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fastening rigid insulation shear value/ site built SIP’s

Robert Bacin| Posted inGeneral Questionson

I am adding 4″ of rigid insulation over my existing cathedral ceiling. The existing structure is a simple gable roof build with 2 x 6 rafters 16″ o.c. with R-19 batt insulation. Each side is 15′ x 52′. The plan is to install the rigid insulation over the existing sheating and add an additional layer of plywood and roof with a 22guage standing seam roof. I will capture the insulation at the perimeter with a taller fascia but several of my construction knowledgeable friends are questioning the lack of shear in the field by screwing down the new sheating with long screws thru the soft ‘rigid’ polyiso foam into the existing sheating.
Notes; 1. Located in southern california
2. Existing sheating was removed and existing framing was improved by adding plywood edge blocking, framing clips to rafters and blocking @ wall plates and ridge beam.

The concept of installing solid lumber sleepers on the existing deck at 8′ of 12′ on center vertically or 4′ on center horizonally is one suggestion but is this needed?

Any thoughts?

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Replies

  1. Sean @ SLS||#1

    You have to screw into the rafters / trusses which while not impossible, is hard to do when you start getting past 4" of insulation

    As for the shear & seismic concerns, you are probably going to have to consult with & get a sign off from an engineer no matter which way you go.

  2. Jesse Lizer||#2

    Robert
    你说你是putting foam over existing sheathing....but then you say you have removed it? If your existing sheathing remains, then that is your shear diaphram right there. However if it was removed, my guess would be to either replace it with new or use an engineered X bracing shear design.

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