roof insulation

hi. i'm new to the forum so pardon me if this is the same old question. seems for the cost (or less) of structural sips i can deck my purlin roof with 2x t and g and apply foam to that. i'd box that in, apply strapping and then a metal roof. would appreciate any input. sources for raw foam, eps vs urethane, etc. thanks, a.

thanks for the reply. any recommended reading for roof systems?

Not yet but the engineering and research is over half done. I'll tentatively be setting sod either in spring of 2009 or 2012 depending on which frame gets cut next... Feel free to stop by and help move the 20'+ 10x15 white oak timbers.

I helped an owner builder with a living roof building based on the main house at earthwood
http://www.cordwoodmasonry.com/
It was an octagon frame with a central stone chimney. Roof slope was 7.5in/12ft. We used 10x12 pine and hemlock beams with posts at 8ft apart. The walls are round strawbale. It was an engineering experience for sure. The roof material went like this: t&g decking/rubber membrane carefully glued together/gravel/chopped straw/topsoil/seed.

Did you use Butyl rubber or EDPM? I've been planning on an open ended Butyl box... Don't think UV will be a concern.

How thick was your T&G decking? I had a double layer of 1.5" square edged decking in mind when I ran the numbers for the rafters and purlins.

Since I'm not paying an engineer for this one I'll be overbuilding everything (left off the corrections for beams over 12" deep etc.). I'm also planning on a lot of moisture issues as the roof will come to ground level on the east end and drain off through rubble trenches supporting the posts along the north and south 'walls'.

The decking was a single layer of 1.5" pine t&g.
hmmmmm I believe the membrane was butyl, but not sure. It was very rubbery and came from a inner tube factory and was quite heavy, I had three rolls on my truck and it's the only time my tires ever rubbed on my farm built flat bed....

Sounds like Butyl to me, I watched a crew roll it out with a pair of bulldozers to create a six acre drying pond. 40' wide rolls, one roll per semi.

Traditional living roof in Stockholm, Sweden.
One of my favorite houses...

This is pretty cool, too...

http://www.skansen.se/pages/?ID=679

Alan, I built a roof very similar to what you are describing. The downside is that it is very labor intensive compared to SIPs. Even if your labor is cheap, there is some cost associated with having your t&g and frame exposed to the elements while you build this roof sandwich with materials that are not found at your local home building store. Only after building one house this way, did I understand why attics make so much sense - even on a timberframe house.

If I haven't convinced you to build it otherwise, here's the foam that I used:
http://www.firestonebpco.com/roofing/insulation/
costs more than fiberglass, but the cheapest raw solid insulation that I could find anywhere.

Good luck, let me know if you have any questions. I have pictures of my built-up roof on my blog. For instance:
http://massiehouse.blogspot.com/2006_10_01_archive.html
If the forum allows it, here are a couple of pictures:



-Thomas

ps. have you already seen this thread?
http://www.tfguild.org/ubbcgibin/ultimatebb.cgi?ubb=get_topic;f=24;t=000013

Derek's approach would probably be much better than what I did.

I tried to pretend that the 6 mil vapor barrier was a temporary roof. Not so! The nail holes in the vap barrier allowed moisture in, which was then trapped between the t&g and the vapor barrier and never dried out - warping some of the t&g boards. Also, after about 8 weeks, the vapor barrier disintegrated in the sun light and had to be reapplied in some areas. I should warn that the worst instance of warping (the one that required ripping and trashing 200 square feet of beautiful prefinished 7/8" oak t&g) occurred where I used 90lb role roofing as a temp roof, and the roofing nails did not self-seal where I thought they would. Like one of the slate books says, "a matchstick sized hole in the wrong place (e.g. a valley) can leak gallons of water in a single rain storm." It probably only takes a few spoonfuls of trapped water to warp t&g and/or ruin the finish.

tar/felt paper would probably have worked better - I think it would have let some of the moisture out if it had leaked. But then you are increasing the combustibility of your ceiling, and the tar paper genuinely stinks when it gets hot. What to do?