Hello. This is my first post – I am an architect/builder currently working on a post-beam/conventionally framed custom house in the Upper Peninsula of Michigan.
my question is:
We are just about to install a “warm roof†over a post and beam timber structure. The sandwich is as follows: 1 ½â€ Tongue and Groove, a 10mil VB, 4 layers of 2†rigid insulation (8†total), and 5/8†OSB on top.
I am looking for advice for the best way to install and fasten this system to the t&g. (what type of fasteners, at what spacing etc).
My concern is that with all the holes that will be punched through the VB by screwing this in- the consequent moisture from rain could get down into my beautiful tongue and groove – permanently staining it. We will of course be covering it daily but there is a chance it may still get wet (we are in a high wind location and are doing our best to manage tarps! :- ) At any rate, I would like to minimize the risk – as well as minimize the number of holes in my vapor barrier. Any ideas? Special washers? Some kind of sticky membrane at the screw holes? (with out adding uneven thickness).
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Thank you in advance!
Heather
Replies
I'm sure someone will chime in with better ideas but here are some of my thoughts.
1) Ice and Water or some other kind of bituminous membrane would act as a vapor barrier. It "self heals" around fasteners and would be good at keeping the water out.
2) By the time you put four layers of 2" EPS on and tape them and worry about them blowing around during installation I think it might be easier (and maybe even cheaper) to use structural insulated panels in whatever thickness you need for your required R-value.
Get yourself some cans of spray foam. It is sticky as anything can be and is helpful to seal the individual sheets to each other at the edges. A dab of it here and there will also glue the surfaces of the sheets together. That way you will need far few screws through things.
Do not bother with the throwaway cans unless that is all you can get. A gun with re-useable cylinders is more controlable and the cans go much further.
Google for Enerfoam, but there are several kinds out there.
I have never gone more than 4" with this type roof, but I place 2x4 sleepers for a cold roof effect over the insulation and then run lag screws through them into the rafters. You can do the same by glueing plywood or advantec sheathing onto the insulation - same glue - and then screwing it to the 2x6 under decking.
I use screws from GRK Canada which are available up to 12" long and are some of the strongest, easiest to use possible to buy.
http://www.grkfasteners.com/
it's the third one shown - the RSS
Now about leaking concerns - if you were to use some 30# tarpaper or the synthetic underlayment RooftopgaurdII on your 2x6 decking, it would not tear as easy, and would self-seal a bit better around penetrations than with the plastic. Would also be more fun to work over.
And not to worry too much, if you are running that T&G material up the roof as most do, not much will leak in to stain it except at valleys.
'Course, if you run it horizontally on rafters instead, then it will try to trap every drop of rain that it see right in the joints.
I am curious about why you chose this layering instead of stress-skin panels?
Availability?
On the positive side of things - if you get frustrated with the system and the weather, you won't have far to go to cuss out the architect.
;)
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Wow! Thank you for all the great advice.
Yeah, we've been using the grk fasteners and they are fantastic but...pricey. and yeah we did price sip panels but once again...very pricey. Also, the roof has a complicated hip roof geometry and we were reluctant to "count" on yet another manufacturer to get our design right. (We've had trouble in this departement and we'ver really found that if we want it done right we really have to do it ourselves.) Regardless it does sound good and maybe I'll try it later with a simplier geometry.
Would you tape all of the seams together and would you recommend a tape?
We are running the t&g horizontally so maybe the Ice and Water shield solution is what we will think about. It is a tricky spot. Any other ideas for that? with out making the thickness too irregular across the surface. Its turning out to be not the best detail...
but yeah, this is my first job done wholely by myself and YEah i usually try to beat the rest of the crew to finding the architect with comments compliants and ....feedback! ;- ) but its fun. you have a good sense of humor!
With horizontal run T%G and a cut up roof styule, the Iceand water shiled sounds good
Buttwo concerns - one is cost which you have on your mind.
The other is that the bituminous I&W will off gas to the interior through the joints in the T&G, especially when you have hot air pooling near the ceiling in summer. That odor may or may not bother you. Is this for resale or personal?I would use Rooftopgaurd II. It is almost 100% tear proof. And it has on the backside a surface that is very slightly tacky, like post-it notes, that will modestly self seal around nails and screws, more so than plastic or 30# tarpaper will, and I do not notice it off-gassing like Grace I&WI do not tape seams on roof assemblies like this.
There are a couple reasons for taping normally, neither of which apply, IMO, when I am using the spray foam as a glue/adhesive, and seam sealer all in one as I lay panels in. Primarily, the tape is to seal seams against convection. The underalyment RTG II works as your primary VB inside, and when the joints in the foam are foam-sealed together, there is no avenue left for air movement.Of course, you want to stagger seams as you layer all this in. With several layers of foam, I can only imagine waht sort of staging and chicken ladder setup you will be using for a steep roof. So go slow, think it through to plan the stages, be safe, and good luck
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Of course, you want to stagger seams
Piffin, I am glad you mentioned staggering the seams...
Dr. Lstiburek learned this the hard way on one of his personal projects.
I know of a low income project where this was used and no tape, no foam sprayseals, no staggering - just slap it in and cover it up.Every one of those homes has suffered from "leaks" and mold
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Just out of curiousity, how much more (percentage wise or per sq ft.) did you figure that SIPs would run over your built up roof?
She probably won't know that answer accurately untill she has calculated all the labour that went into this method
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Yeah - if i can figure the cost difference out after all is said and done i will certainly post it to show.
Question regarding the 5/8" sheathing that will be installed over top of the 8" of foam. Which is better or how to decide between using 5/8" Tongue and Groove OSB vs. regular OSB with the Simpson plywood clips (4/sheet). Is there any different? (aside from cost of course)
Thanks!
IMO - no reason for T&G or the H-clips here. You are placing this directly on the foam so it has backing.When you are running it over trusses at 24" OC, you need either the T&G or the H-clips top maintain alignment. and strength sharing.'course it wouldn't hurt to do either but I see no reason for it. Going with all square edge means there will be places you can save on waste if you pay attention to cuts. T&G means the male/female conflicts will cauase materials waste
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You mention washers...
The screws Isuggest are great for the final wood over and down thru, but if you are thinking you want to attach it a bit mechanically at each layer, ther are disc washers available made for roof insulation that are about 2-1/2" Diam with a hole in center. They would be purchased from any roof supplier that handles stuff for low slope roofs like EPDM or BUR.
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I would seriously consider Ice and water shield over the whole roof, the cost increase for that would be minimal considering the benefit.
You should also consider sips, I have used them with the bottom layer pre-stained T&G. The final product was a superior product all around. If not Sips I would consider spraying one of the many foams.
What about the off-gassing situation? or likewise, what about warm humid air getting behind the t&g and mingling with the tarry ice and water shield? could there be some kind of reaction there?
as for the roof guard II - what about all the holes put in it by nailing it in to install it? Would i also want to seal/caulk all of the seams?
THe plastic we already purchase for the VB is actually a 15mil. Really incredible, strong stuff.
Thanks!
maybe you want to go ahead an use it then. I just hate woprking over that stuff if theere is ANY chance I will have to step on it.
That makes the ground rise up fast! And I don't bounce as good as i used to .
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"Our" very wise Piffin hit on the things that came to mind.
But for one.
Roof is 5/8" + 4" + 1.5" which is 6 1/8" total. If you use 5.5" long screws through the osb, they'll only "bite" 5/8" into the T&G. You probably can't use 6" screws as they'll sure-as-shooting come through the grooves and "show."
Good point
In our case we have:
1 7/16" T&G + 8" (of foam) + 5/8" OSB = 10 1/8" plus some for compounding of materials.
In this case we are finding only a 10" screw and will probably have to add something to be able to embedd only an inch or so into the tongue and groove. Where we will be over timbers, we will use 12" screws and go directly into the structure.
SIPs are the way to go .