Install metal roof deck directly over top of existing?

[Ben29]

Due to mid-construction owner modifications, I am in a situation where the steel roof deck (that was already ordered and is on-site) does not attach to a supporting member at one end. To remedy this, the deck supplier suggested that they install new deck (2-span condition) over top of the existing metal deck to bridge the gap. Is this acceptable? I am worried that they won't get a good screw attachment of the top deck to the supporting steel below. This happens to occur right at a moment frame. Portion of the shop drawing is attached...

I was going to suggest that they just cut the existing deck as required, but I don't want to make more work for them if I don't have to.

 

[TheDW]

I'd prefer to continue the new deck farther right to the next joist, making it a 3 span condition, and neglecting the partial span. Otherwise it looks like you'd be using cantilevered deck as deck support.

Edit: and I'd think that the connection to supporting steel wouldn't be any different than a normal lap over a support.

 

[WinelandV]

Is the gap the 1'-6" dimension to the right of the moment frame? Or is the gap on the left side of the moment frame? The drawing isn't clear as to where the gap is.

 

[dik]

I'd go three two span and the new deck would simply cover the existing 'cantilever' deck. Concur with Koot below...

Dik

 

[KootK]

I'm fine with the concept and fine with two span as long as the numbers work. Like daywalker, I don't see an issue with the nested deck fastening so long as you're not trying to fasten over a deck location that is already nested.

 

[RPMG]

I think Ben's concerns are legitimate. There could be 3/32 to 1/8" gap between the deck plies, for diaphragm shear at the moment frame. You could consider adding rod bracing.

 

[jayrod12]

I would be more confident on a screw attachment to supporting structure than a weld. Especially where nested. They buy a longer screw as required to ensure you get good tapping and bite. You should be able to have a reasonable visual confirmation of how much screw is sticking out of the bottom of the deck whether there are significant gaps or not.

 

[JAE]

We've nested decking before with no problem - similar to jayrod12 - use screws to ensure a proper connection.

Our use of double decks was in areas where joists were spaced further than "typical" to accommodate larger, joist-hung mechanical units between joists. Instead of using a heavier gage deck, or a deeper deck, we simply doubled it.