Metal Panels Over Existing, 25-Year Old Mansard Roof

Kevin D.
5/4/2011
Before I begin, my apologies in advance for the myriad of background info and questions. I recently purchased a 1986 home with a Mansard roof that has its original, 3-tab shingles (see attached photo). There is evidence of past roof leaks in various rooms of the home, so I'm sure at least some roof decking will need to be replaced. The roofing contractor I'm hiring uses 29-gauge, low-rib panels installed over my existing shingles using ferring strips (1x4's or 2x4's) mounted 2' o.c. to create a batten system. He will replace any bad decking he finds, but beyond that my existing roof will remain. My questions/thoughts: 1) I am worried that, by not removing all my asphalt shingles, bad decking that APPEARS to be fine will be missed. Is this a legitimate concern? Or will a professional roofer be able to determine which decking is fine and which decking needs to be replaced, regardless of whether there are asphalt shingles in his way? 2) The above aside, is there a good reason for me to pay extra (for labor and dumpster haul) to have my existing shingles removed before installing the batten system and metal panels? The perfectionist in me doesn't like the thought of old shingles being under my nice metal roof, but the frugal part of me doesn't like the thought of paying extra for something that is unnecessary. 3) If the existing asphalt shingles remain, does there NEED to be a new underlayment installed on top of the shingles and beneath the ferring strips? Is this step needed if I'm using a batten system? Or is it simply a nice "second level" of protection, but not a must have? 4) Beneath my ferring strips (on top of my asphalt shingles or new underlayment, depending on the answer to question #3), is it worth paying a few hundred extra to have a radiant barrier (see atticfoil.com) installed? As I've understood it, a metal roof that's light in color is a radiant barrier itself. If that's the case, is there any benefit to having AtticFoil installed beneath the ferring strips? Or would it be a worthwhile purchase since the two layers of radiant barrier would make my roof SUPER energy efficient? Thank you so much for input. Kevin
Todd Miller
Classic Products, Inc.
5/5/2011
Good questions. My thoughts are below on your enumerated questions: 1) He should be able to recognize bad decking. On the other hand, as long as the battens are being attached through the decking and into the rafters, the condition of the decking becomes less critical. 2) In my 30 years experience, a big benefit of metal roofing is its low weight which allows installation over existing shingles. This allows a homeower to spend their money on a better roof rather than tear off and disposal costs. 3) I always like a new underlayment and I feel that it is required by the International Building and Residential Codes. 4) A radiant barrier would be helpful provided it faces an airspace. Probably more helpful though would be cross battens. Putting down vertical battens first as sleepers across your rafters and then the horizontal battens would allow you to ventilate up the roof, in the resulting chamber, bringing air in at the eave and exhausting out at the ridge, This would probably have more energy efficiency benefit than the radiant barrier.
Kevin D.
5/5/2011
Mr. Miller,   Thank you so much for your detailed response. I have a few follow-up questions I'd like to run by you. For clarity, I'll preserve our enumerated format:   1) and 2) Makes sense. Thanks!   3) Since this "underlayment" will be going on TOP of asphalt shingles (as opposed to underneath, per usual), do I need to use a special type of underlayment? Will 15 lb or 30 lb felt work? Should I use a "breathable" underlayment such as Deck-Armor by GAK? Should I use a synthetic ice and water barrier, such as Owens Corning WeatherLock, in the valleys and eaves? What is your recommendation?   4) Just so I understand: Battens would be placed vertically, across my rafters, and drilled through my decking and into the rafters. Battens then would be placed horizontally, on top of the vertical battens, which would provide ventilation underneath.   4a) Would these horizontal battens simply be drilled to the vertical battens, or would they need to be drilled all the way into the rafters as well?   4b) Do the metal panels attach to the horizontal battens or the vertical battens (or does that depend on the panels being used)? I assume  they attach to the horizontal battens; otherwise, I can't imagine why horizontal battens would even be needed.   4c) What size ferring strips should be used to create the cross-batten system? If the horizontal strips lay on top of the vertical strips (assuming I am understanding correctly), using 2" strips would result 4 inches of air space beneath the metal panels. Is that too much?  How much air space should I provide?   (A new question)   5) With there being evidence of roof leaks, attic mold is a concern. The attic ventilation is very poor and the home is in the hot and humid South. During the re-roofing, I will address the ventilation issue. But being a Mansard, my actual attic space is two-feet tall at its highest level. Getting into the attic and investigating -- much less scrubbing and removing -- any mold on the underside of my roof decking is a Herculean task.   With only bad decking being replaced, "good" decking could, potentially, have mold on its underside, correct? Once I am able to investigate, if it turns out I do have a mold issue, is replacing all my decking a cost-effective solution (since I'm re-roofing anyway)? I realize this may likely be outside your level of expertise, but any advice you can provide regarding attic mold from leaky roofs and/or substandard ventilation would be greatly appreciated.   Thanks again.   Kevin
Todd Miller
Classic Products, Inc.
5/5/2011
Thanks for the questions. I prefer one of the synthetics or 30-pound felt as underlayment. Ice and watershield should be used where required by code. Breathable underlayments are not a bad idea where no attic ventilation is present but if it is applied over your old shingles, keep in mind they are not very breathable. The horizontal battens could ideally be attached through to the decking and/or rafters. Generally a 1" or even 1/2" vented airspace is very good. It does not take a huge amount to create a thermal break. I would strongly suggest investigating the presence of mold. If there is mold, then that needs to be addressed. Adding ventilation is important. Adding it through cross battening is good and helpful but the main attic still needs to be vented. I understand the design of your home though and realize it is difficult. All Best.
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