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.

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

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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Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
EastWestData (www.EastWestData.com )

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

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

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"If con is the opposite of pro, is Congress the opposite of progress?"
Richard Rushing, HCRI
Duncanville, Tx.

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

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.

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Eric Mayberry # 6328
Down Home Inspections

Save those shakes. They make excellent kindling.

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Critical Home Inspection Services
www.Home2Spec.com

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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Accurate Inspections & Consulting, Inc.
847/736-9975
Markus Keller

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.

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.

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Dave

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.

Attached Images

skipplusosb.jpg (130.6 KB, 43 views)

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Richard Moore
Rest Assured Inspection Services - www.rainspect.com
Seattle, Washington

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). (-;

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
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
EastWestData (www.EastWestData.com )

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".

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.

Of course...any decking requires the proper nails. (see the links below for recommended nailing.)

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

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Richard Moore
Rest Assured Inspection Services - www.rainspect.com
Seattle, Washington

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
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
EastWestData (www.EastWestData.com )

Thanks for the input.
Larry W.
South Carolina