Metal over old roof if ...: "I've read many of the threads in this forum (very ..." - Guest User (10/24/2001) | Roofing over existing roof | Archive

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I've read many of the threads in this forum (very helpful) but still have these questions We are putting a new roof on a cabin in NH this weekend. I'm hoping I might get a quick reply w/ some advice on ventiallation and removing the existing roof first. The old roof is one layer of asphalt shingles over 1" foam over T&G (1x6?) over the roof rafters. There may be tar paper in that mix somewhere too. (Sorry but the cabin is 3hr away so I cannot check right now.) Inside is cathedral ceilings w/ fully exposed rafters and underside of the roof decking. A few leaks cropped up last year and large parts of the roof have a light greenish tinge (algae?). The cabin is in a wooded area and does not get a lot of direct sun. If I tear off the roof I'd probably end up trashing the insulation we have (more expense to replace it). Access to the cabin is by a 1000' foot path so getting the old roof out would be a real pain. I know I need to add a ventilation before applying the metal roof. I'm thinking of running 2x4s up the shingles (vent channel) then put 1x4 purlins across the roof to attach the roof panels. I'm guessing I'd put 30lb tar paper over the 2x4s and under the purlins? Is that enough of a vapor barrier to protect the underside of the panels? 1) Do I need to remove the existing shingles. A friend w/ some roofing experience suggested that the old roof would rot under the shingles and make a hidden rotting mess possible ruining the decking. 2) Does the above vetillation plan seem adequate w/ or w/o the old roof? 3) Since it is NH, how might I add insulation outside before roofing - tear off old roof and add 1-3" of foam below the above proposed ventillation (2x4s/paper/1x4s/panel)? - add above exising shingles under 2x4s? - Faced/Unfaced? - type of foam? Thanks. _Mike	Guest User 10/24/2001	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/362/find/
I am so sorry for the delayed resonse. This was the week of our major national trade industry trade show, MetalCon. As a result, most of us have been out of our offices and quite busy. Really, you have this all thought through well and you have a good plan. Make sure that the metal roofing panel you choose is appropriate for installation over lathe and then be sure to adhere to the manufacturer's suggestions as far as lathe spacing, underlayment, etc. Generally, I'd put the new underlayment on top of the existing shingles rather than between the metal and the lathe. Putting it between the metal and the lathe would make it prone to tearing and, in heat, the felt might stick to the metal and then tear when the metal expands and contracts. The only thing I'd add to your equation is that this airspace you are creating needs to be vented. You need to allow space to have soffit vents at the bottom and then ridge vent at the top. This will allow moisture to escape and, of course, eliminate any problems that could result from collected moisture. All in all, you've done a fine job on your research. All Best.	Todd Miller Isaiah Industries, Inc. 10/26/2001	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/363/find/
I am thinking about putting a metal roof on my home. At the present time I was thinking of having it installed righ over the existing three in one shingles. I also was planing to use no lath or other materiai between metal roof and existing shingles. Would that be feasable.	Guest User 5/5/2002	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/364/find/
Many of today's residential metal roofs are very appropriate for installation over existing shingles. This is due to the low weight and unique design of these products. There are some products which require lathe boards and they work very well also. You're still getting a low weight and very durable roof system. I always, though, recommend the use of an appropriate underlayment between the old shingles and new roof. This serves several purposes including protecting the back side of the new metal panels from the abrasive granules of the old shingles. Typical underlayments are 30-pound felt and also some newer poly materials such as Triflex 30 and RoofTopGuard. In all cases, follow the recommendations of the manufacturer of the metal roof system you choose.	Todd Miller Isaiah Industries, Inc. 5/6/2002	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/365/find/

Metal over old roof if old roof wet