Author Topic: 20 x 20 w bedroom in Ohio  (Read 22935 times)

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Offline MountainDon

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Re: 20 x 20 w bedroom in Ohio
« Reply #100 on: May 03, 2018, 02:54:09 PM »
.......Planning to use something along the lines of 8 inch GRK screws to hold the strapping down. I have 6 inches of insulation, then 3/4 inch strapping vertically then 3/4 inch strapping again horizontally.  .......

I didn't see this until today.....  The 6 inches of foam plus two separate layers of 3/4" furring with 8 inch screws doesn't add up to me.  6 + .75 + .75 = 7.5" total thickness to be penetrated by the screws, not to mention the osb or whatever the roof sheathing is. Generally speaking, you should have 1.5" minimum penetration into the underlying framing. Perhaps I don't understand how the screws are to be used; what thickness is meant to be penetrated.  ??? 

I'm not familiar with tables that would suggest the size and number of screws to use for roof sheathing + foam insulation, etc. However, there are tables for fastener size and number for foam wall insulation, installed on tthe exterior like we did when retro-insulating our home walls. Fastenmaster makes a variety of structurally rated screws and has a table for the Headlok series, for walls.   That doc is downloadable from here.  Just thought I'd mention that in case it might be of use to someone.

Fastenmaster has a large assortment of resources for their products.  I don't know about the same for GRK, but that may be only because I've never searched for any docs for their products.

Just because something has been done and has not failed, doesnít mean it is good design.

Offline Mike 870

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Re: 20 x 20 w bedroom in Ohio
« Reply #101 on: May 04, 2018, 06:51:00 AM »
Thanks Nathan and Don,

I really could not find much info on this at all.  Most of what I found was on the forestry forum from guys who had build their own timber frame house.  Seems most people use SIPS, which just aren't possible on my site.  The 8 inch screws will go through 3/4 strapping then 6 inches of insulation before they bite into the sheathing boards and rafters so I guess I'm short by 1/4 inch.  I could always use a forester bit and bore out 1/4 inch of the strapping.  The screws have a pretty decent size head on them.  Once that first set of strapping is fastened another 3/4 inch will go perpendicular (horizontal) across the roof.  The second set runs the direction it needs to be for the roof.  I've got a layer of 30lb felt under the insulation, really it was because I've lost some knots and have decent size gaps between the boards in the dry winter air.


Offline NathanS

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Re: 20 x 20 w bedroom in Ohio
« Reply #102 on: May 05, 2018, 03:34:34 AM »
I used a spade bit to predrill all the holes through my 1x4 furring. Those screws would probably split the furring quite a bit if not predrilled, also could be a little risky trying to get the head of the screw flush with the wood. The nice thing with the spade bit is you could start all the screws on the ground, that way two hands is enough when you're trying to install.

I assumed fastenmaster or GRK would have a screw schedule for roofs like they do for walls, maybe not. The question mark is snow load, otherwise you could use the same schedule they recommend for walls.

This article is behind a paywall but would be worth a 1 month subscription or free trial. Martin Holladay will consult with engineers and professors when he writes these articles.
http://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top-roof-sheathing

There is another guide called "REMOTE" that has good info on screws but I think it was mainly for walls.


I also marked the stud locations at the bottom of the wall before installing the foam. I then used a T square.. or level.. and marker to mark all my stud locations.

Some of the prep work gets pretty tedious but you really want to make it as easy as possible when you're putting in the big stuff.

Offline Mike 870

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Re: 20 x 20 w bedroom in Ohio
« Reply #103 on: May 06, 2018, 03:43:11 AM »
Snow load is 20 psf, but I donít think that will really come into play here.  The only job of the screws is to hold down the polyiso and the strapping.  There really wonít be much downward force.  The toughest job they will have is holding things down in high wind.   Iíll need to predrill the holes.  I have some really good leftover pine for strapping the stuff is much nicer than your typical box store pine.  Almost too nice to use but I donít really have another use for it.  I have more than enough screws, so I should be fine.  Iím only accountable to myself on this build so...  heading to the cabin tonight so I can get an early start tomorrow.  Hopefully Iíll have a video on the process.

Offline dablack

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Re: 20 x 20 w bedroom in Ohio
« Reply #104 on: May 07, 2018, 05:00:24 AM »
Won't your siding attach to the 3/4" strapping?  If so, then those screws will be not only holding up your polyiso and strapping but also your siding.  That is a big moment arm (6.75") for so little bite into the house (1.25").  I would want more bite with so much weight hanging on the screws.

Offline Mike 870

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Re: 20 x 20 w bedroom in Ohio
« Reply #105 on: May 07, 2018, 05:38:00 PM »


Here's what I got done so far.  Hopefully this picture helps it make more sense.  It's not for siding it's for a metal roof.  I still have a two foot strip to add at the top before I can finish this front section.  The screws are biting into the ash rafters, it's solid I've been climbing all over it today.  At the bottom and the sides the strapping fastens to built up 2x4's. 

Offline dablack

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Re: 20 x 20 w bedroom in Ohio
« Reply #106 on: May 08, 2018, 04:29:05 AM »
That's a horse of a different color!

I think what you have done looks pretty good and if you have been climbing all over it and feel good about it, then do it to it!

Just for historical info, if someone else reads this thread and wants to copy what you are doing, I would want a little more bite with the fasteners.  In this instance, gravity is helping and the angle between the downward pull (gravity) and the fasteners are less due to it being a roof, but with siding, the angle would be 90 degrees and I wouldn't really feel comfortable with this at all. 

Offline Mike 870

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Re: 20 x 20 w bedroom in Ohio
« Reply #107 on: May 08, 2018, 08:12:10 AM »
I think most people just use SIP panels on top of their roof, but my site would be very difficult for that as well as the logistics of it being a decent drive from my house.

 In hindsight I would notch in some boards into my rafters themselves that run horizontally that way I have more area to fasten to because the rafters being 4 foot centers is kind of far apart for the strapping.  I have strapping on 2 ft centers but those ones only bite in 3/4 of an inch into the pine board ceiling.  I had to fasten those ones so that they would not penetrate through the sheathing.  So every other vertical strap is solid and bites into Ash hardwood, and the ones in between are not so solid but held down by the horizontal straps.   Overall all once everything joins up its pretty secure though. It's not ideal but a good learning experience. 

Offline Mike 870

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Re: 20 x 20 w bedroom in Ohio
« Reply #108 on: May 10, 2018, 01:59:20 AM »
Here's yesterday's progress. I have a time-lapse of putting up the felt, strapping insulation and roof coming soon.




Offline Mike 870

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Re: 20 x 20 w bedroom in Ohio
« Reply #109 on: May 11, 2018, 09:48:07 AM »
Hereís the video of insulating and roofing the front side of the cabin: