Ventilation space on a SIP roof covered by metal roofing
I have a 12/12 pitch cathedral SIP roof that has rot at the seams of the OSB sheathing. Located near Green Bay, Wisconsin. I will need to take off shingles, find and repair gaps between panels, replace rotten patches of OSB, and install a ventilated roof.
Would a metal roof over 2×4 vertical spacers be a good choice? Where could I get drawings and info about Metal roofs? What kind of tape is available for panel seams? What kind of waterproof or permeable membranes should I use over the SIPs and under the spacing strips? Would there be a possibility of condensation from the metal dripping back to the OSB sheathing in the winter?
Thanks for any help on this. Allan Poole
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Replies
Allan,
The first issue to address is the fact that with SIPs, the OSB facing is structural. If you have OSB rot on a SIP roof, it's essential to get an engineer and the SIP manufacturer involved to help design a safe method of repair.
Q. "Would a metal roof over 2x4 vertical spacers be a good choice?"
A. I'm not sure what you mean by "vertical spacers," but it is certainly a good idea to include ventilation channels between your SIPs and your roofing -- especially if you have already had problems with condensation and rot. The image reproduced below shows suggested details.
For more information on ventilation channels above SIP roofs, see Air Sealing SIP Seams.
Q. "Where could I get drawings and info about metal roofs?"
A. You can find drawings (details) here on the GBA site, in our detail library. You can also consult an architect, or you can consult manufacturers of metal roofing.
Q. "What kind of tape is available for panel seams?"
A. Most SIP manufacturers also distribute tape for SIP seams. One popular tape is distributed by R-Control. Here is a link to a page with more information: R-Control SIP tape.
Q. "What kind of waterproof or permeable membranes should I use over the SIPs and under the spacing strips?"
A. I would recommend the use of a vapor-permeable roofing underlayment -- for example, asphalt felt or Solitex Mento 1000.
Q. "Would there be a possibility of condensation from the metal dripping back to the OSB sheathing in the winter?"
A. Yes, of course. That's why it's essential to include roofing underlayment.
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Martin's advice is quite sound as usual, but I am going to anticipate subsequent questions you might have.
If one utilizes a high rib style metal roof can you eliminate the second layer of sheathing/underlayment by attaching the metal directly to the furring? Maybe even directly to the SIPs assuming that the depth of the ribs would provide enough airflow? I looked high and low for those answers, and never did find a definitive source. Ultimately I decided the response should be no, for these reasons:
1. Likely the furring would end up having to be turned 90 degrees thus preventing any moisture between layers from running down and out the roof. (I did see one design where the furring was turned 45 degrees, clever, although I think tough to work with screw pattern wise.)
2. Metal alone is noisy in the rain, application on the sheathing seems to dampen that impact noise. Fortunately I learned this ahead of time as we built our pole barn first.
3 Yes, moisture does condense on metal roofs under the right circumstances, happens at our house quite often, even enough for flow down onto the gutters. So it seems smart to let any water underneath drip down on to the second layer of impervious underlayment versus on to the permeable membrane atop the SIPs.
I will note that we did not go with vented drip edge as shown in the above drawing, but rather with Cor-A-Vent behind the fascia. This allowed us to be sure that no ice or snow trapped atop our gutters would impede airflow. I also recall that our calculations showed that we couldn't get the proper volume, balance, and direction of airflow eave versus ridge due to the small holes in the vented drip edge.
BTW, the last time this kind of repair came up I hinted to Martin that it would be neat if an article were written that described the entire process, engineering review through completion. Maybe you can keep us abreast of your progress!
Allan:
I'm no expert, but a couple of observations. The vertical "spacers" (furring,nailing strips), would work fine if it fits a nailing/screwing pattern. I've used lots of the exposed screw roofing with good success, but you would need horizontal furring for this type to screw on or next to the ribs every 9". I'm not sure what standing seam roofing needs. While it is true that horizontal furring is more restrictive to letting condensation moisture drain down, Martin has typically said it works well enough (still provides ventilation,and I think the amount of moisture would be nominal). If you are not comfortable the it would, you could cross hatch using horizontal then vertical about for the fastening surface. IMO, 2x4's are overkill here and 1x;s are fine, since they are lying on or crossing rafters and don't seem to have much of a structural function, but rather a fastening one. But 2x4's have more "meat" for the fastening, if you are uncomfortable or maybe in a high wind area.
Martin thanks for the quick response. My thinking was that with a metal roof, could I create the bottom to ridge ventilation space with wood strips, that would also serve as a fastening base for the metal roof? That way I could forgo the second layer of sheathing. But it sounds like metal roofing is normally installed on horizontal strips or sheathing. M I right?
Thank you Andrew and Howard! I will write a piece on my experience through this project. That is a great idea. These panels were sold to us by a company called R Control, the company is now called ACH Foam Technology. I have asked them to help me with a solution because this can't be the first time this problem has occurred, but so far they have been unwilling. At the very least, I can share my experience so others can avoid my misfortune. The house was built in 1997.
Allan,
Many (but not all) types of metal roofing can be installed on purlins or furring strips that are installed parallel to the ridge.
As Andrew and Howard have pointed out, if you want to avoid the expense of plywood or OSB sheathing above your ventilation channels, the usual solution is to install two layers of furring strips. The first layer is perpendicular to the ridge, to allow for airflow from the eaves to the ridge. The second layer is parallel to the ridge, to facilitate the installation of the roofing panels.
Although it's possible to skip the lower layer of furring strips -- I've done it -- you get imperfect air flow if you do that. You don't want to cut corners in your case, because you are repairing a disastrous failure. You need robust air movement through these new ventilation channels.