metal roof questions?

[phalynx]

I am planning on putting on my metal roof myself.  My intentions were to use 1x4's 24" o/c as purlins and attach the roof one panel at a time, working my way down the roof line while standing on the 2nd floor.  My roof rafters are 2x6, 24" o/c and are 13.5' long total.  My plan is to attach the sheetmetal directly to the purlins.  Is there any kind of vapor barrier that I need or will the metal do all I would need.  I plan on putting in r-21 in the rafters.  Will it need to be vented?  I have heard that you don't need to vent a metal roof. 

I just want to do it right.  Thoughts, input, appreciated.

[glenn kangiser]

Just went through all of this on Christina's place.

Looks like 30lb or #30 felt to stop condensation.  Jimmy Cason also has the problem.  You could do it and have it foamed like Christina's place. http://countryplans.com/smf/index.php?topic=2991.msg43311#msg43311

Another option we do on steel buildings would be vinyl backed insulation sandwiched between the purlins and sheeting, although I haven't heard of it done on wood.  It is my opinion it would work, but no guarantees.

[MountainDon]

If we're talking a house/cabin then yes, you need a barrier. Water vapor will condense on the underside of a metal roof if it's allowed to touch the metal. You need to keep the air away from the metal. Possibly the best way to do this is like Okie_Bob and Christina have done. Spray on foam. Icynene or other.

I'm going to do a metal roof. My method is going to be to construct a normal rafter supported roof with sheathing nailed in place to form a nice rigid roof structure. The wind blows hard at times up in my mountains. The metal will go over the #30 roofing felt, over the sheathing. Maybe that's just me.

Going the purlin route, without spray foaming the underside of the metal roof you will need a vapor barrier and an air space between the R21 fiberglass or cellulose in between your rafters. I've got a picture someplace I'll dig out and post.

[MountainDon]

A question for you... is code and/or inspectors involved? Christina's plans were interrupted by her code inspector. She had to modify her plans and her budget.

Here's the picture...



This is shown with shingles but if you simply make adjustments for deleting the sheathing and shingles and insert metal roof and purlins the principles remain the same. You need the air space and the air entrance and exit points, UNLESS you seal the metal up with foam.

[phalynx]

no code/inspectors involved at all.  But, after reading the posts Glenn linked to, I think I need a vapor barrier.  I don't get it, but it appears I need it.  Why would tar paper stop sweating but metal won't?  Odd.

I don't really want to spray in foam, it would be very cost prohibitive.

[phalynx]

any tips on installing the metal correctly?  I am going to use a snap-lock metal panels.  They are 16.25" wide and will be 14' long.  I am really concerned  about it not being square or drifting off square.  With 14' long pieces, I can't really follow a chalk line. 

[glenn kangiser]

The ability of the steel to change temperature rapidly cooling in the evening with warm air underneath causes rain inside similar to a glass of icewater on a warm day.  The warm air condenses (reaches it's dew point- turns to liquid) on the bottom of the metal making a pretty massive amount of water. 

The felt does a couple things.  It keeps the mass of warm air away from the bottom of the cold metal - slightly insulating, and if there is condensation on the metal, it will be drained safely outside of the house on top of the felt.  If by chance a bit gets through - the felt will wick it out and allow vapor to slowly pass, besides allowing your venting to work without having to deal with the liquid rain.

[glenn kangiser]

any tips on installing the metal correctly?  I am going to use a snap-lock metal panels.  They are 16.25" wide and will be 14' long.  I am really concerned  about it not being square or drifting off square.  With 14' long pieces, I can't really follow a chalk line. 

How about making a jig from 2x4's and screwing it to your rafter tails to stop the sheets evenly, then remove it when done.

[phalynx]

Man Glenn,, you are handy!  I bet that would work pretty well.

[glenn kangiser]

Thanks.  I have built around 300 steel buildings so have an idea or two of what could work -- even on a house.

[rwanders]

I am also considering using spray foam (closed cell polyurethane) but I plan to place the metal roof over plywood or OSB sheathing. I am in Alaska so insulation is critical---would the foam allow me to omit an additional vapor barrier and also omit the usual ventilation space between the insulation and the roof sheathing?

[Ernest T. Bass]

If you have a good vapor barrier on the inside of the insulation, why would there be any warm, moist air under the metal when it cools off at night? Or are we just talking about outside air that enters under the metal during the day? Is venting really a good thing, if that's the case?

I'm wondering if I should use felt under the metal roof of this cabin that we will be building. The metal will be installed on runners over EPS foam insulation, so a little condensation isn't going to hurt the foam. Might it rot the runners, though?

[MountainDon]

I don't think there should be any problem with what you call runners. (Horizontal strips, if I interpret correctly?)

I base this on the fact that it is common practice here, when constructing a home with a tile roof, to build the roof in the following order... sheathing, roof wrap or building felt, wood nailing strips, then the roofing tiles. The strips are not continuous but are in sections with gaps to allow for drainage whenever necessary.

[MountainDon]

Here's an interesting technique aimed at building in a very cold climate (Alaska would qualify I think  ). This is from Building Science Corp. They have produced several design techniques aimed specifically at differing climates. There is no one size fits all according to them. All their designs are aimed at energy conservation. They have many PDF documents available for download.

Not shown in the diagram is the ridge vent.



That's a LOT of EPS foam!! 

[ergodesk]

Hey MountainDon,

I always say the more EPS Foam the better.
The right insulation now will save a bunch later.

Build Smarter....EPS Structural Air
http://ergodesk.blogspot.com
http://styrihomenews.blogspot.com

[Ernest T. Bass]

Yep, Don. 1''x4'' wood strips to tie the foam down and secure the metal to.

[Redoverfarm]

phalynx  If your metal has a rib or peak then it should not run off if it is not forced. It should lay perfect for the next piece to lock into.  The main thing about metal is getting started square. If it is not then the end result can be very much to your dislike.  If you sheeting or perlins are the same distance from you end rafter then decide how much overhang to aloow for the tin. I use 1" and 1-1-1/2" . Set a string line blocked out on each end to the desired overhang. Then move your tin after it is overlaped on the previous piece to that string line.  Do not try to have too much of an overhang on the rafter tails or you will have to add extra facia to make the gutters work right.

If you want to double check yourself half way through the process you can pull it some to get back on track. Measure from the sheet edge to the last rafter or sheeting edge on top and bottom. If it is the same you are good to go.  If you see that it is running off then when installing the next sheet attach the top or bottom( depending on which way it is short)and before screwing down that end just lift it slightly out of the groove while pushing toward the direction that you need and then attach the screw. You can also pull in this fashion by going tthe opposite direction.  You would be surprised how much you can gain just a little at a time.


I know you can"t stretch metal but Oh yes you can of sorts.

[rdpecken]

any tips on installing the metal correctly?  I am going to use a snap-lock metal panels.  They are 16.25" wide and will be 14' long.  I am really concerned  about it not being square or drifting off square.  With 14' long pieces, I can't really follow a chalk line. 

The way we did it was to make sure that the first piece was as square with the roof as possible. That is the one that will determine how the rest of the panels end up, as they are all snapped togeter and have very little ability to move laterally.
First, we ran a string along the eave, and another up the gable to the ridge.

We marked 4' out on the long eave string, and 3' up on the short gable string. Then we adjusted the top of the short string to and from the gable end until we measured 5' diagonally between the marks. This established a square edge on the gable end. We lined the first panel edge up against that string, about 1/2" in from the gable end.

After placing the first panel, the rest just snapped/screwed in place, measuring the overhang at the bottom to keep it uniform for each panel.
The last panel wasn't more than a half inch or so off of square (could be the roof, rather than the panels).  It all worked very well, and was much easier than any shingle roof I have been involved with.

[glenn kangiser]

phalynx  If your metal has a rib or peak then it should not run off if it is not forced. It should lay perfect for the next piece to lock into.  The main thing about metal is getting started square. If it is not then the end result can be very much to your dislike.  If you sheeting or perlins are the same distance from you end rafter then decide how much overhang to aloow for the tin. I use 1" and 1-1-1/2" . Set a string line blocked out on each end to the desired overhang. Then move your tin after it is overlaped on the previous piece to that string line.  Do not try to have too much of an overhang on the rafter tails or you will have to add extra facia to make the gutters work right.

If you want to double check yourself half way through the process you can pull it some to get back on track. Measure from the sheet edge to the last rafter or sheeting edge on top and bottom. If it is the same you are good to go.  If you see that it is running off then when installing the next sheet attach the top or bottom( depending on which way it is short)and before screwing down that end just lift it slightly out of the groove while pushing toward the direction that you need and then attach the screw. You can also pull in this fashion by going tthe opposite direction.  You would be surprised how much you can gain just a little at a time.


I know you can"t stretch metal but Oh yes you can of sorts.

The pan deck sheeting will be a little harder to stretch or shrink , but John is right -- you can stretch or shrink a bit by how tight the ribs or standing seams are pulled or pushed allowing you to pull them the same distance from a check line top and bottom.  In low wind conditions I sometimes screw a few sheets together without fastening  to the purlins to prevent sawtooth then measure the eave or match a string line and check line, then fasten the front edge in the proper place and go back and install the rest of the screws.  That was on steel buildings - low pitch -- steep pitch is much harder so that will probably be of no use to you.  Still wanted to get the info out though.

[Redoverfarm]

rdpecken 

What color do you call that shown in the picture? Unusual but I like it.  I am sort of tired of the Forrest Green and reds used today. Thats why I went with a burnished slate.

[phalynx]

I just realized.  I bought the edge trim pieces like you showed in the green roof picture.  What I didn't think about was the overhang.  I planned on having just a 6" overhang.  I was just going to extend the purlins 6" over the gable end wall.  I was then going to attach the sheet metal to it.  Now, the problem I see is I didn't create a facia board on the gable ends like you did.  Will this be a problem with the edge trim pieces or will it work fine/look funny.  

[glenn kangiser]

I think it will need the board or be floppy and not able to be squared.

[Redoverfarm]

Glenn I would imagine that the tin you used was a little heavier than residential tin (29 ga) that would allow you to walk between perlins without it bending.  I just worked off the perlins. Tried to work from the floor and scaffolding but a real pain moving the scaffolding every couple of sheets plus you really couldn't reach all the screws either.  But my porch was 3/12. The cabin main I used my Duetz and a ladder in the bucket laid against the roof sheeting.

[glenn kangiser]

Hey MountainDon,

I always say the more EPS Foam the better.
The right insulation now will save a bunch later.

Build Smarter....EPS Structural Air
http://ergodesk.blogspot.com
http://styrihomenews.blogspot.com

Interestng info, ergodesk.  Second link didn't work.

[glenn kangiser]

We used 26 gauge - occasional 29 on odd jobs.  We worked on the roof on 1/12 or 2/12 pitch.  When I did my garage --12/12 I worked off the purlins and a fold down ladder along with a forklift meat catcher at the bottom where possible.