Installing metal roofing on my 24 x 28

Started by grover, August 11, 2013, 07:56:59 AM

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grover

My original plan was to have the trusses, then the osb sheathing  which is usually 7/16, then the 30# tar paper then the metal.  Well, me being ignorant about metal roofing, I thought you just lay a piece of metal and your screws go through the metal and into a truss.  Screws every 24" and everything is great.  I found out yesterday that this is not the case.  Makes perfect sense now why I see many putting the purlins on horizontally.  This gives something to screw into without trying to hit a truss.
The question is now how should I do it?  To back up a little I am using scissor trusses that are 12/12 pitch outside and 8/12 inside so all of my insulation will be at the 8/12 part of the ceiling next to living area.  I wanted to use the osb decking just to make everything a little more solid and to resist racking of the trusses in a high wind.
Any advice on how to install the metal?  Osb, no osb? osb with purlins?  I'm confused now.

Redoverfarm

Grover on unheated space such as a porch or outside shed I would probably use purlins.  But on insulated space I went with OSB, titanium felt and steel roofing.  Titaninum was used to protect the OSB for a longer time between the OSB being installed and the roofing tin. If there is no extended period of time between the two then regular 30# felt will suffice.  I like to put a row of Ice/Storm guard on the eve edge and also any valleys. I would not use 7/16" but rather 5/8" OSB.  One problem with purlins and no OSB/felt is it will be prone to insects and critters making access into the insulation cavity and some condensation from the steel onto the insulation.  Most in this area install the metal directly to the OSB and eliminate the purlins for heated living space.  The roofing manufacturers approve this.  Another draw back to purlins and OSB used in combination is that it does not allow an escape route for condensation. 

There is virtually no way to install the metal and be able to hit your rafters with the installation of the screws.  It is acceptable and done all the time to attach it to the OSB.  Most DIY's put way too many screws in their roofing when installing.  The more you put the more chance for leakage.  Every 24-36" horizontal rows of screws in the middle (eve to ridge) will suffice.  On the eve one screw every valley space.  On the ridge area just enough to hold the tin in place as the ridge attachment will attached and penetrate those sheets.  No screws in the ridge peaks of the tin because it will not allow for the metal to expand and contract. Yes it will.  I can hear mine in the AM & PM moving opposite direction with the temperature change. Just my $.02 worth.


MountainDon

OSB for the structural rigidity and to remove the metal surface from the condensation plane. 

Even over open porches, etc. EXCEPT where the bottom is exposed to weather you should use plywood. Looks better and OSB is not approved for weather exposure like under open eves or a porch roof.

No purlins anywhere.

#30 felt or synthetic weather resistant barrier.

Metal roof panels screwed to the OSB on top of the felt.

Just because something has been done and has not failed, doesn't mean it is good design.

grover

So I assume it is pretty commonly accepted to screw the metal to the osb but the osb needs to be more than 7/16?  I'm pretty sure the lumber salesman has had 7/16 osb delivered to the site although I didn't measure it.
The only other solution I can think of is pulins on the trusses and then the osb but I would lose some of the structural rigidity that the osb directly to the trusses gives me.
If we go with purlins with no osb I would think I would need some sort of foam board under the metal to eliminate the condensation issue.

Confusing

MountainDon

5/8 OSB makes for a finer roof, but my entire neighborhood is done in 7/16 and there are a good number of metal paneled roofs included in that. Metal screwed to the OSB.   Nobody's roof peeled off in the recent high winds; gusts hit 77 here.

OSB should be nailed to the trusses; 6" on edges, 12" in the field.

Condensation in our attic (on the underside of the OSB) has never been a problem. I do see point condensation on a few screws at times, but that is all. OTOH, dew on the upper exterior surface of the metal is a commonplace thing. If the attic is ventilated properly there should be no issues with condensing on the OSB.

Purlins are not needed when OSB sheathing is used. OSB solves potential problems and makes a stronger roof diaphragm.  My opinion and I stick with it.

Add to that; not all metal roofing panels are structural rated. They should be if the roof is not sheathed with OSB/plywood.


Just because something has been done and has not failed, doesn't mean it is good design.


JRR

Opinions vary.  I like sheathing ... plywood, not OSB.  And I like purlins, adds stiffness, ... and makes for easy overhangs at the ends of the roof.   Foam insulation can be added between sheathing and purlins.  I still use the plain old corrugated style and get the heaviest gauge I can afford.

Don_P

If the roof is vented under the sheathing as is typical, foam up above the sheathing is not doing a whole lot for you and purlins for the overhang are going to peck out pretty quickly as overhangs and snow loads increase. I can see the logic but it doesn't seem to be the best bang for the buck. I use 5/8 osb typically but plywood is fine, 7/16 doesn't do much for me personally, I'd take it back and trade it. Typically "venting" the underside of a metal roof is what causes condensation. Seal it to the sheathing and vent the underside of the sheathing. I use the foam closures made for the metal  at the top and bottom to seal the metal to the deck and avoid letting cool nighttime high humidity air in there to condense.

JRR

My last project was a cathedral ceiling, therefore insulation over the sheathing worked quite well.  I used 5/4" decking as purlins ... and the overhangs were about 1 1/2ft ...stiff enough for any snow we will ever get in Tenn.  (Actually, high winds are what we have to build for.)

I have personally never observed metal roofing "sweating" underneath ... in barns or porches.  Doesn't mean it doesn't happen, of course .... but I think it is a small occurrence.  (Now if you have a barbeque in a metal roofed barn, in winter, then yes ... get the umbrellas out!)  I take the opposite stance on venting, and "super-vent" the metal.  Atop the sheathing went 4" of rigid insulation, then felt, then hold-off strips (again, 5/4" decking material) attached thru the insulation to, and aligned with, the rafters, then the purlins ... etc, etc.  The edges get stuffed with metal screening to keep out birds and friends.  I figure a free-breathing system will quickly and easily get any rid of any condensation should any occur.  This method rejects some of the summer-heat which we usually have a lot of ... but not this rainy year!

Don_P

I agree with foam over an exposed rafter cathedral as one good approach, we call that "built up" or "wrap and strap", sheathing (usually exposed T&G), foam sheets with edges between layers offset and taped, then purlins up and down the roof forming the venting channel and ply with metal or shingles above. But, this is a scissor trussed roof. The insulation is typically under the top chords, on the ceiling chords, with venting inside the truss assembly.

The problem with venting the underside of metal directly is when moist air vents under the metal and the metal is below the dew point for that humidity level, it's one of those psychrometric equations, the moisture condenses on the cool surface. If you see dew on top of the metal there is more than likely dew on the underside of the exposed metal, and that does vary with climate... or rather with relative humidity and dew point. I seal the metal to the sheathing, vent under the sheathing, and avoid the entire issue in any climate or humidity situation. The wood is less sensitive to those short term temperature swings and when it does happen the wood acts as a "hygric buffer" absorbing the small amount of moisture and then drying back out when the conditions change rather than having liquid moisture on an impervious sheet of metal that can lead to corrosion. Actually in dirt floored barns with exposed metal roofing I've seen pockmarks in the dirt from the daily morning "rain" inside. The purlins in my sawmill are heavily water stained from this and the metal is often covered with beads of water in the morning, no walls, super vented, the damp air contacts the cool metal and condenses.

With the type of built up roof I described above, the T&G is usually 2x6 on top of beam rafters and I'll extend the T&G out to form the overhang. I've had an engineer walk me through the math for cantilevered flatways purlins and lookouts and can probably do a post on that given an example of snow or wind loads in another thread if it is of interest. We do tend to underbuild those areas, and I've got plenty of guilty examples in my past. They are a frequent place where houses begin to come apart in wind events.


jamesp

#9
so a slightly OT query, regarding re-doing a roof on a house.  Its a simple gable roof (small 700sq ft house), ceiling space is small.  I suspect the ceiling doesn't have much insulation, especially near the eves/edge of the roof.  Shingles need replacing, preferably with metal.

Is it feasible to add more insulation to the ceiling, likely blocking any ventilation that might remain.  Then add foam above the existing roof sheathing, then a ~1" air space above the foam, then OSB/ply, then metal.....almost be converting the roof to a cathedral ceiling. 
I assume a vapor barrier would be needed between the living space & the now non-vented ceiling (which is feasible as the inside ceiling could do with replacing also).

Don_P

A double vapor barrier with loose insulation trapped between, sounds like a bad idea. If you want to go unvented I'd remove the existing insulation and the ceiling and sprayfoam the underside of the roof sheathing. I think that would work better and be less expensive than what you are proposing.

jamesp

ok fair enough, surprised spray foam would be cheaper than rigid foam though(?). 
I hoped there was a way to add insulation to the top of the sheathing, as the shingles need to come off anyway.  I haven't looked in the ceiling yet (place is a cabin I'm considering purchasing) & I don't think there is much/any insulation about the eves, & I don't think there is much room to add any.  Otherwise the easy thing would be just to add insulation to the ceiling.

Don_P

Sprayfoam is not cheaper than sheet foam but it is likely cheaper than sheet foam, purlins and another layer of sheathing. If the roof is trussed or lightly raftered think about the additional load as well. If you add more insulation to what is there install air chutes (prop-R-vent or similar) fromm the outside wall line up above the insulation to preserve the vent space. Cardboard or thin plywood from the wall top to the underside of the chutes will keep the insulation from spilling into the soffits and prevent airwashing thru the edge of the insulation.

Took a pic inside the sawshed this morning;