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Proposed roof assembly for historic home

optimax | Posted in General Questions on

I own a historic home built in 1905 in California, climate zone 10. The roof currently has three layers of roofing: two layers of wood shingles and one layer of asphalt shingles. My plan is remove the top layer of asphalt shingles and the middle layer of wood shingles, but leave the lower layer of wood shingles that is against the skip sheathing; this will allow me to retain the authentic appearance looking up at the underside of the roof assembly from the living space below. I obviously need to re-roof and have a number of conditions that I would like to accomplish in the process. I have included proposed drawings for your consideration. Because this is a historic home, I would like to maintain, as much as possible, the authentic look of the home.


 1) Keep thickness of roof assembly as thin as possible while maximizing the insulation value.  I am adding a molding (see proposed drawings) to help camouflage the increased thickness. 

2) Weight is a major consideration; must minimize the weight of the roof assembly.  Let’s just say, structural engineering of the roof assembly left a lot to be desired 120 years ago.  The proposed roof system only weighs 4.37 lbs./ sq. ft.; that is all most 2.0 lbs. lighter per square foot than the existing 3-layer roof that is currently on the house.

3) The roof type would likely be considered a cathedral, as the underside of the roof structure is the finished ceiling of the living space below. I would like to retain this authentic “look”, therefore, no insulation or finished ceiling (drywall) on the inside of the roof assembly.  Only the underside of the skip sheathing and wood shingles are seen when you look up from the living space.

 4) Will there be a moisture concern or breathability issues?

 5) Is venting of the roof assembly necessary?

 6) I will be using the recommended installation instructions for the polyiso rigid insulation and the standing seam metal roof.

 Please let me know if this proposed roof assembly will work; if not, what do I need to change.  I would like to get this projected started.

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  1. Expert Member
    Joshua Salinger | | #1


    I'm going to assume you are in CZ3 as most of CA is in this zone- there are a few spots of CZ4 and CZ5, so let me know if you are in one of these.

    At the cross section of your drawings, I am not seeing the existing rafters, but I will assume they are located below the skip sheathing and are at 24" OC. The other thing I see on the section but not in plan view is the 2x4 3/4" framing above the skip sheathing. Is your intention to just have this at the eaves or also in the field above the shingles? The plan view drawings make it look like this member is only at the eaves.

    You will need something to attach the metal roof to. This can either be 1x4 purlins or sheathing that is secured to the rafters below with long screws. I recommend sheathing as it will make it easier to detail any penetrations, and make the roof more durable from debris falling and allow you to use a thinner gauge roofing. If you go with purlins you may want to consider setting the first course perpendicular to the eaves and the second course parallel to the eaves so any condensation or water can drain.

    Assuming you can get your insulation panels to lay flat I would stagger the panels to offset the seams. If you are going with polyiso, use the foil faced variety to create a class I vapor barrier. Tape the seams with foil tape to keep moisture from getting to the bottom of the sheathing or the bottom of the metal roofing. This will also double as your air barrier in the field. It will be tricky to detail the air barrier at the roof/wall connection at the eaves and rake. I'm not sure exactly how to best detail this without more information.

    The biggest issue I see though is that the 4" of polyiso doesn't meet the minimum code requirements for roof insulation. In CZ 3 you will need a minimum of R-38. This would translate to roughly 6" or (3) 2" layers of polyiso. This would certainly make your roof thicker and would have aesthetic repercussions, not to mention the added cost of the long fasteners.

    Ultimately, it is tough to keep the look of the skip sheathing and shingles as viewed from the living space below. Trying to balance that with the exterior aesthetics of a historic home may be really tough. It may make the most sense to simply consider a hybrid approach of insulating the intra-rafter spaces with fluffy insulation, removing the shingles altogether adding plywood or a membrane as the air barrier and then a minimum R-20 of class 1 vapor retarding foam above that. This loses the interior aesthetic, but it would keep your home from degradation issues, meet code and keep your home safe.

  2. optimax | | #2


    I'm not sure what climate zone I am in; the property is in Redlands, CA.
    Yes, the rafters are below the skip sheathing. They are full dimension 2 x 4 Douglas fir on 32" center. That's why weight is a concern.
    Yes, the 2 x 4 3/4 framing above the skip sheathing is only on the eaves and rakes; nothing in the field.
    Yes, I plan on staggering the polyiso and tape the seams.
    The polyiso would be fastened to the skip sheathing per manufacturing instructions.
    The standing seam metal roof would also be fastened to the skip sheathing with long enough anchors to fasten to the skip sheathing as well.
    Anything more than 4 inches of added thickness would likely not be aesthetically acceptable.
    Unfortunately, this leaves me with a dilemma.

    Rick Payne

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