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RE: Insulrock diaphragm

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This appears to be a gypsum roof deck system. There were many manufacturers of gypsum systems in the 1960's. The particular system that was used varied region to region. My advice would be to find an old ICBO Research Report on Insulrock. I have an old US Gypsum Research Report on Thermofill and Pyrofill. Generally the gypsum formboards were just there to support the cast in place gypsum, and contributed little to the strength of the diaphragm system. What you are referring to were often called "form boards". The sequence of construction for the gypsum structural deck consisted of:
1.  The bulb tee or truss tee's were welded to the structural steel
2. The form boards were placed on the bulb / truss tees (The form boards were held back from the tees to allow the cast in place gypsum to flow around the exposed portion of the tee's)
3.  Wire mesh was placed on the form board and tee's.
4.  Gypsum concrete was placed on the deck system.

The system worked pretty well unless you got a leak in the roofing system.

Allowable shear loads ranged from 510 to 1200 pounds per lineal foot.

Harold Sprague

From: "Will H" <haynewp(--nospam--at)>
Reply-To: <seaint(--nospam--at)>
To: seaint(--nospam--at)
Subject: Insulrock diaphragm
Date: Mon, 03 Jan 2005 10:22:49 -0500

I have an existing building from the 1960's that used Insulrock roof panels on bulb tee purlins. I don't see how this can be considered a structural diaphragm, but it is the only thing that has been distributing the lateral loads on the building for 40 years.

Does anyone have any experience with this product? By looking at the document below, I think the planks are only resting on the tees and clipped to the main steel beams but the drawings don't specify.

thanks for any help

Will Haynes, P.E.

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