Author Topic: Metal roofing; what is really the correct way to install ?  (Read 12067 times)

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Offline Arky217

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I plan to put a metal roof on the retirement home that I'm building. The roof will be trusses with 2 foot spacing. Over the trusses will be Avantech Zip roofing panels. The Zip panels will have the seams taped over with Avantech's Zip panel tape. (They claim this will make the roof rain tight up to 120 days).  The insulation will be cellulose in the ceiling.

I have heard just about every way there is to install metal roofing panels to prevent condensation:

Some say to put down a 15# or 30# layer of felt over the sheathing, then the metal panels; in other words, not to use purlins or the metal will condensate like crazy; enough so that the water will find it's way through the layer of felt.

Others say just the opposite, that putting metal panels over sheathing (or sheathing with felt) will enhance and trap condensation and accelerate corrosion of the under side of the metal; and that you should put down 1x4 purlins (with drain holes cut into the bottom of the purlins to allow trapped water to run down the roof), then the metal over the purlins.

Avantech says that with their Zip panels, just put the metal directly over their sheathing. That the superior resin in their Zip panels makes even a layer of felt not necessary.

So far, I am leaning toward Avantech's recommendations, (but I still wonder how any trapped condensation will escape from between the sheathing and the metal, what with the metal tightly against the panels).

What say you metal roofing experts ?

My only experience with metal roofing panels is:

I build a 12x12 storage shed. I used boards for the roof. Put felt over the boards, then the metal. That was 3 years ago; I haven't noticed any problems yet.

About the same time, I built a 20x24 carport. Just like the commercial ones, I put the panels horizonally directly over the metal 'trusses'. There are times when the condensation is so severe that it appears like it's raining under the carport.

Thanks,
Arky

Offline Redoverfarm

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Re: Metal roofing; what is really the correct way to install ?
« Reply #1 on: May 08, 2010, 03:47:50 PM »
Arky the keyword is "condensation".  A properly insulated and ventilated roof should not have condensation.  Condensation is the product of warm meets cold.

Offline MountainDon

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Re: Metal roofing; what is really the correct way to install ?
« Reply #2 on: May 08, 2010, 04:11:31 PM »
The metal roofing we have came with a 40 year warranty against corrosion perforation as well warranted for the same period for the color finish. There were no stipulations about being installed over sheathing, tar paper, synthetic wraps, or anything. It seems most metal roofs around here are installed over a synthetic wrap over OSB sheathing. If Advantec states their product is water resistant enough you're probably okay to follow their installation process. ??? Of course there's also a good chance that my paranoia gene might kick in hard and I'd do wrap anyhow.  ??? 

For whatever it's worth, here's an anecdotal story. When I installed the metal roofing on the shed I left one panel un-installed for months. On the day I got the last panel cut to size and installed the morning was very cool with lots of dew condensed on the upper surface of the metal that had been sitting there for some time and on the exposed tar paper. Out of curiosity I had a look under the loosened edge of the panel that was already installed. The surface under the metal was dry. That's an un-insulated roof and hunidity that was likely in the 59% range; maybe higher, it rained later that day.
Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline n74tg

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Re: Metal roofing; what is really the correct way to install ?
« Reply #3 on: May 09, 2010, 06:42:38 AM »
As I put my metal roofing over 2x4 purlins, which seems to be agreed to by some and disagreed by others, I decided to google "metal roof over purlins", which led to an interesting metal roofing forum.

You might be interested in reading some of the posts, I think your question is covered.

http://www.metalroofing.com/v2/forums/index.cfm?action=mboard.members.ViewCategories
My house building blog:

http://n74tg.blogspot.com/

Offline MushCreek

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Re: Metal roofing; what is really the correct way to install ?
« Reply #4 on: May 09, 2010, 08:32:28 AM »
When putting metal over a sheathed roof, what I've seen done is to run vertical furring strips on top of the sheathing, on top of the trusses, then put purlins cross-ways. This creates the channel to drain moisture away easily. What I plan to do is sheath my roof, put a wrap or tar paper (or Advantech), put down at least 1" foam, which will be secured by the vertical furring, then install purlins, and the tin. One nice thing about purlins is they make it a lot easier to climb a steep roof, at least until the tin is on. Basically, it's a ladder, especially when spaced off the deck by the vertical furring. The foam will serve as a thermal break for the roof, and a secondary layer of waterproofing, should everything else fail (I'm a belt and suspenders kinda guy). I've been in tin-roof barns that practically rained down on you, but the air inside was hot and humid, and of course, there was no sheathing.
Jay

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Offline Don_P

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Re: Metal roofing; what is really the correct way to install ?
« Reply #5 on: May 16, 2010, 06:50:38 AM »
Things sweat because humid air contacts a surface that is below dew point. Wipe off your sweating tea glass and place it in a plastic bag. If the humid air cannot reach it it will stop sweating fast.

If the underside of the metal is not in contact with humid air it cannot sweat. My vote is solid sheathing with tarpaper and end plugs top and bottom. Vent the roof under the sheathing. Attempting to vent the underside of the metal simply guarantees that it will condense and drip often.

Offline lonelytree

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Re: Metal roofing; what is really the correct way to install ?
« Reply #6 on: May 17, 2010, 12:36:20 AM »
If you are in an area with ice damming <sp> use 2 rows of ice shield and standing seam metal roofing. That way you only have one row of screws that can be taken out by ice/snow.

Offline JRR

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Re: Metal roofing; what is really the correct way to install ?
« Reply #7 on: May 17, 2010, 05:26:50 AM »
I hope to put some corrugated metal roofing in place this summer.  Much like already described; vertical spacers atop the trusses and sheathing, then 2x4 purlins crossways.  I will have felt and foam board insulation directly against the sheathing (under the spacers).

I'm toying around with the idea of first installing "every other" piece of metal, then coming back and installing the "filler" pieces on top.  I've never done it this way, and it will take a bit of careful measuring ... but I think it will make future repairs of the roofing much easier if any are ever needed.

Offline Dave Sparks

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Re: Metal roofing; what is really the correct way to install ?
« Reply #8 on: May 17, 2010, 06:41:52 AM »
In case this has been said I will say it again... for a class A fire rating tar paper cannot be used and the material should be fiberglass. Never really understood this one but if it was that hot I probably would already be dead.
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Offline Redoverfarm

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Re: Metal roofing; what is really the correct way to install ?
« Reply #9 on: May 17, 2010, 06:54:58 AM »
I hope to put some corrugated metal roofing in place this summer.  Much like already described; vertical spacers atop the trusses and sheathing, then 2x4 purlins crossways.  I will have felt and foam board insulation directly against the sheathing (under the spacers).

I'm toying around with the idea of first installing "every other" piece of metal, then coming back and installing the "filler" pieces on top.  I've never done it this way, and it will take a bit of careful measuring ... but I think it will make future repairs of the roofing much easier if any are ever needed.

I think that would be a nightmare to make everything work out.  Even consecutive installation requires some "tweaking" to make the sheets fit right.  There is no problem with removing one sheet later with convientional installation.  

With convientional installation you will work off of an "unfinished" surface all the way from one end to the other.  In your sequence you would be working off of a finished sheet every other sheet.  Not to mention accidental scratches but a slick work surface is almost impossible to work off of.  Pre-drilling screw location and a "chicken ladder" is about the easiest that I have found if you are working off of a sheeted roof.  With purlins the ladder is already in place.  Just predrill the sheets at the mid-point of the 2X4 width.  The worst problem you will encounter will be the last sheet to which you will not have platform from which to work and a couple ladders or scaffolding are a must. 

Offline JRR

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Re: Metal roofing; what is really the correct way to install ?
« Reply #10 on: May 17, 2010, 01:09:27 PM »
You are right ... and I am guilty of over-simplifying.  I would still install one sheet at a time across the roof, just make every other joint "tucked under" instead of laid over.  Might be helping mixing metal with poly sheets on barn/shop work.  As I say, I've never tried it ....and the plan may pass quietly with time.

I have trouble dimpling metal by walking or working on it, as the final edge is near.  I use sheets of plywood and rubber tumble mats to minimize the risk.  (Guess I could try losing some weight .... Nah!!)

Offline Redoverfarm

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Re: Metal roofing; what is really the correct way to install ?
« Reply #11 on: May 17, 2010, 01:39:51 PM »
You are right ... and I am guilty of over-simplifying. I would still install one sheet at a time across the roof, just make every other joint "tucked under" instead of laid over.  Might be helping mixing metal with poly sheets on barn/shop work.  As I say, I've never tried it ....and the plan may pass quietly with time.

I have trouble dimpling metal by walking or working on it, as the final edge is near.  I use sheets of plywood and rubber tumble mats to minimize the risk.  (Guess I could try losing some weight .... Nah!!)

JRR when I have done it I usually start at one end making sure that it is laying square with the sheeting/roof.  Attach the farthest rib down then each subsequent sheet is laid on the next outer rib.  In essence you are covering the previous rib with a new sheet.  Continue in that respect and there is no need to lift and insert the next sheet.  I find it helpful to put a stop (if working alone) on the eve or a string line across the bottom eve (If you are lucky enough for a helper) to keep the bottom edge straight. I block out that string or stop to allow sufficent overhand. The top is not critical that it is cut exactly to the ridge as the cap has a wide flange which will cover several inches and if you are puttting a ridge vent you will want to allow room for air movement.  I use about 3' spacing between rows of screws with the bottom being attached fairly close to the eve/gutter.  The top only has to have a bare minimun as you will be doubling in that area when you attach the cap.  Word to the wise.  Do not put any screws into the peak of the metal near the top where the ridge is attached.  It will prevent the ridge from lying flat on the metal.  In fact I do not put the screws in the peaks of the metal only flat areas.  

Offline Ernest T. Bass

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Re: Metal roofing; what is really the correct way to install ?
« Reply #12 on: May 17, 2010, 09:08:33 PM »
There would be no problem removing individual panels for future repairs with each panel overlapping. A sheet could still be unscrewed and lifted out from under it's sister. This method is preferred to reduce the risk of wind damage/uplift.. You should always start laying sheets on the edge furthest from the prevailing winds, so that the overlaps are facing away.

Also, most sheets will have one side that has a larger metal lip extending out from the rib. This is supposed to be the rib that goes under the next sheet; the extra metal helps support the joint when it is installed over open framing.

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