is already done. Plus my wife wanted the tub, but we always take showers. Tubs are great for washing venetian blinds, dogs, etc. The long dimension of the tub is centered over the center supporting wall and is against the exterior wall on the gable end where the first floor joist runs parallel on the double top plates of the wall below. The other joists running under the tub have been reinforced.


  27. #27
    William Kreth's Avatar
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    Default Re: OSB on top of Skip Sheathing?

    So my wife hired a guy to do our roof. I thought he was our CONCRETE guy, apparently he does it all. Well after he removed the Wood shake, his workers were up there on the roof about to lay the felt right on top of the Skip Sheathing without laying down OSB/Plywood base. I immediately stopped everything and called the foreman who was not on site at the moment. his worker claimed they were going to lay think felt on the Sheathing then the asphalt shingles and all would be OK. I disagreed since the other estimates I had all claimed there would be a solid base added on top of the skip sheathing then felt, then asphalt tiles. So does anyone know of a situation where this would have been an acceptable method? Was I being a Di*k?

    I don't know exactly the spacing of the skip sheathing but I'm guessing about 5 inches or so. It just didnt seem like a good idea. in any event he agreed to lay OSB first, so I'm less concerned, just curious. On a side note, while pulling staples and nails from the skip, before the OSB showed up, his worker stepped on a weak board and fell through the roof and put his foot through my ceiling. I looked up through the hole at him and said "See, THIS is why I Need Plywood" I don't think he saw the irony in it though.

    Last edited by William Kreth; 09-21-2009 at 05:31 PM. Reason: spelling

  28. #28
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    Default Re: OSB on top of Skip Sheathing?

    Quote Originally Posted by William Kreth View Post
    the asphalt shingles and all would be OK.
    With the composition asphalt shingles you need solid sheathing underneath them.

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  29. #29
    Mitchell Toelle's Avatar
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    Default Re: OSB on top of Skip Sheathing?

    Quote Originally Posted by William Kreth View Post
    So my wife hired a guy to do our roof. I thought he was our CONCRETE guy, apparently he does it all. Well after he removed the Wood shake, his workers were up there on the roof about to lay the felt right on top of the Skip Sheathing without laying down OSB/Plywood base. I immediately stopped everything and called the foreman who was not on site at the moment. his worker claimed they were going to lay think felt on the Sheathing then the asphalt shingles and all would be OK. I disagreed since the other estimates I had all claimed there would be a solid base added on top of the skip sheathing then felt, then asphalt tiles. So does anyone know of a situation where this would have been an acceptable method? Was I being a Di*k?

    I don't know exactly the spacing of the skip sheathing but I'm guessing about 5 inches or so. It just didnt seem like a good idea. in any event he agreed to lay OSB first, so I'm less concerned, just curious. On a side note, while pulling staples and nails from the skip, before the OSB showed up, his worker stepped on a weak board and fell through the roof and put his foot through my ceiling. I looked up through the hole at him and said "See, THIS is why I Need Plywood" I don't think he saw the irony in it though.
    You're in California. As far as I know a permit is required to be issued when replacing a roof covering. Some of the reasons for this are: AHJ needs to verify nailing at the new sheathing, review of proper moisture barrier install, review of proper flashing and drip edge, etc.

    Pull a permit. This will help you when it comes to monitoring your Contractor/ Concrete guy and his methods.


  30. #30
    Ted Menelly's Avatar
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    Default Re: OSB on top of cedar shake roof

    As most have said.

    Strip it to the framing.............period. Any future sale all that will become painfully aware when they want thousands upon thousands of dollars for removal all the multiple old layers along with the new sheathing and roof covering. Think of the cost involved now (that is why you are not doing it right ,is it not) and now add on some thousands more for removing the new layers.

    Enough said. Stop the roofer now before it has gone to far and spend the extra money now, evven if you have to borrow it.


  31. #31
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    Default Re: OSB on top of cedar shake roof

    I have let the contractor install 1X's in between the existing 1X4's so the deck would be solid.

    It is normal practice around my area that when wood shakes were installed the lathing was spaced the width of a 1X4 apart. The contractor would lay down a solid row of 1X4's up the rafters, then nail every other 1X. Then they would remove the 1X from between the others, leaving a space that another 1X would fit into.

    I have also allowed OSB or plywood to be installed on top of the existing 1X's and required them to break the joints on top of a rafter. Then nail the sheathing directly over the rafters into the 1X's. There would only be a 3.5" space where the sheathing was not over a solid piece. Two 8d nails in each 1X4 on the edges, one in every 1X in the field. Not a problem in my opinion.


  32. #32
    William Kreth's Avatar
    William Kreth Guest

    Default Re: OSB on top of cedar shake roof

    I think I MIS labeled this post, I did not mean to imply the guy was going to lay OSB on top of my Wood shake, he has taken it down to the Skip, replaced any questionable 1x with newer pieces and is laying OSB on top of the skip sheathing. In the meantime I have told him to hold off on any more work until I find out about the permitting process. I don't want to get in trouble. Thanks for the Advice everybody.

    Last edited by William Kreth; 09-22-2009 at 09:23 AM. Reason: forgot a word

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