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Metal Roofing used as Diaphragm

Metal Roofing used as Diaphragm

Metal Roofing used as Diaphragm

I am designing a non-insulated building where standing seam metal roofing is specified.  Is a structural sheathing substrate (i.e. plywood or cdx) my only option for developing the roof as a diaphragm, or are there standing seam metal roof products rated to provide shear resistance required for diaphragm design?  I am aware that the National Frame Builders Association has developed the "1996 Diaphragm Test" which serves to show the acceptable use of light gage metal roofing and siding as lateral force resistance systems for their specific configurations.  However, the structure I am designing is not a post frame structure, and I do not want to extrapolate data just to make this work.  An alternative option would be to know if there is a steel deck product available that functions as a roofing finish and provides equivalent water intrusion protection as standing seam metal roofing.  In this case I could design the diaphragm as per Steel Deck Institute diaphragm design manual.

Any advice is appreciated.

RE: Metal Roofing used as Diaphragm

Standing seam is a weather barrier and architectural treatment.  I've never seen any used as a diaphragm.  The way they assemble it (seams mechanically clamped) makes it unlikely in my mind that they would have much shear capacity.
But it would be nice to hear different.

RE: Metal Roofing used as Diaphragm

Profesionally, I am not aware of any, but I havce never had the impetus to use the material for a diaphragm either.  Knowing the material geometry and installation, I would not put much, if any reliance on the lateral strength due to the high vertical ribs and resulting tendency to deflect.

Personally, I would rely on the CDX and forget the standing seam.  

Mike McCann
MMC Engineering

RE: Metal Roofing used as Diaphragm

There are a few standing seam roof systems on the market that have a stated diaphragm capacity, but I would not rely on the panel due to the rib construction and buckling of the flat (pan) of the panel.  Best best is to use CDX or a through fastened panel.  

What type of panel spans are you dealing with?  Is thermal expansion the primary concern with slotting of screw holes causing a leak?  Or is the general installation where over/underrunning of screws leads to leaks?

RE: Metal Roofing used as Diaphragm

Agree with Mike and JC...not much reason to use standing seam as diaphragm.  The standing seam is uni-direction...the remainder of the sheet offers very little in shear or compression to develop a diaphragm.

If you need diaphragm, consider plywood or corrugated decking with standing seam applied to top.


RE: Metal Roofing used as Diaphragm

The roof sheeting fixed to purlins does provide some capacity to act as a diaphragm but how can you justify that something with a thickness less than 1mm with ribs has the capacity to transfer loads in-plane. There is a residential steel framing institute in my region that have been trying to get a PhD on-board to conduct research tests into metal sheeting used as a diaphragm but I don't think it has got of the ground.

RE: Metal Roofing used as Diaphragm

Wei Wen Yu's book has a section discussing sheathing. There you can find conditions that ensure diaphragm action for some cases. And you will easily note that it is almost impossible to meet those conditions with standing seam roofing.

RE: Metal Roofing used as Diaphragm

They did some unofficial testing at James cook uni a while back on trimdeck 0.42 BMT, the problem was that the sheeting was good for shear when loaded in shear alone, I think they got 1.5kn or something per screw (don't quote me).

Then they decided to do a bit of fatigue work and the found that the purlin spacing had to decrease significantly because the sheeting once it reached plastic fatigue (I don't really know what to call it) lost a lot of its capacity. Hence really wasn't worth the effort for any building which used purlins, but the little garage sheds use this to their advantage, but I believe stramit or lysart did some further private testing to ensure compliance.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: Metal Roofing used as Diaphragm

Thanks for all the comments.  This reinforces my original thinking.  It helps to have a few people agree with me before I tell a client he has to install 20,000+ sq ft of structural roof sheathing.  The project is for a mini-storage development where I am using cold form steel purlins at 24" o.c.  It seemed like an ideal project to check if the roofing could perform as a diaphragm, but in all likelihood it just isn't possible.  Thanks again.


RE: Metal Roofing used as Diaphragm

I would think that a horizontal truss using roof bracing would be more economical that plywood sheathing.  In Australia, we use purlins and metal roofing for almost all of our commercial projects, and never use sheathing, but rather use discrete bracing members to form a truss.  Why are your purlins so closely spaced?

RE: Metal Roofing used as Diaphragm

7/16" wood structural panel substrate (as I currently have proposed) has a recommended roof span rating of 24".  Agreeably, if I used structural metal roofing with a braced roof system as hokie66 proposes, I can increase my purlin spacing (because structural metal roofing allows for much longer span ratings).  However, for the local area, diaphragm framing is a far more typical construction practice.  I think I will stay with the wood substrate.  Thanks for the input.

RE: Metal Roofing used as Diaphragm

Why don't you try x-bracing rods in the plane of the roof?

Depending on the size of the building, one or two bays in each direction should do it.

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