metal roofing idea

[nathan.principe]

Ok, I realize this is the third thread Ive started in terms of my roof! lol, so bear with me.  To bring you up to speed, I am going to install a metal roof on my cabin and because of the height and steep pitch I have opted not to sheath the roof with ply wood, instead I have installed purlins across the rafter at 24"centers.  I have been made aware of the potential for moisture to form on the underside of the metal roof and have kicked around a few ideas for underlayment and have gotten some good advice from this forum so far.  again, for safety reasons I can not put underlayment completely across the whole roof at one time because this would stop me from being able to install the metal roof peice by peice from the inside out.  I had the suggestion given to me to install felt in vertical strips across the purlins under each metal panel as I went along. 

My new ideas is to use the same method only replace the felt paper with 1/2" rigid foam panels and tape the seams as I went along.  The advantage I see to this is that the foam I will be using is advertised as water proof and will offer me an extra r-2 in the roof, the cost is minimal, at only $7 for a 4x8 sheet.  I will cut the foam into 2'x8' strips to install vertically under the metal panels as I go along.  What do you think? am I on to something here?

[Don_P]

The metal panel manufacturer would be the best person to answer. I suspect it will void any waranty and will leak at the fasteners prematurely. I think the foam will have too much "give". The fasteners will also be spanning through a soft material so that is not your roof bracing panel and the purlins are also not providing rack bracing... a flexible roof might result. Remember vertical tarpaper is also non code, it is meant to be applied horizontally with seams lapping. I'd lean towards figuring out how to do it normally, tried and true, or hiring it out to someone who is comfortable doing it that way.

[nathan.principe]

Even if the tar paper is installed correctly, horizontal with laping seams, how will it stand up to the moisture from condensation?  I fell like it will absorb the moisture instead of shedding it, know what I mean? 

[nathan.principe]

also, in terms of racking, couldnt there be a small interior beam of some sort installed at a 45 degree angle spanning from the ridge to the gable wall?  I supose if done right could add some visual interest from the interior

[glenn kangiser]

Where the tarpaper is pinched against the wood, there will not be enough air with moisture to condense.

One of the great things about tarpaper is that if it gets too wet it will wrinkle a bit and drain or wick the condensed water out of the space if too wet,  then dry and go back to being dry tarpaper waiting for the next moisture.

House wraps will not do that and if they get water behind them they trap it causing rot and mold problems.

[Don_P]

Yes Nathan a diagonal "wind brace" is a traditional way of bracing a board sheathed roof. Glenn's points about tarpaper are right on. If the metal is sealed from moist air there is nothing there to condense. When the metal is "vented" then moist air contacts the cool metal and condenses. Tarpaper has the ability to absorb and store a small amount of moisture and then release it when drying conditions return. It is pretty easy to make a "chicken ladder" to hang over the ridge and move it along just ahead of the metal sheets as you work. The last sheet is always the tough one as is the ridge... which is the same problem with your proposed method.

[MountainDon]

Nathan, I was going to ask "what about the ridge?" that requires someone to be right up there on the top anyhow.

The reason I hired out my metal roof was I didn't like being up there so I do appreciate your seeking alternative ways and means.


Regarding the foam, I've done a little more searching for info.

One thing I noticed on buildingscience.com was that every method of using rigid foam sheets on a roof has sheathing applied over the foam. Sometimes only 3/8, but always sheathing on top of the foam and roofing paper on top of that, then the roofing material.

I also found that there are. and have been. concerns about foam sheet shrinkage. (The foams have been reformulated several times over the years and some wonder how we'll know when they get it right.) Because of the shrinkage not all tapes are suitable. Tyvek tape, as an example, expands/shrinks at a different rate than foam and eventually will pull loose as indicated by tests. Someone also noted that tape in the test lab and tape applied on the real world job, where dust etc is blowing around may not perform the same.

[cmsilvay]

Maybe I am not getting what you are trying to do but if you were to use roof jacks as you moved along while also using fall suppression rigging I think in the end it would work out easier. You are going to have to get to ridge to cap it. Working through the purlins sounds easier then it is. Those sheets are bulky and I can forsee damaged panels from the angles you will be working from. Also no matter how well thought out and planned this might be there is always the what if factor that will happen. Better to be prepared( its the Boy Scout in me) If you are not comfortable working on something it might be a good idea to think about geting a sub. Sometime the time saved never mind the safety factor makes it pay of in the end

[nathan.principe]

So what you all are saying is it might work?....... JK you all make valid points, Im going to stick with useing felt under the metal.  I had planned on using a chicken ladder for the ridge and last panels, just wanted to minimize being up there.  I do think Ill risk-it for a biscuit with the felt being installed vertical tho

[MountainDon]

Nathan, do you know if the metal panels are rated as structural? Some of the re-reading of info I have laying around seems to indicate that some panels are not rated for structural use. Not sure how widespread that is.   It also leans heavily in the direction of applying what they refer to as "architectural panels" directly over sheathing with roofing felt between the metal panel and the sheathing.

http://www.steelroofing.com/  Lots of info there from a manufacturer of Galvalume steel metal.

[cmsilvay]

Hey I was thinking (i know its dangerous) but I have seen this done around here in barns. I believe the double buble reflective is sold in rolls at most big box and lumber yrds.

http://www.abmartin.net/html/abm_storage___installation_ins.html

Condensation

  Condensation can a problem for metal roofing over open purlins especially for residential applications. It is recommended to lay a double bubble reflective insulation over the trusses or rafters then install the purlins on top of the double bubble reflective insulation. Tape all joints and staple the reflective insulation to the bottom of the purlins. The insulation will be most effective with at least a 1 inch air space between the insulation and the metal roof sheeting.

  If roof purlins are already installed, lay the insulation on top of the purlins. Then fasten the sheeting through the insulation into the purlins. The double bubble insulation will greatly reduce the condensation plus reflect radiant heat (from the sun), thereby keeping your buildings cooler in the summer and warmer in the winter.

[MountainDon]

  If roof purlins are already installed, lay the insulation on top of the purlins. Then fasten the sheeting through the insulation into the purlins.

Gotta wonder about the "squish" factor between the roof panel and the purlin doing that. 

[nathan.principe]

MtnDon, this probably comes as no surprise to you, but Im going for the cheap stuff! haha, surely they are not structural, Im torn between 2 choices
http://www.lowes.com/pd_96189-28906-716702706873_0__?productId=3047973&Ntt=metal+roofing&pl=1&currentURL=/pl__0__s%3FNtt%3Dmetal%2Broofing

http://www.lowes.com/ProductDisplay?partNumber=12475-28906-4736008000&langId=-1&storeId=10151&productId=3050351&catalogId=10051&cmRelshp=sim&rel=nofollow&cId=PDIO1

As don-p suggests I will be adding sway braces to the framing to minimize movement

cmsilvey, I am definitely a thinking out of the box type person, so Im going to research you suggestion and also make some cost comparisons


next, a thought came to mind on how I can install the felt properly( if I go that route) with out sacrificing my " from the inside out method", I can pre cut the felt to the correct horizontal lengths and start by attaching them all at one end, with the correct lap, and keep them rolled up, I will unroll them as I move across the roof

[Don_P]

I wondered the same thing MD mentioned about structural metal when 2' wide was mentioned. 5V is the only 2' panel available locally and mfgr's instructions specify solid sheathing only for residential applications... although I've seen plenty on purlins that has performed pretty well long term. The squish factor was also part of my concern with foam, will the screw maintain seal?

Most folks here seem to gravitate to metal roofing and I kinda wonder why. It is more dangerous to work on, and I'm not really sure that it has a long enough service life to justify its' cost. I was in the building supply the other day and he showed me some pics of a fading metal roof about 10 years old. He was bitten by the onduline failures and has decided to stop supplying metal roofing for now. It is the current fad but there is nothing wrong with good shingles, they are easier to work with, easier to walk on, flashing is easier to do correctly in detailed areas, there is no condensation problem.

Back in the day my Dad decided to do something that at the time was the latest hot tip. We were shingling directly over high density foil faced polyiso using long roofing nails. By the time that was found to be a bad idea he had a few roofs to replace, sort of tempers my out of the box thoughts, I like to research them carefully. The foil was reflecting heat onto the backside of the shingles and causing premature failure. The method MD posted is what we evolved to. The bubble wrap sounds like it has the possibility for paint failure from the same cause, don't know  . I enjoy steep roofs less every year, but Dad is still working on them, he did take a slide this year. At 76 he is probably more comfortable than me up there, or he's just plain nuts  .

[MountainDon]

The ribbed panel, being Galvalume instead of plain galvanized will have a longer life. The ribbed is also marginally thicker, but at 29 gauge is about as thin as any ribbed panel made.

Do you have a Metal Mart near you? They are a Texas outfit with stores all over. When I bought they were very competitive on 26 gauge (thicker than 29 gauge)  They cut the panels to the exact length you specify so that eliminates cutting those 144" panels and having to make joints. (They cut the flat metal and then roll the panel profile you select.) They will not cut angles so you still need to cut valleys. They also have all the drip and rake edges plus have all the valley and other flashing pieces anyone might need available in 10 foot lengths and the exact color. You can also get flat 10 foot x 41" pieces for special flashing or other needs. I used those for some special flat panels on a small gazebo roof at home.

[MountainDon]

Most folks here seem to gravitate to metal roofing and I kinda wonder why.

If I would have had my own way I would have used the same shingles on the cabin that I used on the gazebo. They are top rated for fire resistance. However, my vote didn't seem to carry as much weight as the other one involved.      The gazebo was shingled because I did not want to deal with cutting all those angle cuts. I couldn't use that argument on the simple gable roof on the cabin.   Heck I even managed to size the length so none of the panels had to be cut. The only cut on the cabin was the perforation for the chimney and bathroom vent (drill sawed).


One thing I noticed last week is that after the metal roof has been installed a while (3+ years in the case of the suburban home) it is not as slippery to walk on. I had to do an antenna adjustment and was pleasantly surprised.

[nathan.principe]

gonna go with the galvalume ribbed panel, and 30# felt installed properly.  My next question is, if I leave a minimum 1" (or more) overhang with the metal panel, is a drip edge necessary?

[Don_P]

Correct, not necessary, the trims are where metal gets expensive.

[JRR]

30# felt over purlins, instead of sheathing, sounds difficult.  I would poke foot-holes all through such, I'm afraid.

Roofing metal must be much thinner than it once was.  I have a neighbor whose house probably dates to the '30's or '40's ... rusty old metal roofing, galvanizing long gone.  He gets up and smears on some paint every few years, but it is mostly rust brown.  He claims no leaks.  It looks to be 24 gauge or heavier, but I'm not sure.  When I last ordered metal roofing, 24 gauge was available.  I went with the 26 gauge, which was still a "special order".

[desimulacra]

Just to put my two cents in  I dunno about felt but it will not provide any reflectivity or as far as I know a measurable R factor . I gotta go with cmsilvay that foam stuff may be the cats meow. However I used the single bubble wrap and it worked in my workshop and how !!! It is never hotter inside my shop in the summer than outside by the "feel" test. (I have never measured) In contrast my brother had a shop the same size with the fiberglass insulation and it is unbearable when shut up and then entered on a hot sunny day. In case someone wonders my shop is much tighter with less airflow. I laid the bubble stuff across the stringers so it is exposed to the inside.
I have never had a problem with condensation and the stuff does add a small amount of insulation factor. I also had a shop with the Fiberglas insulation twice (first and last) At this time (6 yrs) I would never build a shop using anything but the bubble wrap, unless a newer miracle product (that foam?) comes out