another roofing question

[nathan.principe]

It has occurred to me that I might have built a structure thats impossible for me to roof with out professional help ( which I couldnt afford even if I wanted too), the pitch is too steep, The building is very high off the ground, I dont think I will be able to get plywood up there safely.  So I have done some research and I am seriously considering installing the metal roofing over 1"x4" white wood furring strips @ 24" oc across the rafters, w/o any type sheathing or vapor barrior.  My reasoning is that I will be able to install the furring strips and metal roofing w/o actually getting on the roof, by using ladders and such from the interior.  I plan on using butyl sealing tape at all seams and neoprene washer screws in predrilled holes ( maybe even a dab of roofing sealent before driving the screw) the screws will be installed at the peak of the corrugation as apposed the valley.  My roof is a 12 in 12 pitch so I feel this lessens the chance of leaks.

I understand the roof will have to be well vented to minimize moisture from condensation, and obviously there is a risk of a leak occurring at the screw locations.  I am less worried about the seams as I feel the butyl tape will do a good job.

with that being said, is there anything that I havent thought about that could go wrong, it will have to be pretty strong evidence for me to change my mind.  At this point a " I dont know if id do that if I was you" probably wont sway me.  With that being said Im sure Mtn Don wont dissapoint me by not chiming in with a scientific chart or something!!! haha j/k

here is some of my inspiration, and yes Im sure this is not ment for residental application, but whatever
http://www.fabral.com/downloads/details-postframe.pdf

[MountainDon]

I don't have any charts. 

I understand the issue of the steep pitch, slippery metal, height and all that. That was one reason we went with a 5.5:12 pitch. And then I chickened out and had an unemployed 'jack-of-all-trades' install the metal for $250 cash. (He had previous metal roofing experience and this was a very simple roof.)

I would not be so concerned about rain leaks as I would be concerned about the condensation issue. I would be concerned about how to absolutely prevent condensation from dripping into the insulation.  Speaking of which, what roof insulation system do you envision?


I know you have no inspector issues, IIRC, but one owner-builder here in WA or OR had to have foam sprayed on the underside of the metal as the only way her code department would permit a metal roof on purlins.

[speedfunk]

Agreed Steep roofs are no fun.  Thats we we did 3/12...ahh .. I am to wimpy for 12/12 

If you are going to do that can you leave your self room to slide something inbetween the vertical purlins (under roof), so if you start seeing an isssue with condensation you can fix it without ripping off the roof.  Maybe like some ripped pices of OSB board with felt on it or something?

[Don_P]

I've used felt over the purlins... it just got less safe again.
First thought on what you propose is, no sway bracing. The purlins are not bracing the roof the way plywood does. you propose replacing the plywood sheathing diaphragm with metal.  but, by screwing the peaks the metal is not a diaphragm either... part of the reason post frame requires screws in the flats. Read the linked instructions you posted, they are developing diaphragm by specifying a screwing method. The roof is likely to move enough to wallow out the screwholes if screwed through the peaks. Postframe is a minimalist system with parts designed very "efficiently", highly stressed. One reason you see so many failures of them is that when someone muffs a detail there isn't a whole lot of redundancy to take the load a different way.

edit, went and dug up one of their papers up to expand on the above.
chart's at the bottom,  
http://postframeadvantage.com/i4a/pages/index.cfm?pageid=3813

[dug]

Phalynx came up with an interesting method using purlins and felt paper which he ran vertically, allowing him to work from underneath as you suggested.

http://countryplans.com/smf/index.php?topic=2525.60

I had considered doing this but went with sheathing because of some of the concerns that Don mentioned.

I would advise against screwing the metal down in the peaks though. I agree that on the surface it seems to make more sense that way but unless you have ultra thick metal there is no way to compress the neoprene washer without deforming the metal when fastening from the peaks. Also you have 1 1/2 in. of the screw just free floating and as Don said movement in the roof would wallow out the holes and create leaks that you are trying to avoid.

[nathan.principe]

OK, I can deffinatly see the concern of screwing them in place at the valley as opposed to the peak, I did see in the link that I posted that they did mention doing it that way but didnt really explain why ( I thought I was being crafty  ).  I will go the route of screwing in the valley.

Mtn Don, My plan for insulation is to use R13 ( although I know this is not recommended) for the roof as well as the walls bc of the thickness, It will give me an extra 2" of ventilation space in between the rafters, not to mention the extra 1" of vent space I will get from the purlin height.  Condensation leaking on the insulation is a concern of mine, and I had thought that after teh roof was installed and before insulation went up that I would cut strips of synthetic underlayment a little greater than the width of the rafter spacing and tuck it up and staple the strips spanning between the rafters to create a barrier. That was of coarse before dug chimed in with phalynx's method of laying felt in vertical segments.  I think I will offset the starter strip of felt from the roofing pattern, and alternate the installation between the 2 ( felt then metal, felt then metal, felt then metal, etc).  To further ease the installation I will probably precut the felt on the ground as to not have to deal with the weight.

[TheWire]

I put on my metal roof 9/12 pitch with 2 ladders.  1 Extension ladder to get to the eaves and one long 1 piece ladder with rubber padding along the back.  This ladder laid against the roof and was held in place by a rope from the other side.  I felt quite safe when on the ladder and had a harness with a rope over the ridge when I needed to leave the ladder.  The sheeting was done with helpers passing up the sheets from the inside and using 2x4 cleats to stand on.  I also used synthetic underlayment which was grippy enough to hold me on the the roof enough though I had the harness.