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grover
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# Posted: 11 Aug 2013 08:57am
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My original plan was to have the trusses, then the osb sheathing which is usually 7/16, then the 30# tar paper then the metal. Well, me being ignorant about metal roofing, I thought you just lay a piece of metal and your screws go through the metal and into a truss. Screws every 24" and everything is great. I found out yesterday that this is not the case. Makes perfect sense now why I see many putting the purlins on horizontally. This gives something to screw into without trying to hit a truss.
The question is now how should I do it? To back up a little I am using scissor trusses that are 12/12 pitch outside and 8/12 inside so all of my insulation will be at the 8/12 part of the ceiling next to living area. I wanted to use the osb decking just to make everything a little more solid and to resist racking of the trusses in a high wind.
Any advice on how to install the metal? Osb, no osb? osb with purlins? I'm confused now.
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OwenChristensen
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# Posted: 11 Aug 2013 10:48am
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I screw to the OSB three or four screws across about three feet apart. I predrill all the sheets at once.
O
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grover
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# Posted: 11 Aug 2013 12:01pm
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What thickness of OSB is needed? I think the lumber salesman delivered 7/16 although I didn't measure it.
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bldginsp
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# Posted: 11 Aug 2013 12:09pm - Edited by: bldginsp
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I'd get info from the roofing manufacturer. I expect they'll tell you 7/16" OSB is fine to screw to, and they'll tell how many screws to use and in what locations. That is, greatest distance between horizontal rows, and spacing along each row. Check out the PDF below on metal roof installation.
There are two types of metal roofing installations- those intended to keep the covered space completely dry and those not, as in a barn. If you want to keep it completely dry, applying sheathing helps a great deal because you can carefully lay underlayment, which ultimately is the final waterproofing layer.
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OwenChristensen
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# Posted: 11 Aug 2013 12:39pm
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I use 7/16 ths. Common sense will tell you that in a high wind area, more screws are needed than in a deep woods or low wind area. Make sure your screws aren't too long. If they go past the threads, they might not hold the rubber washers tight to the metal. Some people like to put the screws in the top of the ribs, as less water runs over the screw hole. I think that deforms the metal, so I put mine in the flat.
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ICC
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# Posted: 11 Aug 2013 01:16pm - Edited by: ICC
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screws in the flat
just tight enough to slightly squish the rubber part of the washer
Usually 1 " screws is the length used for 7/16 sheathing.
The screw must protrude some on the underside.
read the mfg instructions. what they say must be done to have a warranty.
There are special metal to metal screws for securing the rib overlap to the next sheet. They do not go into the wood.
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bluewillo
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# Posted: 11 Aug 2013 05:25pm
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Can I tag along with this discussion? My cabin was already roughed in (meaning the outside construction was done....but nada for inside..no insulation, water, electric).
My metal roof is the only thing above the boards (I know zip about pitches, but there are no trusses.). It's just the 2x6s and the metal up there.
Best insulation advice? My biggest concern is moisture...I'm allergic to mold and mildew. Husband, whose place this won't be and doesn't offer any advice except !@#$$%^& "too much work! too much money to do this right#^&*(....oh, men! get between me and my dream, LOL! Like that's gonna stop me!) has only said it will be horridly expensive. I don't know how to figure sqare footage for the ceiling (maybe, just treat it like a wall, l x w) for the estimate that I tried to get for blown in foam.
so, what do you all suggest? pink fluffy stuff? There's no OSB between the metal and the studs...
been reading here for month, now all please join in the laughter at the newbie! ;)
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WY_mark
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# Posted: 11 Aug 2013 07:09pm
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bluewillo, check into open cell spray foam insulation.
Grover, your roof should have come with a spec sheet, or the manufacturer will have one you can download. different profiles get different fastener patterns. depending on climate many metal roofs want ice and water shield under them, not tar paper (they're not the same).
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terracore
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# Posted: 11 Aug 2013 07:22pm - Edited by: terracore
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In Hawaii the metal roofing is attached directly to the trusses/rafters. No OSB or tar paper is used.
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grover
Member
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# Posted: 11 Aug 2013 10:25pm
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Blue,
Tell us more about what you have. Sounds like you have 2 x 6 rafters?? That's not very big so I hope the structure is not very big. The spray foam stuff is really good but expensive.
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MtnDon
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# Posted: 11 Aug 2013 11:03pm - Edited by: MtnDon
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Quoting: bluewillo no OSB between the metal and the studs...
The problem with this is moisture from the air will condense on the underside of the metal. Then it's anybodies guess whether it will run down the metal and drop off safely or, Most Likely, drop straight down in a miniature interior rain. If there is fiberglass or cellulose insulation under it that ends up being bad news in the form of potential mold, mildew.
More information is needed for what to do for the best and the healthiest solution.
- Climate or general area of the country.
- Will there be a ceiling attached to the rafters providing a vaulted cathedral ceiling effect OR will there be an attic space left above a horizontal ceiling?
WY_mark is correct in that spray on foam is a good solution to the condensation issue on the underside of metal.
2x6 rafters may be fine as far as the structural requirements. However, the snow load would need to be known as well as the span between the support walls; wood species and grade if known, otherwise we use a 'normal' figure. Are there rafter ties included in the design/build? Those connect wall tops horizontally and are nailed to the rafter tails.
They may be strong enough but may not offer space for sufficient insulation for maximum comfort.
Also, no OSB or plywood sheathing over the rafters means the roof is not as rigid as it might need to be to withstand high wind forces. The sheathing would also prevent the condensation on the underside. There is no cheap way to build a proper roof; leaving out parts makes an inferior product.
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Oilerfan
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# Posted: 12 Aug 2013 12:13pm
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I just installed my metal roofing this weekend for my cabin. I am an engineer (not structual...who loves to over design!!) so I went with 3/4" ply with tar paper prior to the metal roofing. I installed screws like ICC mentioned. I remember back in my summer construction days we installed screws on the standing seams but I didn't do that for my install.
I think you can also put a more swanky roof felt down but I thought that might be overkill. Tar paper will be fine for my cabin!
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grover
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# Posted: 12 Aug 2013 01:56pm
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Wow! 3/4 for roof decking seems like overkill. I's hate to be the one lugging those sheets up a ladder but I'm sure those screws will hold.
The debate over where to put the screws (on the flat or on a ridge) seems to be a big debate too. I'm thinking on the flat but what do I know.
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VC_fan
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# Posted: 12 Aug 2013 04:55pm
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Blue, etc - I'll brace for being chastised (that's what usually happens) but my cabin is a very, very temporary use structure and I don't expect to leave it to my kids' grandkids. Its materials cost much less than the average used car. It's a dry place in the woods and never intended to be a showplace. It was supposed to be about a 5 year diversion but it's now been standing about 14 years and is doing just fine. Had I had more time, money, and help I would have done as MtnDon and some of the others suggest - good advice. But I used 2x6 pre-cut rafters (with bottom joists and several rafter ties) to span the 16 feet, purlins over that and then the metal directly over that. Working by myself 2 hours from home and a half mile from the nearest other person made lugging 4x8 sheets of anything up on the 2nd story roof something I just didn't care to do. Afterwards, I put in bat insulation and then put lightweight (8 mm) interior plywood over that. Perfect? No. Best? No. Ideal amount of airspace above the insulation? Uh-huh. Safe? Yes. Sturdy? Plenty. For my purposes it's just fine. I'm in southeast Oho (pretty moist during big parts of the year) directly in the woods under a huge oak tree and have had no moisture nightmares. If I do, I've got so little money in it I'll just redo it if it really bugs me. So my advice is make sure you know what your intent is for the structure before you commit to making so much work for yourself that you may give up on the whole thing. OK, let the verbal beatings begin!
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bluewillo
Member
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# Posted: 12 Aug 2013 06:16pm
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Thanks all for your responses, and to Grover, my apologies for hijacking your thread. I've started my own. VC_fan, your I like your answer best....but this is meant to be my permanent home...I mean to be buried on that 5 acres! 
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terracore
Member
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# Posted: 12 Aug 2013 08:52pm
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Quoting: MtnDon The problem with this is moisture from the air will condense on the underside of the metal. Then it's anybodies guess whether it will run down the metal and drop off safely or, Most Likely, drop straight down in a miniature interior rain. If there is fiberglass or cellulose insulation under it that ends up being bad news in the form of potential mold, mildew.More information is needed for what to do for the best and the healthiest solution. - Climate or general area of the country.- Will there be a ceiling attached to the rafters providing a vaulted cathedral ceiling effect OR will there be an attic space left above a horizontal ceiling?WY_mark is correct in that spray on foam is a good solution to the condensation issue on the underside of metal.2x6 rafters may be fine as far as the structural requirements. However, the snow load would need to be known as well as the span between the support walls; wood species and grade if known, otherwise we use a 'normal' figure. Are there rafter ties included in the design/build? Those connect wall tops horizontally and are nailed to the rafter tails.They may be strong enough but may not offer space for sufficient insulation for maximum comfort. Also, no OSB or plywood sheathing over the rafters means the roof is not as rigid as it might need to be to withstand high wind forces. The sheathing would also prevent the condensation on the underside. There is no cheap way to build a proper roof; leaving out parts makes an inferior product.
What you say about condensation makes a lot of sense. It doesn't get cold enough here for water to condensate in this manner.
Hawaii code requires a lot of hurricane strapping for the roof, so I'm not worried about the winds.
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MtnDon
Member
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# Posted: 12 Aug 2013 09:42pm
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Insulation and vapor barriers is the one construction area where best practice varies tremendously with climate zone. Probably the most varied. Things that are recommended in the north are almost guaranteed to cause trouble in many southern situations.
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creeky
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# Posted: 13 Aug 2013 09:21am
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I put 1" polyiso insulation, tinfoil faced, under my last metal roof. Next time I do it I will foil all the edges. There's a season of small wasps that like to chew the polyiso.
What I like is that when the sun shines on the metal roof the polyiso reflects the heat. The warmed air between roof and polyiso rises out the ridge vent. Cool air is drawn from the roof eaves.
My building faces sse and it has yet to get more than 1 degree above ambient temp.
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grover
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# Posted: 18 Aug 2013 12:45pm
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I'll try a pic that shows where we are as of yesterday, 08-17-13.
One shows the trusses with purlins 24" on center. The plan is to put the osb on top of the purlins, 30 felt, then the metal.
Cabin_081713.jpg
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Trusses.jpg
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ICC
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# Posted: 18 Aug 2013 09:35pm
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I don't understand why the purlins are there at all. Especially with the OSB on top where it becomes difficult to impossible to nail all the OSB panel edges to gain the strength that gives.
Trusses. No overhanging eves? How much insulation will you be able to get in there near the eves? Venting?
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grover
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# Posted: 18 Aug 2013 10:22pm
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I was a little skeptical about screwing the metal to 7/16 osb. Now the screws will go through the osb and into the 2x4 purlins which are 24" on center. Something that is missing in the pictures would be the lookout rafters which will hold the osb on the outside edge of the roof. 1 ft overhang on each of the 4 sides counting the porch.
(The 2x4 purlins are not on the front yet in the pic.)
The front, which you see in the first pic, will also get an 8 ft wide porch so 2x6's will be used as rafters on the shed porch. This will give the porch a 3/12 pitch and it will get vents at the transition point between the 2 pitches and the porch ceiling will have to have vents too.
Not sure how much insulation I can get in the eaves. Plan is to use the plastic baffles in the eaves to retain an air flow up gap to the vented ridge cap.
I think the only thing different in what we are doing and how most pole barns are done is my trusses are 24" on center and we are adding decking.
Hope all that makes sense.
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grover
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# Posted: 20 Aug 2013 09:36pm
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Does it make sense?
I hope it does.
I thought it did.
I think it does.
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ICC
Member
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# Posted: 20 Aug 2013 11:16pm - Edited by: ICC
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All I can say is I have 7 buildings of my own scattered about the ranch. Assorted sizes and all with ribbed metal panel roofing screwed into 7/16" OSB, with the OSB nailed to rafters or with the big barn, nailed to trusses. In the past couple years we've recorded a half dozen or so wind events with sustained gusts of 55 to 65 mph and the grand daddy of 89 mph this summer. We've had many trees go over, some living, some dead. Fortunately none fell on anything important. A few still hanging and needing attention.
None of the metal roofing has ever shown wind related problems.
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Montanan
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# Posted: 23 Aug 2013 07:57pm
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Our roof consists of hand hewn true 2x6 pine boards placed vertically and that is the interior ceiling. On top of that- to the exterior- we laid tar paper and then horizontal rows of 2x4s all the way up, at 4' intervals, to attach the metal roofing screws. We used 4x8 foam sheets of insulation and placed them between every row of 2x4s. We put the roof metal on top of that and put the screws in the flat, as ICC said. It's been only 2 years but so far, so good. We're warm and dry.
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