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Sheathing necessary with metal roof? Options
#1 Posted : Thursday, March 26, 2009 7:11:11 PM
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Posts: 134,494

Hi,  I'm building a small 500 square foot straw bale house and the roof is going to be a ridge beam with rafters.  I'm going to put a metal roof on it, but I was wondering if I had to put sheathing down on the rafter fist.  It seems like it would be a waste of wood if I didn't.

My idea is to have the rafters be 12" wide so that when I put some sort of sheathing on the bottom I can push cellulose insulation into the space between the roof and the internal sheathing, which I was considering using something like old burlap bags for, just stapled to the rafters.  Obviously I'm going for the most environmentally and cheapest way possible.

Any ideas?

#2 Posted : Thursday, March 26, 2009 7:11:11 PM
Rank: Guest

Posts: 134,494

Hi Tao

I build my own roof, so I can tell you what I think is important.  First, 12" spacing between rafters is too close together.  While building you will not be able to squeeze between the rafters as you work.  I'm a big guy and I had a bit of trouble squeezing between 16" centers.  But, as your rafter spacing gets bigger, you need larger wood members for strength.  This is where you should sit down and consult conventional building codes to determine the size and strength the rafters need.  Take this very seriously.  You only have to be under your roof for one single storm to realize just how important keeping with code is.  My cabin has a 24 X 24 footprint, and I used 2X8" rafters with 16" spacing.  Here's a pic during construction to give you an idea of how it went.


You want sheathing on top of the rafters for rigidity.  It is really surprising just how flimsy each peice of wood feels untill you have them all nailed together.  For maximum strength and rigidity I glued, nailed, and added double earthquake anchors to each rafter.  I thought I needed it.  I found out how much that was true after an 18" oak tree just 50 feet from the cabin was blown over in a windstorm. 

After the sheathing was in place, I covered the whole roof with tar paper, and finally the steel roofing.  This I tied down with rubber washer roofing screws.  I am insulating with fiberglass.  Yes, it is energy intensive, however, it is going to stay in place for 50-100 years, so I think the energy cost is acceptable.  Animal problems are high in my area and the less at home they are in my roof the better.  I'm also very concerned about fire danger, and that much less flameable components in the roof is conforting.

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