Replies

1. 
   Hokuto | Oct 05, 2011 12:10pm | #1

   We just had a steel roof installed over the existing asphalt shingles; the roofer said he thought using purlins (or furring) was not good, since more condensation could form underneath, so he installed the steel directly against the shingles. So far so good.

   http://s27.photobucket.com/albums/c197/Peregrinor/ww/2010_housework/P5287889crw1.jpg

2. 
   Piffin | Oct 05, 2011 05:46pm | #2

   six of one and half dozen of the other - pros and comns both ways.

   For cold climate, I derfinitely want it on purlins to create a cold roof. That has advantages for you in FL too, as it helps keep int attic a bit cooler and wqill reduce your cooling loads.

   dsadvantages - you can get wind under it with purlins so you need to check local regulations to see iof that is permitted. Another concern is "oil canning", esp with the lighter 29ga screw down panels. Less of a concern with heavy standinf seam or 24-26ga. Ecen less of a concern when on firm sheathing. Use tarpaper or syunthetic underlay regardless for on the sheathing

   I would never want the metal rioof directly on the old asphalt shingles. That will wear it out from the back side and have more ratehr than less condensation problems. That other roofer was screwed up

3. 
   [email protected] | Oct 06, 2011 02:11pm | #3

   flat on the deck

   I have it installed on the decking, with a layer of 30-lb felt, and under two layers of rosin paper to provide a slip face.  But, I'm trying to make the buildings "fire smart" for the wildland urban interface.  So the less air can circulate under the roof, the better. 

   I also have it specified to fill the openings at the bottoms of the ridges with a coarse stainless steel wool to prevent embers migrating into the void. 

   If I read your post correctly, you have 5/8-inch decking on now, and they are saying thye want to install 1X4s to raise the roof off the decking.  I don't see how this will not cause a problem with condensation on the underside.  The moist air will still come in through the ribs, and cause condensation on the underside.  The 1X4s will trap any condensate in the void.  I'm guessing that would cause rot of the decking and the 1X4s at the top edge where they meet. 

1. 
   DanH | Oct 06, 2011 07:51pm | #4

   I think you overstate the problem of condensation.  The air that infiltrates under the roofing will be the same air as on the outside, and will have the same dewpoint.  Some slight concensation may occur when there is a "dew fall" (which basically means a clear night when radiant cooling can cool the roof surface to below the dew point), but relatively little will occur on the back side of the roofing because there's simply so little air.  And as soon as the sun hits the roofing and the dew evaporates on top, it will be gone below (actually a bit sooner, due to heat escaping through the roof).

2. 
   Piffin | Oct 07, 2011 06:46pm | #5

   While you are guessing, I am going from experience. It does not do that. The cold roof scenario vents out the moisture.

4. 
   onyxxteriors | Nov 21, 2011 09:34am | #6

   metal roof install

   First, call your local code inforcement to make sure you do not have any restrictions in your area.  If the roof has no roen decking and only one layer of roofing you can usually apply an insulating barrier (1/4" foam core insulation) and just apply right over the existing roof.  Applying right over the shingles will rub the back of the metal roof and eventually holes in the panels.  If there is two layers or more of roof, the old roof must be taken off down to the decking and then the weather / insulating barrier applied to avoid any burs rubbing on the roof.  I have never suggested the furring strips you speak of as too much air under the panels can cause them to loosen and eventualy blow off.  All the metal roof panels we install are an inch long.  This allows the end of the panel to be folded over the drip edge and locked on to prevent high winds from snagging the panels.

   
5. 
   BossHog | Nov 21, 2011 09:47pm | #7

   I think there's a minimum embedment for the screws that hold the sheet metal down. I'm going from memory, but it's something you should probably check.  Especially since you're in a high wind area.

   I'm thinking our metal supplier (Fabral) requires either 3/4" or a full inch. Something I probably oughta check on myself.