Steel deck laps

[xez]

Hello all,

I apologize if this is the wrong forum for this question - wasn't sure where it was appropriate.

I'm new to steel decking (e.g. B-deck) and was wondering if sealant is typically used at the seams, overlaps, endlaps at the steel deck since polyiso rigid board and roof membranes are installed over the top of deck.

Thanks in advance.

 

[KootK]

Have no fear, this is surely the perfect place for your question. I've been at this a couple of decades and, to my knowledge:

1) The deck joints are not sealed as you suggested and;

2) Elements of the roof system, other than the deck itself, typically function as the building envelope.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[xez]

Thank you for the quick response KootK.

I was looking into potential causes for condensation in decking and couldn't find many sources regarding the joints.

 

[MIStructE_IRE]

As for condensation forming on the underside of roof decks, this has become an issue throughout school buildings in my particular country (Ireland) so your mileage may vary... It has nothing to do with deck laps here, its the degree of air tightness others are requesting of the building. Its ridiculous in my view - years ago it was all about ventilation...now its all about air tightness! Causing big condensation problems on the underside of roof decks however which is revealing itself on ceilings.

 

[dhengr]

Xez:
The condensation is caused by the fact that there is too much humidity in the air in your bldg. and that the underside of the deck is being allowed to cool enough to become a condensation surface. The underside of the deck is reaching the dewpoint temp. for the air in the bldg. You have to do something in the way of mech. ventilation and dehumidification to lower the moisture vapor in the contained air. As MIStructE_IRE suggests you can’t make bldgs.. more air tight without providing some mech. ventilation system to control these issues. Have bldg. changes occurred which are causing these problems all of a sudden? You might also provide some fans which would circulate the air up at the elevation of the roof deck.

 

[xez]

dhengr/MIStructE_IRE - I don't have a building that currently has condensation issues, but I was trying to understand potential drawbacks in a steel deck when it comes to condensation. My assumption is that unsealed laps, fastener holes, and penetrations would be a factor. I understand that building use, temperature, humidity, dewpoint, mechanical systems play a large role. When would a vapor barrier between the insulation and deck be used?

 

[BridgeSmith]

I would think sealed laps would be worse, as any moisture that got through the concrete above would be trapped on top of the steel decking. The bottom of the deck is not the place to be trying to stop leaks. I believe condensation would form on the underside of the decking, so sealing the laps will not help there either.

 

[DBronson]

XEZ:

Have a look at the linked paper. Discusses the proper way to seal metal roof decks:

Link

Note that he recommends using paperless gypsum board sheathing to support the air barrier. Paperless gyp board won't support mold growth.

Regards,

DB