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Standing seam metal roof over asphalt shingle roof

fruhmenschen | Posted in General Discussion on August 14, 2011 09:48am

I am currently dealing with a roof I orginally built 15 year ago on a post and beam frame home. The roof in question is a shed roof that meets a back wall with a clear story window installed  into the back wall.There is also a side wall that butts up against the shed roof .The side wall travels the lengthof the 7-12 pitch shed roof until it meets the back wall mentioned above.The measurements of the shed roof are 12 feet wide and 8feet long stretch that runs into the back wall.There is a permanent Velux 4 foot skylight in the center. There are currently 30 year asphalt shingles covering the roof.I have regular Grace ice and water shield going up two feet from the drip edge and two feet ice and water bent so 1 foot goes up my sidewall and the other 1 foot lays on top of the shed roof. This runs up the entire 8 foot span from drip edge to back wall. The side wall clapboards are step flashed over the ice and water shield.The shed roof was framed with 2×0 rafters  .The rafters are filled with 10 ” high density fiberglass insulation with proper vent capped by 5/8 ” plywood. The inside of the rafters are covered with 1″ foiled face thermax as a vapor barrier covered by sheetrock. Every winter I experience ice dams and leaking. Please note there are two shed roofs on this house , one on each side if you were looking at the house from the front.

QUESTION SHOULD I STRAP THE SHED ROOF VERTICALLY AND LAY NEW PLYWOOD SHEATHING COVERED WITH HIGH TEMP ICE AND WATER SHIELD TO LAY MY STANDING SEAM ROOF ONTO?THIS SHOULD KEEP THE NEW ROOF COLD IN THE WINTER TO PREVENT THE ICE DAM. THE STANDING SEAM METAL ROOF WILL NOW BE ALMOST AT THE SAME HEIGHT AS THE VELUX SKYLIGHT. WILL THIS CAUSE PROBLEMS?

 

PLAN B  QUESTION: SHOULD I OPEN THE OLD ROOF ALONG THE ENTIRE DRIP EDGE GOING UP 2 FEETAND PLACE 2′ POLYSTYRENE BLUE FOAM BOARD BETWEEN THE ROOF RAFTERS , TAKING OUT THE PROPER VENT AND MAKING THE 2″ FOAM FIT TIGHT BETWEEN THE RAFTERS IN EFFECT MAKING THEENTIRE 12 FEET OF DRIP EDGE COLD IN THE WINTER?THEN PLACE MY STANDING SEAM ON TOP OF THE OLD ROOF PLYWOOD SHEATHING? THIS WILL PLACE THE STANDING SEAM SOME  4″ BELOW THE VELUX SKYLIGHT.

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  1. Scott | Aug 14, 2011 11:49pm | #1

    Yeesh... sorry, but the all caps text makes it harder to read.

    As for the ice dams and leaking, it may be pimarily an insulation issue. Changing the roofing may help, but you may also have success with insulation, which might be less expensive.

    1. fruhmenschen | Aug 15, 2011 12:59am | #2

      Scott: When you say it could be an insulation problem, I tend to agree with you.

      The issues for me are :

      1. should I go back into the old roof , remove the proper vent for better draft

      and lay my metal roof over the old roof

      2. should I go back into the old roof, rmove the proper vent foam and insert 2" panels of blue polystrene

      and replace the old plywood then put on metal roof

      3. should I leave existing roof and strap over it vertically to create air channels

      then sheath with plywood and put my metal roof on.

      keep in mind by strapping/sheathing over old roof my standing seam metal roof will now almost be at the same level/height as my 4x4 foot velux skylight. What problems will this cause?

      1. Piffin | Aug 15, 2011 12:07pm | #3

        Regardless which option you mention using for the roof, you will need to pull the skylight and re-install it.

        Unless you were to use I&W entirely by tearing off the existing shingles and installing the metal to that surface. Your skylight flashing has to integrate with the roof material.

        I am aftreraid I don't really understan d all the text description without a drawing or photo.

        But it sounds like the problem is more in your insulation/ventilation package than in the roofing. Cathedral type ceilings are notorious for condensation problems and ice damns

        1. fruhmenschen | Aug 15, 2011 05:06pm | #4

          This shed roof is located on the 1st floor in the kitchen  so all the heat rises because there is open architecture.Granted some heat will get past the 10 " of fiberglass and 1" of thermax vapor barrier. I agree that for whatever reasons the soffiits are not venting my roof properly in the winter.

          How do you feel about enclosing the remainder of the 10" rafter space by fitting 2" blue polydtryene board between the rafters?

          1. Piffin | Aug 15, 2011 06:48pm | #5

            I think if you are going to tear things up on the ionterior, you shuld use sporayed polyurethene foam or blown cellulose and forget venting. Use a good vapor retarder.

            But first spend time stuying the building science website and details there

          2. Scott | Aug 15, 2011 11:22pm | #6

            >>>But first spend time

            >>>But first spend time stuying the building science website and details there

            Zackly what Piff said:

            http://www.buildingscience.com/

            There's lots of good info there. You need to dig for it, and be willing to learn by reading (which can be tough for some people), but if you persevere it's worth it.

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