I builit a sip home in Charleston SC about 9 years ago. None of the interior joints were taped and a vapor barrier was installed over the whole roof. Needless to say I've got some rot issues. The panels are 4' wide 8 1/4" eps with (2) 2x8 splines. The panels rest on a ridge beam and the top wall plate. The spans very, but most are about 11' 7". After talking with the panel Mfg., it's my understanding that the splines are NOT structural due to our location and spanning distance. The outer layer of OSB is pretty bad about 1' down or so, and maybe 4' down each spline. The interior OSB is perfectly fine. I've got to take some pictures and send to the panel mfg but I wanted to get some input from you folks. Any info is appreciated.

Sometimes it's possible to repair these by laying down some 2x4 furring to created a vented "over-roof", which would keep the OSB dry, as long as you don't put anything more vapor retardent than #15 felt over the patched up areas. If it's easy to strip the vapor barrier, do, and replace it with #15 felt, with the more serious weather resistant barrier on the over-deck.

But if the splines aren't structural as-is an over-roof approach may not do it.  Assuming it doesn't need more structural capacity to be added, if the steel roofing can be mounted on purlins it would save a bit of cost relative to an over-roof, since it wouldn't need the decking layer.

If you have to re-engineer it a bit for capacity, it's worth beefing it up sufficiently to handle the capacity of solar panels, should you decide to add some in the future. The cost of rooftop solar has fallen by almost 3/4 since 2008, with no end in sight (2/3 of the cost in the US is currently comes from the "soft costs", and when/where solar has become a commodity those can be reduced dramatically and quickly.) Verifying and building in additional structural capacity now is a marginal bump in the repair cost, but it would pretty expensive to add in the future.

I agree - make it breathable up to some point where exterior air can flow through. Do something to seal the SIP joints - maybe jam in flexible gaskets (can seal even with 200+% movement) or pour in thickened epoxy or urethane glue (blocks the gap and then prevents movement). Spray foam and caulks aren't worth much when it comes to thin gaps or movement. Then tape over it.

Hopefully the SIP manufacturer has some input wrt structural SIP repair. I'll say only that routing out the rotted OSB and gluing in replacement OSB is probably better than what you have now.

I don't think that's fixable

What is the vapor barrier under the metal roof?

The vapor barrier under the metal roof is Tamco metal and tile. In know all of the reasons we're haviing the problem but What I'm trying to figure out is if its fixable without removing the whole panel. Since it's the top skin thats damaged has anyone had any experience with removing the osb where it's compromised and glueing back down a rew piece of? The Splines themsellves are damaged but don't provide any real support anyway so I feel like they could be removed and foamed. Trying to get the insurance company on board is another challenge if anyone has any thoughts there.

IMO, the splines also transfer force from one panel to the other. In your case, they are probably still doing that for the interior side skin. I also see them providing a significant portion of your required strength. 2x8 at 11'7", 24" (vs 48") spacing suggests maybe 50%, even if the percentage of a intact SIP panel may be much lower (ie, perhaps 2x8 splines are normally structurally insignificant).

That might even be a solution - instead of removing splines, add more to the point where the roof would be strong enough even without the SIP.

it's lipstick on a pig!, those panels are shot. They either need replaced with SIP's or rafters, there is no practical way to re-attach a rotted skin.

So tell me if I understand what went wrong here. The seams were not taped but there's a vapor barrier on the exterior side of the OSB panel, under a metal roof. Moisture from inside the house is making it's way through the seams, condensing, and therefore getting the OSB wet?

There was a huge study on this phenomena. If one doesn't tape/air seal the interior SIP joints, hot air will rise, leak past the joints and condense once it hits the cold outside surface, turn into moisture and start to rot the OSB.

The solution is to always tape the interior SIP joints with something like SIGA Rissan. Then as a belt and suspender approach, vent the roof ABOVE the SIP by using purlons or secondary cold roof. Use a breathable roof membrane on top of the SIP like SIGA Majcoat.

Once that is done, your OSB SIP roof will last a lifetime. Remember, all wood eventually gets wet, the important thing building science has learned is that it has to have an opportunity and means to dry. Remove that drying ability and you end up with rot. That's why old 1900's buildings never really rotted that bad because while they were leaky, they allowed air movement to dry wood framing members. Modern bad building practices took away some ability for the wood to dry, so you now have rot.

Ridge areas on roofs are the worst spot because that is the highest point

Here is the article on the SIP rot study

Lbear, great article. As with anything proper building technique is critical. Amazing what some extra cans of foam and a tight vapor barrier would have saved. Sounds like Chris is right. Replacement is the only option. As far as insurance claims, it's the builders liability.

It's a good reminder that a single air barrier on the cold side isn't adequate. A house might do very well on a blower door test and still have interior to cold surface back to interior air movement (or the reverse with A/C). Best to air seal both sides (walls and roof). And build for "breathable to both sides, more so to the cold side".

My architect developed a great detail for a SIP roof. It has the interior taped and sealed with SIGA Rissan, then furred down for a layer of Roxul mineral batts and finally sealed with drywall. The top layer is sealed with SIGA WigLuv at the seams, layered over with SIGA Majcoat breathable roofing membrane, then 2x4 purlons are used for vent channels and another layer of 5/8" OSB is placed over that to which the metal roof attaches to. It's an R-60 roof that allows for vaulted ceiling and 24" overhangs.

No doubt, this requires more time and money to do but it's a bullet-proof system for the SIPs. It's a belt and suspender approach and it's still A LOT cheaper than replacing a SIP roof 10 years later. The SIGA tape is not expensive, more time consuming than anything. The 2x4 purlons are inexpensive and the 5/8" roof sheathing layer is where it's pricier.

Perhaps SIP manufacturers should make SIPs with built-in vent channels. For example, a waffle pattern in the top 1" of foam. A layer of OSB, 2x lumber and then another layer of OSB seems inefficient.

Jonr, your proposed panels are already available from numerous panel manufactures. They are referred to as vented or cold roof panels. Foam or wood spacers are placed in between the foam and the sheathing creating the air space. The height of the space can be dictated. Some of the manufactures cut grooves into the foam and then apply the sheathing which creates the air space. See the images below

Yes, somewhat like that but actual SIPs vs non-structural nail base.

The problem is not with the SIP, it's with the treatment of the joints of the SIP

Sure, but nothing is ever done perfectly. For cold climates, most references recommend, as Lbear did, venting (ie, breathable to the exterior) AND sealing the joints well. The latter is easy, but if the former involves 3 layers of OSB, it gets expensive.

Posted By RGates on 27 May 2016 08:47 PM
Jonr, your proposed panels are already available from numerous panel manufactures. They are referred to as vented or cold roof panels. Foam or wood spacers are placed in between the foam and the sheathing creating the air space. The height of the space can be dictated. Some of the manufactures cut grooves into the foam and then apply the sheathing which creates the air space. See the images below

Those are not SIPS. It's a foamboard panel but it is not a structural panel that can carry roof and dead loads like a SIP can.

Unless I am looking at it incorrectly, that is not a SIP

Posted By jonr on 28 May 2016 11:46 AM
Sure, but nothing is ever done perfectly. For cold climates, most references recommend, as Lbear did, venting (ie, breathable to the exterior) AND sealing the joints well. The latter is easy, but if the former involves 3 layers of OSB, it gets expensive.

It would only be ONE additional layer of OSB on the cold roof side.

The SIP would be: 7/16" OSB + rigid foam (EPS or PU) + 7/16" OSB

Then on the exterior would be 2x4 furring channels and then the 5/8" OSB which is attached to the furring channels. So an additional layer of OSB which is the "sacrificial layer" to which the roof material would be attached to.