install over spaced sheathing?

John Pautler
9/2/2016
Gentlemen, I want to replace my 32 year old cedar shake roofs with standing seam metal. The shakes are installed over spaced sheathing (1x4's with 1.75" spaces between the boards). I have narrowed my decision down to two contractors, both of which use a 24 gauge Kynar product that is mechanically seamed. One of the contractors insists on first overlaying the spaced sheathing with OSB, thus making a solid surface. His reasoning is that while the spaced 1x4's will provide an adequate surface to mount the clips/panels to, potential pressure differences caused by wind can/will promote leaking. The other contractor, who uses a supplier that is a member of Metal Roofing Alliance, insists that a solid sheeting is not required and they can install directly on top of my existing spaced sheathing. Their warranty will cover this type of instillation. Please see the attached picture of the underside of the roof as it currently is. I have between 75 and 80 squares so the difference in price is considerable. Thanks in advance for any advice, wisdom and opinions that you can give. John
Dick Bus
9/2/2016
first of all thanks for selecting a metal roof. installing over skip sheathing is fine. The underlayment is very important. Grace Ice and Water Shield or Titanium UDL50 is recommended along the eave up to 3'0" beyond the inside of the exterior wall; as well as all valley's. the rest of the roof would be covered with Titanium PSU30 or something similar. if your pitch is greater that 3/12 I would recommend that you use a snap lock panel with expansion clips as opposed to a mechanical seamed panel. the other important thing is verify the details that the contractor will be using. Ask him ahead of time and confirm with the panel manufacturer. The members of the Metal Roofing Alliance have agreed to promote the best metal roof for the residential market.
Todd Miller
Classic Products, Inc.
9/2/2016
Thanks. I want to second what Dick said. I agree. I do want to weigh in on an additional point though. Sometimes folks read responses on here and it's easy to take things out of context if you do not read the question well and understand all of the variables. John is in Tucson which maintains low humidity. However, if he was in a damp climate and if his attic did not have good ventilation, I would not support anything other than an installation over solid deck -- too much opportunity for condensation issues otherwise on a residential application. One additional note: Wood shakes on spaced sheathing naturally breathe a great deal -- more than a metal roof likely will. In applications like this, it is good to evaluate ventilation needs and perhaps increase the attic ventilation, both intake and exhaust.
John Pautler
9/2/2016
Thanks for the quick replies. Please elaborate why you recommend snap lock with clips rather than mechanical lock with clips. I have a decent amount of pitch (see attached pics). Also, about the venting: The MAIN HOUSE has 2 cupolas with two vents each on opposing sides of each cupola, also has gable vents. No eve vents. DETATCHED GARAGE has 1 cupola with same vent style as house, no gable vents, no eve vents, and is fully drywalled inside. BARN has1 cupola with same vent style as home and detatched garage, no gable vents, no eve vents, no drywall and rather open with regard to airtightness of doors, windows, seals etc.....its a barn. I have never experienced a poorly vented attic situation so I don't know what to look for. Is it possible to tell from this description, and attached pictures, if I have adequate ventilation for a metal roof? Should I consider ridge venting and/or eve venting? Many thanks! John
John Pautler
9/2/2016
pic of barn & detatched garage
Todd Miller
Classic Products, Inc.
9/2/2016
Thanks John. Beautiful home! I will let Dick weigh in on the standing seam question ... my experience with mechanically seamed systems is very miniscule. On the ventilation issue ... being in Tucson, I am not overly concerned about condensation. However, a well vented arctic does help with energy efficiency and, sealing up your attic with a metal roof, you could actually see your attic get warmer than it was with wood shakes. A smart idea would be a metal roof with reflective pigment for energy savings -- something that meets Energy Star requirements. A great source of venting information is www.airvent.com Good ventilation will help with energy efficiency. Good ventilation requires intake vents usually in the eave soffits, and exhaust vents usually at the ridge. It sound like you have gable vents now ... if you could with the new roof switch to eave and ridge vents, that will be more beneficial for you.
Todd Miller
Classic Products, Inc.
9/2/2016
I should add ... the cupola vents are probably helping but rather than really get good air flow throughout the attic, you're just getting airflow from the gable vents to the cupolas
Guest User
9/9/2017
I have replaced the old roof decking/ceiling with new 1 x 12 T&G knotty pine. The old knotty pine was crack split and in bad condition. On to the new roof deck I have put in 2 x 4" framing with the void between each at 3-1/2" deep. I will fill the void with 3-1/2 foil faced (front and back) ridged foam insulation. This will create a even surface all the way across the roof. My question is: Can I put skip sheathing down properly space to meet manufacturers requirements for spacing. I can fireproof tape each framing stud and help seal up the edge of the insulation and creating a fire barrier across the entire surface. It is my understanding that ventilation is not required with ridge foam foil faced insulation when using spaced sheathing to attach a metal roof. If Standing beam flat panel will not work in this situation I could use a style of metal panel that will allow the airspace to breath. Can I use skip sheathing or properly spaced sheathing for this roof?
Info @windowsonwashington.net
An informed customer is our best customer.
9/10/2017
If you are putting down and additional layer, why not cross batten the roof for a bit of above deck ventilation? If not, I would prefer to see solid decking here with detailed seams for air tightness.
Guest User
9/12/2017
Thank you Eric. The rafters were from the old school and are spaced at 30"s. Width of the roof deck is 32' (1' overhang on each end) the width of the building is 30' so the 16' knotty pine takes two boards to do one row on the roof deck and 15' of each board covering the 30' wide building. Because of the amount of waste that would come from OSB to meet building code nailing patterns and offset sheathing due to the substructure raft spans. I was looking at skip sheathing and had questions about ridged foam insulation and venting. From what I understand that with solid ridged foam insulation properly installed (3-1/2 foil faced) that condensation wouldn't be a problem. There is also the option to use a standing seam metal roofing with air flow with a design that isn't flat across the plane. Part of the panel is raised in two places. The other consideration is the weight with solid decking. I want to keep the roof light and tight. The batten could be an option but the ridged foam creates a strong vapor barrier. With additional fire resistant underlayment and pressure treated skip sheathing and little condensation it will probably work. The ridged foam insulation doesn't hold water vapor like the old fiberglass insulation. Thank you

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