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Community and Q&A

Chainsaw retrofit overhangs

James Fugate | Posted in Energy Efficiency and Durability on

I am rehabbing a 1900 farm house near Rochester, NY (Zone 5).  We are replacing the roof, with much of it being restructured as well.  Because of a mixture of cathedral ceilings, knee walls with sloped ceilings, and dormers, I am opting for a hot roof with two layers of rigid insulation (1-1/2″ + 3″ polyiso) on top of the sheathing and cellulose in the rafter bays.

In order to get a tight house and to reduce thermal bridging, I want to construct the new roofs without overhangs and cut off the rafter tails on the roofs that are staying.  Then, the overhangs will be constructed afterwards.

My question is related to constructing the overhangs.  My framer is recommending that 2×4 outriggers for the overhangs be added either directly to the roof deck, or on top of the first layer of insulation instead of on top of the full depth of rigid insulation.  That way, the rafters will be easier to find and shorter screws can be used.  The framer is insisting that the total R-value of the roof assembly will be minimally affected.

Is this safe?  Am I risking the sheathing at those points dropping below the dew point?  Repairing rotten sheathing on a hot roof is NOT something I want to do.

Thanks in advanced.

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Replies

  1. Joel Cheely | | #1

    I'm not picturing the assembly, but I cannot imaging trying to run a fastener (probably a ledgerlok or similar) through the long way of a 2x4 then through insulation and successfully attaching to framing below. Even if it can be done the iso insulation would be crushed by the considerable force of the fastener on the relatively small footprint of the 2x4. How wide of an overhang?

    1. James Fugate | | #2

      The 2x4 will actually be on the flat and the overhang will be 2'-0". Being mostly in tension, the top member of the overhang can lay flat. The bottom member of the overhang, creating the soffit, will be in compression. Wish I had detail image to post.

      Anyway, my framer has the same concern as you regarding the difficulty of screwing through multiple layers of rigid insulation and the sheathing to hit a 1-1/2" wide rafter below. Minimum 10 inch screws would be required. That's why he wants to embed the 2x4 within the insulation rather than lay it on top, like I see most details. I just want to make sure this is OK or not.

  2. Expert Member
    Akos | | #3

    James,

    Wood is a good insulator, a 2x4 stud has a very small cross section, my gut says that it would not be able to transfer enough heat to cause issues.

    Yes the sheathing near the edge by the stud would be a bit colder for the first couple of inches in from the edge. One you are about 16" or 20" inches in the R value of the wood is about the same as your foam, no more heat loss. If you are still worried, you can always wrap the first foot or so of your outriggers in foam.

    This is more of an issue with exposed structural metal that runs into the conditioned space.

  3. Walter Ahlgrim | | #4

    Build it the way the engineer designed it.

    Often contractors get away without the engineer because they are copying engineered assemblies. But what you are doing is very different.

    Let’s say you two build this roof on your own and it collapses under the weight of a big snow, who is going to pay to repair it?

    My wild guess is you will never save enough energy with this plan to pay the extra construction costs.

    Walta

  4. Expert Member
    Peter Yost | | #5

    Hi James - great chainsaw retrofit resource here, including a good cross-section on the ladder-framed new eave overhang/soffit assembly: http://www.hhefficiency.com/blog/2013/8/25/mass-save-der-program-guide.

    Peter

  5. Doug McEvers | | #6

    More information on the subject of retrofits.

    http://thesustainablehome.net/harold-orrs-superinsulated-retrofits/

  6. Joel Cheely | | #7

    I think I would attach the 2x first then cover with insulation. Then shim up the 2x at the overhang.

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