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A little more information would be helpful such as geographic location, building use, slope and material. First recommendation is to contact the manufacturer. I will assume that you are installing a vertical rib metal panel of some nature.
All roof materials condensate to the underside. Metal roofing transfers hot and cold faster so in some areas may cause more dew point crossings than an asphalt shingle on a plywood deck.
The new IBC/IRC building codes call for a moisture barrier to be installed under the roof covering, minimum 30# felt paper. Some manufacturers and/o building officils allow that the existing asphalt shingles if in good enough condition can act as the moisture barrier.
I say that you are putting on a lifetime roof, so the accessorie items should be equal to the expexpected performance and we recommend using a reinforced underlayment, regardless it is OK to install it over the shingles and then the 1x4's.
Make sure that you investigate and install adequate ventilation. The requirement for attic buildings is 1sf/150sf of ceiling area unless it is balanced between the ridge and eaves and the requirenment drops in half to 1/300. If this is a vaulted ceiling, then you must apply a minimum of 1" vented free air space. This can be accomplished by running 1" strapping vertically over the existing rafters, then installing your underlay with the horizontal 1x4's over. You can get vented eaves and ridge closure to allow the air flow.
So no you do not need insulation but make sure the attic or roof is ventilated to code. Good Luck
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Allan,
We would like to install a metal roof over our cedar shakes.
Our house is a thirty year old bungalow with 2/12 pitch roof. The entire ceiling is vaulted with 2x10 rafters and lined with t&g cedar. It has r10 paperbacked fiberglas insulation stapled between the rafters. The rafters are strapped and shakes applied directly to the strapping.
Can we install the metal roof over the shakes by strapping the roof? What do you mean by "reinforced underlayment"?
We would also like to increase the r factor in the roof. What would you reccomend?
Thanks,
Jeff
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Thanks for the info which makes advice easier and more exact.
R10 insulation is not that much even foe air conditioning in Florida. This is only about 3" thick. This leaves you with a6" air space. The pitch is awfully shallow for a wood shake roof at 2/12 or is that a mistype and should be 12/12.
The problem with upgrading insulation is ensuring that ther is no air space which means removing the wood shakes and parts of the space sheathing to top up the fiberglass. I would take the opportunity and make the investment as it will pay back with rising energy costs.
I would remove the old roof and parts of the spaced sheathing and add in 6" (R20) of insulation. While the paper backed insulation is not the best air barrier ir is better than nothing. Don't forget to consider upgrading you ceiling electrical needs at the same time.
I would then install vertical strapping ovet the rafters, then a TriFlex 30 underlay or equal for a moisture barrier. On lower slopes lap the membrane farther and be sure to tape all the penetrations with house wrap tape. Then strapp yor roof horizontal and choose a metal roof designed to go over strapping.
Choose and interview a contractor wisely and you will be forever thankful that you made the investment. PS, Don't forget to ventilate the air sapce and choose an Energy Star roof. Good Luck.
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My mother is considering buying a home in Seattle which has a metal tile-style roof which was installed over wood shakes. The inspector was concerned that if the metal roof is screwed into the wood shakes, the screws would come loose as the shakes deteriorate further. Also, having moisture trapped under the metal roof was a concern. What potential problems should we be looking for? The current owner can't seem to find her paperwork so we do not know who the installer was, and what kind of warranty there was. In general, is installation over wood shakes ok? Any information would be much appreciated.
Thanks, K Prewitt
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There are many types of shake, shingle, and tile facsimile metal roofs designed and approved for this type of application. Most of these systems, through some method, ensure attachment through the wood shakes to the lathe and/or trusses below.
I have worked with these typs of systems for over 20 years and have never seen any problems develop.
You should have a very sound roof on this house. Just the same, if you like, use this website to find a metal roofing contractor who could be hired to inspect this roof.
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Further to Todd, we have a product similar to what you mentioned and we we are reasonably familiar with some of the other similar products. If you emaied a picture directly I may be able to help you out.
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installing steel roofing , manufacturer recomends to install over 2x4 purlins. trusses underneath purlins. what type of insulation should i use and where is the best place to lay the insulation, over the trusses or stapled between trusses also will an osb board ceiling and then a drop ceiling installed affect this live in PA concerned about condensation thanks folks, Brian Bone
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This specification certainly does not meet the new building code for an occupied building. The code clearly states that a roof assembly is;
Roof covering,
Roof underlayment (moisture barrier)
Vented air space ( balanced between ridge and eave)
Insulation
Interior air barrier
Finished ceiling.
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We live in Southeast Missouri and want to install a metal roof over our existing asphalt roof. Please advise if this can be done. Also, the guage, etc. needed.
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Many of the more heavily formed metal roofs, including the shake, shingle and tile profile panels, can be installed over existing shingles.
You might wnat to contact the manufacturers of some product you like and make this inquiry with them.
I always suggest a layer of underlayment over the old shingles in any event.
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I am about to put a standing seam roof on my little Sunday House in Fredericksburg, Texas and being a former insurance agent for over 40 years we did not like overlayed roof. I want to take off the wood shakes and need to know if I should required them to install plywood decking with 30 lbs roofing felt and not just mount the metal roof to the 1 x4's. What are your thought? Also, what guage of steel? Ridge Row coverings? Thanks.
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The first move should be to contact the manufacturer and get their recommendations and buy a complete package with the proper trims, ventilation and fasteners etc. Also there are insurance rebates for hail tested systems that you may want to look into which require a certain installation.
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Allan, what I really need to know is about putting roofing felt on top of the 1x2 before they put on the metal roof. The contractor said it is not needed. That when they install the roof it will put holes in the felt anyway. Am I getting the feeling that he thinks I am really dumb? What do you say?
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Unless the product has been tested otherwise, all roofing products require an underlayment which principal purpose is to trap any condensation from entering the roof assembly. The felt can go under or over the strapping however it must be above any insulation. I would further be concerned and check with the manufacturer on the use of 1x2 as the strapping as it seems undersized and will probably not meet wind uplift requirements.
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Allan, sorry I do have 1 x 4 s. I said 1 x 2 in error.
However, I wanted to know that if they use the minimum of 30 lbs felt should I also ask that plywood or other material be installed first to the 1 x 4 before they put the felt and metal on top of that?
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