is is possible to attach a metal roof deck to bar joists from below

[WIEngr]

I recently visited the site of a remodeling job and found that the roof deck was missing many of its attachments to the bar joists. I was considering attaching it from below with screws or rivets. Does anyone have any insight on this?

 

[JAE]

You could pre-drill holes through the joist legs and then screw up to the deck - you would want to ensure that the screw was long enough to fully engage the bottom horizntal surface of the deck but not too long to poke above the top surface of the deck - otherwise you run the risk of damaging insulation, or poking though roofing, etc.

I would not weld the joist chord tips to the deck for fear of burning, catching on fire, the insulation above.

The drilling is overhead and time consuming - i.e. costly.

 

[JD2]

CAN you do it? I would have to say yes - with the concerns and detailing that JAE has described.

However - If there is a requirement or concern about wind uplift or diaphragm shear capacity, I would think that attachment from the bottom would not offer the same capacities as attachment from the top - I'm guessing here as I have not read any studies on the matter. With the screw head on the bottom of the joist chord angle, there would be less bearing area for the deck against the screw in uplift (only the area of the threads holding it down). Also there would be less of a "compression" force exerted by the screw against the deck down against the joist chord angle to keep it from buckling under load.

My $.02

 

[msquared48]

Thinking out of the box here a little:

If you are worried about uplift, what about using a toggle-bolt through the roof diaphragm and with a side clip to the joist. The bolt should be able to spread out the insulation to deploy above the decking, and there is no welding or need for drilling the joist flanges. Just have to make sure the bolt does not penetrate the roof membrane.

If you need a shear attachment to develop the diaphragm, that will not work well and the other measures should be used.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MikeHalloran]

Consider welding threaded studs (using a real stud welder, not a torch) to the underside of the decking alongside the joist flanges, and then using clips and nuts.

I guess there's some risk to the insulation, but the far side of the sheet doesn't get quite as hot as the near side, and the heat input is time-limited. Perhaps some testing on a representative section would bring confidence in the process.




Mike Halloran
Pembroke Pines, FL, USA

 

[Brad805]

I had a similar problem not long ago where the hilti gun operator was not paying attention (still trying to solve our case). I am curious how well you guys think the screw solution from below will work? If the screws are installed from below the screw head will not help hold the sheets down and I fear that an over zealous worker will strip the threads in a 22g deck.

I like the rivet idea, but I sure wouldnt want to be the guy paying for it.

Brad

 

[Lutfi]

Yes you can. I have seen intuitive design years back. It was used on a WF beam. I think the same can be done for a joist.

What they done is they used a light gauge strap and formed a “U” with two horizontal legs and they screwed it to the deck.

I hope this helps.


Regards,
Lutfi

 

[Ron]

The method Lutfi describes is workable for uplift. If you need diaphragm, the strap has to be attached to the joist angles.

The gauge of the deck has a lot to do with it as well. For lighter gauge decks (24,26 ga), a lot of the capacity from the top is pullover resistance around the screw head or with weld washers. When fastened from the bottom, you have to use a lot more fasteners because you've lost that pullover capacity.

Another way to do this is to run a light gauge stud track along the underside of the joist top chord angle, mechanically fastening it to the angle at close spacing with powder actuated fasteners or Teks screws. Then fasten the overhanging portion of the stud track to the underside of the deck at very close spacing (2 fasteners per flute) with self-drilling, oversized screws. You can do the same with a 6-inch wide strip of galvanized sheet metal (16 to 18 ga), but stud track is readily available and cheap.

Check capacity, but this should handle both uplift and diaphragm. Remember you'll need to do it on both sides of the joist where you have deck laps and where you need extra capacity.

 

[dik]

Use Teks3 self-drilling screws... and be careful. If you need any load capacity (shear, uplift, etc.), if you have to, you may consider using 'pop rivets'... Only time I've ever used them was because they forgot cross bracing and I used them because they had the brick veneer and steel stud wall up... building's been standing for about 25 years...

Dik