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I am planning an addition/re-roofing to my home and thinking about going with a metal roof. The looks of the metal roof's along with the different color's really make the metal roof stand out. My question is the underlayment any different than a shingle roof? I was planning on 3/4 plywood for the addition, then a heavy felt paper then the standing seem metal roof. The re-roofing would be over the existing osb board after the shingles are removed. Some of the posts mentioned titanium as an underlayment? There was also mention of lathe strips? Do those have to be used on top of plywood/osb? or can metal be installed without the lathe strips? The attic has a good ventilation system and have never had any problems with it (25yrs.). I have really enjoyed the postings and comments from everyone. Thanks. dadzo from Pittsburg Pa.
 

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Commercial Roofing
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I think this is pgriz's area of expertise.

Paul, if you will..........
 

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I know Paul typically uses GIWS, which is the premium of underlayments.

I usually place a sheet of "red rosin paper" between my I&W and my metal when I do standing seam. Don;t ask me why, I have read quite a few specs so I do it :) Check with SMACNA

I've never used lathes, though I know there are some guys who do. I have also never done roofs larger than a porch, dormer or bay window.
 

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Grumpy said:
I know Paul typically uses GIWS, which is the premium of underlayments.

I usually place a sheet of "red rosin paper" between my I&W and my metal when I do standing seam. Don;t ask me why, I have read quite a few specs so I do it :) Check with SMACNA

I've never used lathes, though I know there are some guys who do. I have also never done roofs larger than a porch, dormer or bay window.
Thanks, Grumpy and AaronB, for your confidence in my expertise! I hope it is not mis-placed. :)

dadz04, if you are putting on standing seam, you will most probably need to install it over horizontal battens. However, since horizontal battens create air spaces but not true ventilation, I would suggest that you place vertical battens below the horizontal ones, so that you can have air movement by convection. As for underlayment, it is there to solve a problem. In most cases, Triflex-30 or Titanium UDL should be sufficient, UNLESS you get a situation where you may have pooling water. In that case, pooling water will work its way around a nail hole in Triflex or Titanium, so that's where Grace Ice-and-water shield (GIWS) would be useful. Now it is very unlikely that you will have pooling water under a standing seam roof, and the most moisture you will generally get will come from condensation on the metal underside, so the Triflex or UDL should be sufficient. If you provide for the vertical ventilation that I mentioned above, even the condensation will evaporate pretty quickly.

GIWS is commonly used under shingles because of ice-damming, which allows pooling water to back up and enter between the shingles. If there is enough water, it will effectively be under pressure, and this will be sufficient to cause leakage around nail penetrations in thin underlayments. By using GIWS, which is thick and resilient, you get a gasket effect. Of course GIWS is designed to be adhered to the deck - it does not work well if it is just laid over old shingles, like I have seen done a number of times.

Coming back to metal panels - they do expand and contract, and on a 20-foot panel the maximum length change would be about 11/32 of an inch (using -40F as the low temperature and 180F as the high). If you fasten the panels directly over the sheathing, then the expansion and contraction will alternately push and pull on the screws which are holding the panels down. Something has to give, and if the attachment is very firm, then over time you will see the metal around the screws forming oval-shaped holes. If you put the panels on battens, then these will swivel a little (the screws will be deflected), and there is less chance of the expansion causing the screw-holes to widen into ovals.
 

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Paul, I did not know what the battens were for until now, cuz I have seen them done both ways. Something new every day. Im gonna go sell a standing seam roof now. LOL
 

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tinner666 said:
Actually, on a standing seam roof, the panels are held on by hidden 2" cleats, 12" O.C. And they make a 2 part slider cleat that's supposed to handle all that movement. I allow 1/2" play along drip-edges on rakes and eaves. Frank
Frank is right, plus I have used the less expensive (notice I didn't say cheaper) SS panels that have screw slots built in and not clips. You use flat screws and dont quite crank them home so the screw rides up and down the expansion slots. Kinda like putting on vinyl siding. About the underlay talk, we use titanium, but you know during a roof repair yesterday I took some 5v panels off regular 30 lb felt underlay and it was not stuck to underlay like some claim it does. Roof installed 18 yrs ago in Florida. Mark
 

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Metal Roof Install

I would reccomend on the new construction using a 30# felt plus if you want a red resin paper wouldn't hurt. we install 1" standing seam on residential projects every day. we generally use a 1x nailer on reroofs because it takes out the inconsistancy of the roofing substructure you can "shim" up areas that are sagging, and bridge over areas that are humped. we have always just installed horizontal strips.

we have now started using a lot of "bubble/foil/bubble" insulation UNDER the horizontal strips this creates a great radiant barrier and also adds winter insulation as well you can also use the bfb on top of the felt on the new construction. and directly apply the standing seam roof to that.

GOOD LUCK

:Thumbs:
 
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