Hey yall, I just got a set of plans back from building inspector for a small house I’m about to build. The plans called for a 26ga. 5-vee on a 12:12 pitch, screwed to 2×4 horizontal “purlines”.
My plan was to use the 2×4’s (on 2 ft centers) as a kind of “ladder” to work off of, installing the metal first, and then later cutting osb rips, and nailing them to the underside of the purlines, so I could afterwards install netting and pack the rafter bays with blown cellulose. Any air gaps between the osb rips and the rafters would be sealed. The osb would not contact the metal. There would be a 1-1/2 inch air space between them.
The building dept. made a note on the plans that says not to do that, but instead to install the decking atop the raters, and then put the horizontals atop that, and then the tin.
I am curious as to what the code books say about it, and why. This is a new code requirement in these parts. Most all of the older homes using “tin” roofs were, until now, “skip sheathed”.
I have done it both ways, and feel that it is much easier and safer to do it my way. Since I am doing this one myself, and am getting fragile in my old age, I want to convince the inspection dept to allow it. But I just wanted to air it out here first.
I Thank you all in advance.
Replies
Your profile is blank. nobody can know where this roof is a happening that way. Click on your name here and update/edit it please.
If you are just gathering info for a fight with the building dept wherever you are, you are probably wasting your time.
Wind is a factor, but not in the way BB suggested. The winf can create negative pressure over a 1-1/2" space same as it can over an attic.
But there is a lot of stress on the whole roof system anyplace that experiences high winds. The continuous plane of sheathing directly attached per code nailing schedule creates a shear plane the resist racking or collapse. I would not even consider of doing it your way unless in an area where I could be certaain the winds would never blow over about 20MPH
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>>Your profile is blank. nobody can know where this roof is a happening that way. Click on your name here and update/edit it please.
Sorry. S.E. US. We do have hurricanes and tornadoes.
>>If you are just gathering info for a fight with the building dept wherever you are, you are probably wasting your time.
For now I am just gathering info.
>>Wind is a factor, but not in the way BB suggested. The winf can create negative pressure over a 1-1/2" space same as it can over an attic.
A 1-1/2" airspace would exist in either scenario.
>>But there is a lot of stress on the whole roof system anyplace that experiences high winds. The continuous plane of sheathing directly attached per code nailing schedule creates a shear plane the resist racking or collapse. I would not even consider of doing it your way unless in an area where I could be certaain the winds would never blow over about 20MPH
OK. Would you consider doing it if you added an INTERIOR layer of osb fastened directly to the rafters, underneath the drywall?
In our jurisdiction we have to put house wrap right below the tin. Reason being that any warm humid air that migrates up will condense on the underside of the tin because it is pretty cold in winter. At theat point it will ride down and out on the houswrap or at least be absorbed into the dry winter air ventilating through the ribs.
Have a good day
Cliffy
Thanks Cliffy
PS I really like your personal quote
Don't know if it's the reason or not, but in a previous shop that I had, we had a terrible condensation problem on the bottom of the metal in the winter. The metal was on purlins with encapsulated fiberglass under it. Water would drip out at the tops of skylights and at the wall.
"Let's go to Memphis in the meantime, baby" - John Hiatt.
http://grantlogan.net/
Lots of old metal roof barns around hear literally rain condensation inside durring certin times of the year/
My shop has a light gauge metal roof on let in purlins @2' oc. Interior is drywall with R40 blown in f/g above it. It has a 2' roof overhang with vented soffit panels every 2' and the standard ridge vent. I checked it several time last fall, winter and spring for condensation and it was clear.
Might be just blind luck, but so far that system has worked for me.
Dave
Benjy,
I built exactly to your specs. 12/12 roof-- 2x4 purlins 2'oc - serious hurricane alley.
Piffin advises against omitting the sheathing, and his experience level is eons beyond me, but I feel this is a much better system.
I used treated 2x4s with 2 20p sinkers at each rafter, simpson connectors every 4 ties. I also have 4 (2 per side) 3.5"x 16" gluelams supporting the rafters with ties from the beams to the purlins.
I used foil in the rafter bays for insulation-- tight to the purlins and sealed to the rafters. On the bottom of the rafters I've got bubblewrap foil with 5/16 luan below that. I'm amazed at the coolness of the building- no condensation problems.
Common practice in S.La. is to apply metal panel directly to sheathing- tar paper between- no purlins.
While I didn't see any total roof failures for Rita or Katrina- there was a lot of metal panels gone missing due to screwing into sheathing alone.
Louisiana just adopted IRC2003 and I don't have a copy-- my 2006 IRC allows for "spaced sheathing" on metal roofs.
How the code applies to wind areas in excess of 110mph I'm unsure-- I have a 2 day seminar the 1st week in November to address our high wind area.
The purlins were cca treated--- I've since done 2 other roofs where I put down 1x4 purlins (ACQ) over shingles. 25 miles inland- not near the wind exposure- but I want to go pull a couple of screws (galv.) to check for corrosion.
>>I used treated 2x4s with 2 20p sinkers at each rafter, simpson connectors every 4 ties. I also have 4 (2 per side) 3.5"x 16" gluelams supporting the rafters with ties from the beams to the purlins.
I like the idea of using a long strap from the purline directly over the birdsmouth, running down across the rafter to connect to the plate.
This particular plan has a rafter plate fastened directly to the deck on the second floor, so you could catch that, along with the exterior wall studs below. You could do the same thing with the knee walls, which are about four feet high. The knee walls, along with the collar ties on every rafter, and 3/4 strapping inside all give lateral support, and should resist racking of the building, I would think, but this is part of the reason I am asking this question, because if this is a structural issue, I would think there could be alternatives other than decking the roof with osb.
I can see how condensation that runs down onto horizontal purlines and unprotected insulation or stopping and rafters might cause rot concerns, but if upward migration of moist interior air is blocked, there should be minimal condensation on the underside of the tin. Pressure treated purlines would only partially adress this problem.
Sounds like you got some good info on metal roof construction details but let me ask you a question: You said: >> I am curious as to what the code books say about it, and why. << What code are you talking about? Do you thing that the whole US uses the same building code book? Oh yea.... this is an internationally accessed web site... I wonder if the Australian code applies? Or, maybe we can find a Canadian code that supports your proposed building technique for the house that you are building in south Texas?
hey Matt
I asked about what the code books (plural) say
I wasn't asking what all the ssbc code books say, for instance. They all say the same thing. LOL.
Anyway, in my area I am told that the local inspectors use MOSTLY ssbc, but have been known to riff their own tunes.
Thanks all, for the comebacks.
I just met with the inspector, and he says I can sheet the underside of the rafters, and, problem solved. So I may do that behind the kneewalls, and tounge and groove the ceilings, or some variation.
At any rate I am relieved that I won't have to go to the extra labor and expense of sheeting a 12:12.
At any rate I am relieved that I won't have to go to the extra labor and expense of sheeting a 12:12. How is it less expensive and less labor to apply ripped sheathing to the underside of the purlins between the rafters rather than over the rafters?
hey tx,
I plan to frame and roof this house by myself. Normally I would sub it out, but the high price of lots in my area, coupled with the desire to keep long term mortgage costs as low as possible ( I plan to hold on to it for a rental) made me decide to approach it this way. Decking a 12:12, felting, and installing toe boards is simply more time consuming, not to mention more dangerous. The second floor deck will make a nice stable platform to work off of, and two by four horizontals on two foot centers are relatively easy for a 53 year old to clamber around on. The ridge board will be about only 12 feet off the second floor deck, (24 ft wide house) and the rafters will rest atop a plate installed right on the deck. There will be four foot kneewalls running the full length of the roof , except for one shed dormer, and at the stairwell, and the flat part of the ceiling will be nine feet. I will be able to pass materials up through the rafters, and climb up from inside the building, as opposed to climbing up a ladder from the outside. This house is easy to build--even setting the ridgeboard and the 20 ft 2x8 rafters is a breeze. One man can do it, no problem (except that last piece of tin-LOL). Decking a 12:12 is a different story. The difference in expense is in paying for labor to do something I cannot do- or not paying anything for labor and doing it myself.
I can understand and appreciate the idea of rack resistance. When I first posted this query, I was not really sure if the code change had to do with structural integrity, insulation, or both. The inspector says the change was for better structural integrity, but was fine with my solution- to install osb on the underside of the rafters (this in addition to some kind of stopping installed beneath the purlines for airtightness). I always go way beyond code on hurr. clips, etc anyway.
Ben,
OK, understandable answer and what I figured,
I was 52 on the 18th of October and can appreciate: "and two by four horizontals on two foot centers are relatively easy for a 53 year old to clamber around on. Decking a 12:12 is a different story. The difference in expense is in paying for labor to do something I cannot do- or not paying anything for labor and doing it myself.
Live long and prosper.
I guess someone really doesn't need to know for sure what code is adopted for his/her state, much less own a code book if he/she isn't building on a professional basis. :-)
>>I guess someone really doesn't need to know for sure what code is adopted for his/her state, much less own a code book if he/she isn't building on a professional basis. :-)
What does your codebook say Matt?
http://ecodes.iccsafe.org/nxt/gateway.dll?f=templates$fn=contents-nf.htm$cp=Residential%2Fpart00382%2Fchapter01051%2Fsection%20head01070%2Fsection01132.htm$tt=main-nf.htm$3.0
Hello again ,
Well, the house is framed and the roof covered with 26 ga galvalume on 2x4 purlines, etc.
I have decided to have four inches of polyurethane foam blown to the undersides of the metal, covering the two by four purlines and part of the two by eight rafters.In order to satisfy the inspector, I still have to nail 1/2 inch osb to the underside of the rafters before rocking. The framing inspection has been done, and passed, with the stipulation that a verification of the osb sheathing of the undersides will be done at the final.
I have set up a conference with the inspector on tues morning to discuss the issue, and I am not sure how he will react to the idea of allowing the urethane foam to substitute as rack/resistance (eliminating the plywood step). The foam is stong as h*ll IMO, and the osb to the underside of the rafters is overkill IMO.
I would appreciate any feedback.
Why didn't you sheath the tops of the rafters under, or instead of, the purlins?
80558.1
Sorry, I just now read the whole thread.. I get it.
I've done quite a few metal roofs--some w/ purlins and some w/out. In my opinion, the foam would add plenty of strength to the (already strong) roof structure. FWIW, the metal panels themselves are a sort of sheathing. Any time I've built a structure and strapped--instead of sheathed--the roof, after the first few panels are on, the roof stops shimmying...by the time half the roof has metal, it's as solid as a plywood decked roof.
Let me just add one more thing--4" of urethane foam sprayed under your roof structure would make it a fortress. I don't care what the gummint inspector says, It'd be a ROCK. But, having said that, I sure hope you never need to remove or replace a panel for any reason...imagine the horror of that.
>>I sure hope you never need to remove or replace a panel for any reason...imagine the horror of that.
I've been wondering about that. Anyone here ever had that experience?
I've replaced 5-6 metal roofs due to finish failure. All were polyester finish, I think.http://logancustomcopper.com
http://grantlogan.net/
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>>In my opinion, the foam would add plenty of strength to the (already strong) roof structure.
Thanks Pinko. I agree. I really don't know whether the inspector will go along with it- it probably will depend on his mood and his knowledge of closed cell urethane foam. I don't know that much about it either- this is my first application- but so far I am impressed.
As far as an inspector approving your design, you are outside the prescriptive limits of the code already. IRC, SBCC. I assume you are in a wind zone. So no skip sheathing by prescription. So you are at the inspectors whim.
I've done exactly the same as you've done in the past, and I think it's a great system. But you need a stamped approval if your inspector balks.
Closed cell foam would lend itself to great racking resistance IMO, but there is no engineering data to endorse this that I know of. Check with your foam installer and see if they have any data or relationship with the inspector to plead your case. BTW only 2" of closed cell is the norm in my area.
Personally I would install a baffle on the underside of the purlins to keep the foam off the metal and maintain an airflow to a ridge vent. If you use foil faced polyiso. then you get a radiant barrier also.
Just my .02 Good luck
Only thing you can do is find a licensed structural engineer who agrees with you and will provide a stamped letter to this effect to absolve the inspector of responsibility for any future structural failure. I'd talk to him about X-bracing the underside of the rafters with 20" galvanized flashing prior to sheet rock in lieu of OSB for rack bracing. Good luck, a few hundred spent on an engineer might be worth not having to sheath the whole thing with OSB.
I'm sure there are thousands of roofs build just as you've described, but that doesn't necessarily make it a structurally sound idea. I'm with Piffin, and the building official, and think sheathing the entire roof is a much, much better idea.
Everytime there's a tornado or huricane and some houses are standing while those on either side are piles of rubble there aren't many homeowners of the surviving structures complaining that building officials should have been more lax with their requirments.
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