Roof diaphragm spaced from rafters
Roof diaphragm spaced from rafters
(OP)
I am interested in building my own house in the next year or so, and would like to minimize thermal bridging at framing members as much as possible. Is it structurally feasible to apply 2 1/2" rigid insulation over the rafters, and then screw 5/8" plywood over that, through the insulation, and into the rafters? This moves the diaphragm away from the rafters by the thickness of the insulation, and really puts the fasteners to the test. I would have 5/8" gwb directly on the underside of the rafters, and could even run galvanized strapping diagonally both ways to provide racking resistance. Where I live (central NY), wind generally governs, not seismic.





RE: Roof diaphragm spaced from rafters
You might look into SIP panels - foam glued and sandwiched between two sheets of plywood or OSB. The reputable companies have tested them and gotten various approvals.
RE: Roof diaphragm spaced from rafters
Structurally, it would be better to attach a layer of sheathing directly to the roof framing, then bond a layer of rigid insulation on top of the sheathing, and then bond another layer of sheathing on top of the insulation - essentially creating a stress-skin panel. But there are other things to consider as well. How much heat loss will you really prevent? How will this kind of framing affect the life of your roofing material, as there's no ventilation directly under the sheathing to help keep the roofing material cooler in the Summer? Are you trying to create a cathedral ceiling below? Savings from one consideration creates costs in another.
Ralph
Structures Consulting
Northeast USA
RE: Roof diaphragm spaced from rafters
RE: Roof diaphragm spaced from rafters
Pichy - You have to consider more than just shear in the fasteners. Rigid insulation is still compressible, so you will actually see shear & bending in the fasteners when the entire diaphragm resists lateral loads from wind or earthquake. I suspect the entire diaphragm will tend to "loosen up" over time as a result. I appreciate your goal, but I'm skeptical as to its durability.
Ralph
Structures Consulting
Northeast USA
RE: Roof diaphragm spaced from rafters
The difference in cost is not that much, but the structural performance MUCH BETTER.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Roof diaphragm spaced from rafters
Not a good idea. I'd put sheathing down, then the insulation, then a hardboard over that.
RE: Roof diaphragm spaced from rafters
RE: Roof diaphragm spaced from rafters
Goog point mfrad - sometimes the obvious isn't. Constructability is essential for economy.
Ralph
Structures Consulting
Northeast USA
RE: Roof diaphragm spaced from rafters
RE: Roof diaphragm spaced from rafters
RE: Roof diaphragm spaced from rafters
Reference link for you. http:
RE: Roof diaphragm spaced from rafters
RE: Roof diaphragm spaced from rafters