Does T&G Porch Ceiling require ply. underlay? Does T&G alone act as acceptable sheathing dia 1

[finchaser]

Hello All,

I am working on a residential single family home. The owner would like to replace typical ply. porch ceiling sheathing with tongue & groove. Can the t&g replace the ply. sheathing and still act as the diaphragm? Is it a requirement to install ply. above the t&g in order to have a more rigid horizontal ceiling diaphragm? I can find no specifications stating whether it alone can act as necessary sheathing.

Any help/advice would be great!!!

 

[msquared48]

Depends on the load the diaphragm has to resist. I have used no more than 100 plf in similar circumstances when the T&G was part of a roof diaphragm.

Is there no roof diaphragm to take the lateral force here?

Mike McCann
MMC Engineering

 

[jittles]

If you look in the wind & seismic NDS (and probably the IBC?; I typically go straight to the NDS codes), you will find diaphragm shear values for lumber sheathing.

I believe for straight/parallel applications, it's only good for about 50 plf (ASD), and about 300 plf if you orient the lumber diagonally.

 

[finchaser]

First - Thank you both for your insight. Very helpful.

To: msquared48
I still need to calculate the resisting load. I am trying to avoid projecting an extra cost to the contractor/owner if it isn't necessary to install ply. in conjunction with the t&g. I would love to just use the t&g standalone approach. I do worry that the t&g will not work as a functional diaphragm. In the case of a porch extending outward from exterior walls; I feel you would need the ceiling sheathing to act as a diaphragm for rigidity. I find no IRC/IBC code saying you must install ply. underlay. I will have to look into the NDS values and make a comprehensive judgement if I can't find specifications and codes that allow me to use the t&g alone as the ceiling diaphragm.

 

[msquared48]

I guess what I am implying is that if there is already plywood on a functional roof diaphragm, why worry about the ceiling diaphragm?

Mike McCann
MMC Engineering

 

[finchaser]

We worry because we are in a high wind area and therefore receive a lot of shear load.

 

[XR250]

If you are worried about it, I would just add the layer of sheathing. OSB is cheap. You ain't going to get much calculatable (is that even a word?) diaphragm capacity with finish nails.

 

[msquared48]

finchaser:

I understand that. But what prevents the existing roof diaphragm from taking the load?

Mike McCann
MMC Engineering

 

[BAretired]

What is the thickness of the T&G sheathing?

BA

 

[XR250]

Not only are you counting on finish nails to perform a task they do not do very well, but they will be installed by trim carpenters and they cannot be inspected as they are hidden. Why take the chance if you actually need the diaphragm capacity?

 

[finchaser]

The decision has already been made to install the plywood sheathing. I agree 100% with your comment "ExcelEngineering". There is no use in taking the risk.