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Would it be crazy to use TJI’s over old roof rafters?

nyleph | Posted in Green Building Techniques on

We are buying a REAL fixer upper with roof problems that include falling off box gutters. It needs a new roof anyway and I had thought about foam board as we would like to use the attic space as living. Is there any major flaw to attaching 12 ” TJI’s over the rafters and using cellulose? The “sheathing” is just spaced boards which could be cut after the TJIs are installed for one cavity in each bay. This should also fix the roof line to use regular gutters too, I think. I am wondering what the drawbacks of this are, and if regular venting (baffles against the sheathing) would be sufficient.


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  1. GBA Editor
    Martin Holladay | | #1

    Your plan raises several issues that probably require advice from an engineer. Your existing rafters and roof assembly perform many functions and act as a unit. If you intend to cut out the skip sheathing, you will be weakening the existing roof assembly.

    For your new TJI rafters to perform all of the load-bearing and structural functions required for a roof assembly, they will need to be adequately tied to the existing house. You can't just install them on top of your roof and toe-nail them into place.

    That said, it's certainly possible to implement your plan, as long as it gets the OK from an engineer. If you intend to install cellulose insulation, don't forget the ventilation baffles. Many builders have had good success with the AccuVent. Another option is site-built baffles.

  2. mrbreadpuddin | | #2

    This might be hard to pull off aesthetically, have you thought about what you are going to do for trim on the overhangs and rafter tails?

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