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tcmatt
Member
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# Posted: 23 Sep 2012 07:59
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Mtn Don,
I think one reason to purlin on top of sheeting is to give more structure for the screw to hold onto (more than 1/2" of osb).
Here's how I did my 12:12 pitch...
sheeting (left a few inches at the top for the ridge vent)
ice and water all the way the sheeting
purlins
metal
I think as long as you're not using standing seam metal, but using polebarn metal (with ridges for airflow) you'd be okay.
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Trapper
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# Posted: 29 Oct 2012 20:13
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I am in the process of designing a 16x32 cabin with a loft and cathedral ceiling. I have read the problems on this type of design on this forum and elsewhere.
I think I have rapped my head around the issues and would like some feed back on my plan.
Between the rafters on the inside I will separate the 2x6 rafters with chicken wire or something of that sort to place the batt insulation against creating a space between the insulation and the roof sheeting, this will lead to the peek. I will leave a space under the peak that will vent out through the gable end vents and have vented soffet at the eve.
On top of the roof sheeting I will rip 2x4's and nail them vertically along the rafters to create space, I will then strap the roof and cover it with metal sheeting and vented roof cap.
the inside will be covered with vapor barrier and t&g pine.
Hope this all makes sense. Your feedback will be appreciated.
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MtnDon
Member
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# Posted: 29 Oct 2012 22:11
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I believe it is simpler to use 2x12 for rafters, rather than the smaller 2x6 or whatever. Sheath the rafters with OSB. The 2x12's provides enough space in the rafter bays for using up to R-38C fiberglass insulation The C denotes it is special denser batts meant to only expand to 10 1/4 inches. That will leave a 1 inch air space above the batt material. With a soffit vent at the eve edge of each rafter bay and a full length ridge vent there would be sufficient ventilation in each bay. Then apply the weather resistant barrier and apply the metal roofing panels directly over the sheathing. 5/8 inch sheathing provides sufficient bite if there is concern about that. The flat metal against the WRB keeps out the air; when you keep out the air there is no condensation. Any condensation in the ribs will be minimal and the sheathing is protected by the WRB anyhow.
I can not even begin to count the number of cathedral ceiling builds I've seen where the builder used 2x6 and was later sorry they did. It's true that most cabin size roofs do not need 2x12 for structural reasons, but they do make it much simpler to utilize a decent amount of insulation and still vent the rafter bays with minimal effort.
My thoughts. Yours may differ and that's fine. It's not my roof after all.
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bobbotron
Member
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# Posted: 30 Oct 2012 09:26
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I agree with MtnDon, go 2x12, it makes so much more sense insulation wise. Or at least 2x8 or 2x10.
I'd strongly suggest against the whole loft building thing. They are a pain to build, and working on such steep roofs slows down construction a lot. I would love to have built a 12/3 or 12/4 roof.
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MtnDon
Member
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# Posted: 30 Oct 2012 13:06
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more thought on the question of insulation... The R38C I mentioned will likley be a special order in many places. The same for R30C that is designed for use with 2x10's. Top me part of the planning process is to know or learn what products are readily available off the shelf and what can be had via special order. No sense planning on using R38C and then finding out after the roof is assembled that you can't special order in a small quantity. Sometimes an insulation contractor has the materials and may actually be able to install them at even less than the materials cost for us DIY at a big box store.
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bobbotron
Member
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# Posted: 30 Oct 2012 13:08
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If you can't find batts for 2x12's, is there any problem with get multiple batts of a smaller thickness, and stagger the joints?
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MtnDon
Member
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# Posted: 30 Oct 2012 13:25
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No. Just watch the total thickness as rated by the manufacturer. IF the batts have a vapor resistant paper face I would slash the paper a lot on the inside layer though. That is assuming a VB is recommended and that the VB is installed facing the direction that is recommended. Just to be sure everyone reading understands, there are different zones... some are recommended to have VB on the inside, some on the outside and some none at all.
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Trapper
Member
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# Posted: 30 Oct 2012 19:12
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Thank you for the input. I appreciate all points of view.
I live in Northern Ontario, so we do enjoy some pretty cold weather. From what I understand is once you have a vent space in place, the next factor is to have enough of an R rating to prevent the vapor from reaching the dew point. Correct me if I am wrong.
What would the least R rating be to avoid this problem?
It is a camp I am building and in the colder weather the wood stove will effectively keep the camp warm.
I only plan on building in the spring, but want to finalize my plans in the next little while. At that point I will slowly start shop the sales for my materials.
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MtnDon
Member
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# Posted: 30 Oct 2012 20:20
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Correct, keeping the temperature above dew point does prevent condensation. However, in a vented space the roof sheathing will be cold enough at times to have moisture condense on the underside. The idea of the insulation is to keep the heat below in the heated space after all. That is assuming the ceiling insulation is all fiberglass batt.
Things change when another insulation type is introduced. For example, if there is foam to the exterior side of the fiberglass then there will be a minimum R value for that foam. If the foam has too low an r value the foam may get cold enough to have condensation under some conditions.
I haven't got info specific to ceiling insulation but the issue is similar to those encountered when retrofitting rigid foam insulation to the exterior of walls that are already insulated with fiberglass batts.
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pcroom
Member
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# Posted: 23 Mar 2016 14:23
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My wife's ex husband and his dad built a two story cabin I believe it's like 24x40 with a cathedral ceiling and they used plywood I think then blue board then plywood then shingles they use 3 small ac window units 2 upstairs in the kids rooms 1 downstairs that place stays cool and warm it's on piers the floor has blue board in between like the roof also the walls has fiberglass in between the studs and then blue board running across over the studs then there wall paneling. that place is insulated so good it's crazy!!!!
I plan on running like 1x6 on my roof first for the look of my ceiling for the inside then blue board then plywood tar paper then shingles. That's it... the 24x40 cabin has no mold problem and stay cool and warm here in Missouri that cabin is awesome!!!
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KinAlberta
Member
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# Posted: 25 Mar 2016 12:06 - Edited by: KinAlberta
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Quoting: VTweekender Just my opinion, others may have a different outlook. Using the purlins method will cause problems in the long run with putting insulation underneath, as the metal will sweat and fill the insulation with moisture, leaks and mold problems eventually.. OK for open rafter barns and garages .... I would stop right there and take off the purlins and plywood or OSB the roof then add 1" sheets of insulation board, screw the metal roof flush to that...there will be minimum moisture able to build up underneath if any at all, and any that does will find the groove in the metal roof and run down.
Could the owner just sheath right over the purlins (first filling in the perimeter gaps at the gable ends).
Then maybe run a membrane or some sort of coarse roll roofing/asphalt membrane first. Seems most metal roofs are put over existing shingles and so I imagine any moisture is shed by the underlying old shingles.
Also, there's that black or orange dimpled laminate floor underlay that you can buy. I picked up some surplus material and planned to lay it like roll rolling (overlapping edges) on an old boathouse before I put down metal roofing. (I also have a roll of that orange plastic fencing with the extruded look that I'd considered as an option to slightly lift a metal roof to allow air movement.)
...but this is a vapour barrier too.
http://www.cosella-dorken.com/bvf-ca-en/products/foundation_residential/floor/product s/fl.php
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Cpipitone
Member
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# Posted: 8 Jan 2017 08:26
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I want to do the same with the metal roof on top of 3/4" osb and 30 w tarpaper would you still suggest the 1" board still needed
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